PRIOR ART In the Swiss patent n° CH-A-687024 and in WO-A-96/23865 there is the description of a procedure for industrially carrying out the closed circuit culture of microalgae in which: there is the circulation of a liquid containing the said microalgae, the liquid is: w maintained at a temperature between preset values * injected with C02 and nutritious substances * exposed to solar radiation in a transparent region of the circuit (photosynthesis) deprived of oxygen developed in the liquid itself and w separated from the produced algae, that are then extracted from the closed circuit.

The European patent N° EP-A-0310522 describes a means for the intensive and controlled production by photosynthesis of microorganisms suspended in a liquid medium, comprising at least: a photobioreactor on an expanse of water consisting of a first group of tubes transparent to light where the liquid growth medium circulates, and of a second group of tubes set below the first, in such a way as to submerge or not the photobioreactor a carbonater, fed with pure or dilute CO2 a degasser to eliminate the 02 produced by the microorganisms and * an expansion barrel or vessel to compensate any possible variations in the liquid volume within the photobioreactor Systems (and relative processes, devices and plants) of the type certain have advantageous innovative aspects without however being exempt from inconveniences that have, among other things, limited their practical diffusion: for example, the placing of the photobioreactor (according to EP-A-0310522) on expanses of water that could, in particular, be discharge waters from nuclear reactor reservoirs or tanks, entails risk and danger that in the event of the breaking of at least one element of the photobioreactor there would be bacterial contamination, that would lead to the inevitable substitution of liquids, both culture (within the photobioreactor itself) and cooling (in the reservoir where the photobioreactor is placed). Furthermore the need to use the water in the reservoir as a cooling means, limits the application of this technique to small tanks (mainly designed for this purpose) and would not permit its application to natural, not-calm, expanses of water, namely water always in movement.

The degasser generally consists of a vessel with an inlet and an outlet at the bottom for the culture liquid, and with a U tube open in the upper part of the said vessel to collect the oxygen present in the gas phase above the liquid. The elimination of 02 with a single degasser, particularly of the container or vessel type, does not allow an easy and complete elimination of the gas.

Among the more obvious limits of the known technique we limit ourselves to emphasizing, apart from the inability of operating on expanses of natural water in movement and with wide thermal excursions throughout the year, also the inadequacy of the degassing; inadequate degassing not only results in not reaching the desired high cell concentrations, but also in the reduced exploitation of the photobioreactor; only those parts of the plant where saturation has still not been reached exploit the said photbbioreactor to the full, while in other parts of the said plant saturation is most certain verified.

The aim of the present invention is that of providing a system (with relative procedures, plants and means) that eliminates at least the mentioned inconveniences, and that is, in particular, able to be used on all possible sites (compatible with sufficient solar light irradiation).

Another aim of the invention is to provide a system with a fine control of the culture liquid temperature.

A further aim is to provide a degassing system that is particularly efficient also because of its critically differential distribution.

These and other aims are achieved with the ASP system (and relative procedures, plants and means) according to the invention of which the most notable characteristics are recited in the claims hereinafter, that are considered herein incorporated.

The different aspects and advantages of the invention appear more clearly from the following description of the embodiment form (though illustrative and not limiting) represented in the accompanying drawings in which: * figure 1 is a general block scheme of the ASP plant * figures 2,2', 2"are schematic, partially frontal and section views of three advantageous temperature control subsystems according to the invention * figures 3 and 3'are schematic views, respectively top and lateral views, of the inventive differential disposition of batteries of tubes and degassers according to the invention * figure 4 is a schematic view (in orthogonal projection) of one of the on line degassers according to the invention * figure 5 is a partial sectional view of the principal degasser w figure 6 is the control scheme (according to the invention): x) of the temperature of the culture medium, y) of the pH of the culture medium, z) of the quantity of oxygen dissolved in the culture medium, w) of the cell concentration in the culture medium, j) of the integrity of the tube system, k) of the saline concentration of the culture medium * figures 7 and 7'are diagrams respectively related to the temperature and pH controls of the culture medium figures 8 and 8'are designs illustrating the advantages obtained by using a particular flooring under the photobioreactor ASP according to the invention.

In the general block scheme of figure 1 the numerical references indicate what the following reports, allowing the defining of three generic sectors: section I is water treatment (STA), formed by the units 5,6,7,8,9, where all water purification treatments are performed, water from the source being purifying so that it can be sanitized in the desired manner; sector 11 is biomass growth (SCB) formed by units 13,14,15,16 where there occurs the critical and synergic implemention, that is concurrent, of more factors like direct light, presence of nutrients, appropriate physico-chemical conditions (T, pH), that permit cell growth; section III concerns the separation of the biomass (SSB) and is formed by units 18 and 19, where there is the separation of the produced biomass from the liquid forming the culture medium, and, furthermore, the washing of the said biomass to separate it from residual salts.

The other units not falling within these sections each have a specific function, and cannot be put together in a generic function. The description of the different units follows.

1) an unfed water source (SA) from which water A flows; 2) micronutrient storage (Smic) with the function of conserving the micronutrients in conditions appropriate for non-deterioration, it is at this point that the micronutrients (mic) enter the circuit and from the outlet the micronutrients go to the pre- solubilization unit (PS) and the auxiliary unit of nutrient solubilization (USa); 3) macronutrient storage (Smac) has the function of conserving the macronutrients (mac) in appropriate conditions to preclude the deterioration of the product; the incoming macronutrients are directed towards the principal solubilization unit (USP) and to the auxiliary nutrient solubilization unit (USa); 4) a unit of micronutrient pre-solubilization (PS) where the function is to carry out micronutrient pre-solubilization in water; the dosing of the said micronutrients has to be in very small quantities so it is preferable to add them already in the solution form; the said micronutrient solution in water formed here is sent to the unit of principal solubilization (USP); 5) a filtering unit (UF) that has the function of eliminating particle-type impurities in suspension in both water from the source (SA) and recycled water coming from the biomass separation section (SSB); the water leaving this unit (F) is sent on to the following softening unit (UA); 6) a softening unit (UA) in which the water from the filtering unit (UF) is deprived of temporary and permanent hardness; the thus treated water is sent to an accumulation tank (SA1); 7) an accumulation tank (SA1) with the function of being a plenum chamber or expansion box between the softening unit (UA) and the following osmosis unit (UO); 8) a unit of inverse osmosis (UO) that has the function of eliminating every type of substance dissolved in the water coming from the softening unit (UA), ranging from any salts not eliminated in the softening unit (UA) to the elimination of bacterial load; such an operation gives a yield of 50-60%; the treated water leaving the inverse osmosis unit goes to an accumulation tank (SA2); the residual water is used, subject to sanitization, for the culture of other algae species; 9) an accumulation tank (SA2) where the function is that of storing the osmotized water until the moment when it will be used; the greater part of the water leaving the accumulation tank (SA2) is sent to the principal solubilization unit (USP) while a lesser amount goes to the units of micronutrient pre-solubilization (PS) and secondary nutrient solubilization (USS) and the biomass separation section (SSB) where the biomass is washed; 10) a unit of principal solubilization (USP), where there is the mixing together of water from the accumulation tank (SA2), macronutrients from the macronutrient storage unit (Smac) and the solution from the unit of micronutrient pre-solubilization (PS), the purpose of this mixing being to prepare fresh culture medium for use as the substrate for microorganism growth; the water coming from the principal solubilization unit (USP) is introduced into the line of the culture liquid after the principal degasser (DP) and before the circulation pump (PC); 11) A circulation pump (PC) of the diaphragm-type (but not limiting the use of other types) has the function of moving the culture liquid within the growth circuit; the circulation pump (PC) withdraws the biomass enriched culture liquid LCAB sent from the principal degasser (DP), diluted when necessary with fresh culture medium TCF that comes from the principal solubilization unit (USP), and sends it to the pH control unit (UCHpH); 12) the pH control unit (UCHpH) has the function of regulating the pH of the culture liquid LCABD coming from the circulation pump (PC), to the most appropriate value for the species raised, by injecting an acidic or alkaline gas according to the need; the culture liquid leaving this unit is sent to the cooling tower 1 (TR1); 13) a cooling tower 1 (TR1) has the multiple functions of regulating the temperature of the culture liquid to the most appropriate value for the species being grown, of permitting a further degassing of the culture liquid, backing-up or helping out the principal degasser (DP), and of permitting, if necessary, a further injection of gas for pH control; the culture liquid coming from cooling tower 1 (TR1) goes to the first stage of the photobioreactor ASP (FBR1); 14) a first stage of the photobioreactor ASP (FBR1); according to an aspect of the invention the photobioreactor ASP is divided into m modules (fig. 1 shows only one such module), each module comprising at least two stages and each of these consisting of n batteries of tubes and n'degassers on line (n being equal or different from n'), with n and n'being chosen so that a cooling tower relative to them is sufficient to maintain the temperature within the preset limits for the full segment of the stage; the function of the photobioreactor ASP is to create optimal conditions for the growth of the microorganisms, without the said microorganisms ever coming into contact with the external environment. The presence of on line degassers also allows optimized distribution of the nutrients, according to the criterium of greater additions for bigger cell concentrations; the culture liquid leaving the first stage (FBR1) of the photobioreactor ASP enters the second stage of the photobioreactor ASP (FBR2) and passes through cooling tower 2 (TR2), analogously undergoing all that occurred in the first stage; 15) a cooling tower 2 (TR2) with a function analogous to that of cooling tower 1 (TR1); the culture liquid leaving the cooling tower TR2 enters the second stage of the photobioreactor ASP (FBR2); 16) a second stage of the photobioreactor ASP (FBR2) with a function analogous to that of the first stage (FBR1); the culture liquid leaving the second stage of the photobioreactor ASP (FBR2) goes, as a rule, to the principal degasser (DP) except when the cell concentration reaches a preset value Cmax in which case the liquid is sent to the section of biomass separation (SSB); 17) a principal degasser (DP) where the function is to eliminate from the culture liquid the oxygen produced within the photobioreactor ASP due to the effect of the photosynthetic growth of microorganisms; the culture liquid leaving the principal degasser is sent to the circulation pump (PC); 18) a first stage of biomass separation (SS1) made by a vibrating sieve with a filtering mesh of appropriate size for the cultivated species, its function is to separate the produced biomass from the liquid; 19) the biomass produced is sent to the second stage of biomass separation (SS2) while the water separated out is recycled to the water treatment section (STA); the second stage of biomass separation (SS2) also comprises a vibrating sieve; in this second stage the accumulation tank (SA2) releases osmotized water to wash the biomass free of the salt residues from the first biomass separation stage (SS1); also the washing water from this stage is recycled to the water treatment sector (STA); 20) a storage unit for the gas used in pH control (USG), consisting of a cold evaporator that has the function of storing the gas used to control the pH; gas leaving this unit goes to the pH control unit (UCpH), and is dispensed (if necessary) to the on-line degassers in the two stages of the photobioreactor ASP; 21) a unit of secondary solubilization (USS) where the function is to prepare a concentrated solution of nutrients (macro and micro) to add (if necessary) in correspondence with the on line degassers.

General aspects of automatic control The system ASP (Advanced Sensing Production) of figure 1 appears advantageous for computerized control.

The control system of the plant (shown schematically in figure 6), is, according to the invention, able to monitor, record and correct all the parameters influencing the growth of the microorganisms within the photobioreactor ASP. By preference the controlled critical parameters are five, that is: x) temperature of culture liquid y) pH of culture liquid z) quantity of oxygen in solution in thé culture liquid w) cet ! concentration j) integrity of the tube battery system x) Temperature of culture liquid (see inter alia figures 6 and 7) At this site the functioning of the temperature regulation system, in its cooling version, is shown.

This parameter is of fundamental importance for maintaining the life of the microalgae in the culture medium. There exists a temperature that is optimal for cell <BR> <BR> <BR> <BR> growth, that we shall call Topt; cell growth is maximum in correspondence with this value. Shifting from Topt (in any direction) leads to a slow-down in cell growth. To maintain life it is most important not to exceed a maximum temperature that we shall call Tmax, beyond this temperature cell lysis phenomena begin with a consequent diminution of the live biomass and the possibility of bacterial cell proliferation due to the decomposition of the fraction of biomass lysis.

The existence of a minimum control temperature, that we shall call Trin, comes, instead, from considerations of an economic character, due to the fact that the microalgae grow less and less with decreasing temperature until they actually reach a phase of stasis where in fact there is no longer any biomass production. According to an aspect of the invention the temperature of the culture liquid within the photobioreactor ASP is regulated by increasing or decreasing the amount of heat extracted in the cooling towers, and otherwise increasing or decreasing the flow rate of the liquid (as will be seen later, this second method has more the aim of"leveling" the temperature in the photobioreactor ASP).

Thermometric probes placed in key positions on the photobioreactor ASP read the temperature of the culture liquid and activate the control system.

As shown in figure 6 the temperature of each stage of the photobioreactor ASP is monitored at the beginning and at the end. The initial reading serves to see if the <BR> <BR> <BR> <BR> cooling in the preceding tower has brought the liquid to be ! ow Tm. n, whilst the final reading serves to see whether the liquid has been heated beyond Tmax while passing through the stage of photobioreactor ASP. In any case priority in the control system is given to Tmax (the very survival of the microorganisms depending on it).

By preference the cooling towers (TR1 and TR2) are provided with three stages of cooling that act in cascade, according to the following order (figure 6): First stage: when the temperature corresponding to TC2 and TC4 reaches a given value, that we shall call T, (Topt<T1<Tmax) there is the opening of valves V1 and V2.

Second stage: if despite the opening of valves V1 and V2 the temperature continues to rise and reaches a value T2 (T1'T2'TmaX), there comes into action the first stage of ventilation (VE1).

Third stage: if, despite the opening of valves V1 and V2 and the functioning of the first stage of ventilation (VE 1) the temperature reaches a value T3 (T2<T3<Tmax), there comes into action the second stage of ventilation (VE2).

In correspondence with any possible reaching of Tmax the central processing unit (CPU) activates an irrigation system external to the photobioreactor, pouring water onto the battery of tubes.

It is possible that on particularly hot days and under conditions of very intense light the need to keep the temperature below the Tmax value means that, in correspondence with TC1 and TC3, the temperature has to drop to below Tmm. ! n such an eventuality the central processng unit increases the rotation speed of the circulation pump PA1, obtaining in fact a"levelin « ¢'of thé temperature throughout the hydraulic circuit, as shown in figure 7. y) pH of the culture liquid (see inter alia figures 6 and 7') Also in this case it can happen that certain alga species show a very high growth rate. Here we will describe the functioning of the pH control system for the case of the microalga Spirulina, where growth leads, as a consequence, to an increase in pH, and thus the pH gas corrector that is injected into the medium is carbon anhydride, that as well as having the function of being a carrier for carbon for cell synthesis, also restores the pH to optimal levels. In conditions of very highly stimulated growth it is possible that to keep the pH within the limits, in correspondence with pH3, it is necessary to inject a great deal of carbon anhydride into the circuit until the pH drops, in correspondence with pH1, below the value <BR> <BR> <BR> pHmjn. In such an eventuality pH1 sends the reading to the central processing unit that corrects the carbon anhydride range in V7 and progressively opens the valves V3; V4; V5; V6, V11, V10, V9 (figure 6).

Figure 7'shows the difference between a circumscribed intervention (like opening V7) and a distributed intervention (like that of opening, in addition to V7, also the other valves distributed throughout the photobioreactor ASP). z) Quantity of oxygen in solution in the culture liquid The reaction of cell photosynthesis can be summarized in the relation light Carbon Anhydride + Nutrients < Biomass + Oxygen The presence of large amounts of oxygen in the culture medium is deleterious in that the above mentioned equilibrium shifts to the left. Furthermore consideration must also be given to the fact that many of the substances present on the membrane and within the alga cell are easily oxidized, therefore it is necessary to withdraw the oxygen from the culture medium as soon as possible.

The plant described here has been designed in such a way as to favor the elimination of oxygen from the culture liquid as much as possible. However for greater security, and in order to avoid undesired occurrences, provision has been made for controlling the oxygen concentration in the culture medium. Two probes for measuring the concentration of oxygen have been placed in key points of the plant.

These are CC2 and CC3 (figure 6). In the event of the concentration of oxygen exceeding a maximum level, that we shall call Omx, the probes send this information to the central processing unit, that increases the rate of the culture liquid in the photobioreactor ASP, increasing the number of revolutions of the circulation pump PC. <BR> <P>Increasing the rate of the liquid increases the turbulence that this creates in the principal degasser and also in the on line degassers, precisely favoring the libration of oxygen.

An indicator Cl1 continuously reads the concentration of oxygen at the outlet of the degasser and sends this information to the central processing unit that records it. This measurement is not of great importance for checking purposes but it does show the efficiency of the degasser over time, giving a valid indication of the times past which it is necessary to carry out maintenance work on the degasser itself. w) Cell concentration The operation of extracting the biomass from the culture liquid is economically viable only when a certain concentration, one we shall call CmjnX is exceeded. Furthermore it is advantageous that the concentration never exceeds a certain value, one we shall call Cmax, to avoid entering the stationary phase of cell growth where the phenomena of culture"aging"can occur. For this purpose a probe placed at the outlet CC1 of the photobioreactor ASP is able to continuously read the cell concentration, and as soon as it reaches the value Cmax it signals this to the central processing unit that opens the valve V8 and promotes the effluence of the enriched culture liquid to the sector of biomass separation. At the same time there is the opening of valve V12 through which fresh culture media is introduced into the photobioreactor ASP. The moment the concentration drops again (by the replenishing effect) to below the value Cmin, the valves V8 and V12 close.

Figure 6 shows the control system of cell concentration. j) Integrity of the tube system The reasons that could lead to loss of liquid from the photobioreactor ASP are essentially two.

* breaking of a tube: in such cases the loss of liquid is abrupt 'imperfect seats at tube and degasser junctions: in this case the loss can be very contained but protracted over time In the case of verifying the first case, a pressure gauge, PC1 measures the loss in pressure in the hydraulic circuit, transmitting the data to the central processing unit that activates an alarm system. The broken tube is substituted in the shortest time possible.

Instead in the event of the second case it is unlikely that the pressure gauge will manage to distinguish between the loss in pressure due to a small leak and the oscillations due to the continual turbulence in the hydraulic circuit (noise). In such a case a meter for liquid level LC1 placed in the principal degasser (in a region where there is suitably reduced oscillation of the liquid) is able to evaluate the decrease in liquid in the circuit. This meter transmits the data to the central processing unit that also in this case activates an alarm, indicating the need for repairs.

Figure 6 shows the alarm system in the case of a break in the hydraulic circuit.

Among the more notable advantages of the invention, apart from the above mentioned system of electronic control, we limit ourselves to mentioning the following: Flooring under the batteries of tubes The flooring under the batteries of tubes is of a certain importance. It is realized in very fine white gravel, with a granulometry no greater than 2.5mm. Its function is that of reflecting the light in a diffused manner (figure 8') and not direct (figure 8), in this way reducing any lens effect and the risk of cell photolysis. Therefore the flooring is a structural element of the plant ASP and has its own, very precise, function.

Batteries of tubes It is appropriate to describe here in detail both the arrangement of the transparent tubing and the on line degassers.

Arrangement of the tubes On each stage of each module of the photobioreactor ASP, the transparent tubes are mounted in two superimposed rows (figure 3'). This arrangement has been made to achieve the following goals: * Shading the lower tubes by the upper tubes. Such an arrangement allows the reduction of the lens effect in the lower tubes, in that the light falling on them is more diffused (partly deviated by the upper tubes and partly reflected by the flooring) than direct.

This leads not only to a more contained heating of the culture liquid, with respect to if the tubes were assembled on a single level, but also to a lesser amount of direct radiant energy, obtaining in this way reduced risk of photolysis in the cellular material.

The two-level arrangement of the tubes also has the aim of diminishing temperature excursions due to wind. In fact the distance between the two levels is such that it creates (for horizontal wind) a partial impediment to the free passage of air between the two rows, this leads to a decrease in the velocity of the wind and, in fact, in heat exchange (both in heating and cooling) between the air and the culture liquid.

On line degassers (figure 4) The degassers are arranged along the batteries of tubes of the photobioreactor ASP on the basis of a pseudo exponential law (figure 3). Such a law is the result of different considerations, based in particular on the following assumptions -the law that describes the growth of all one series of microorganisms (among which those that grow in the photobioreactors ASP) is a law of exponential character -the quantity of oxygen produced by the photosynthetic reaction of cell synthesis is in relation to the quantity of biomass produced, having assumed for such a relation a linear law Given: -K, a constant depending on the flow rate in the battery of tubes and on the kinetic constant of the synthesis reaction of the biomass -K2 a constant depending on the type of culture liquid, and, to a lesser degree on the pressure -CF a constant depending on the cultivated species -K3 a constant depending on the type of culture liquid and the pressure outside the photobioreactor ASP the relation that describes the arrangement of the degassers is the following: xn = (1/K,) *In [(K2*CF/exp (K, *an))/((K2*CF/exp (K, *an))-K3)] « (1) where xn indicates the distance at which the degassers have to be positioned starting from the last crossed by the liquid, an is described in the following numerical succession, an=exp(Ki%=i.n-i)Xn)(2) and a is an experimental constant.

On the basis of such a careful arrangement of on line degassers it is possible to support the freeing of oxygen from part of the culture liquid.

These devices, apart from permitting the liberation of the oxygen, carry out a series of functions that are listed as follows * they act as compensators with regard to'water hammer', that is the jerks in pressure due to the starting and stopping of the circulation pump * they constitute a"feeding point"of the culture, in the sense that they are provided with an opening 1 for the intake of nutrient salts. Furthermore through distributor 2 it is possible to introduce gas for the regulation of the culture liquid pH. This fact permits the maintenance of nutrient concentration and a pH of around the optimal value throughout the full length of the photobioreactor ASP. In the absence of such a system it would in fact be necessary to introduce salts in only one point of the circuit, with the consequence that the concentration of the salts would vary between a maximum and a minimum, the further away they are the longer is the photobioreactor ASP. This would lead to phenomena inhibitory to growth, through an excess of nutrients in the region of higher concentration and a lack of them in the region of lower concentration, and hence lack of growth.

* they allow the homogenization of the culture medium from the point of both physical parameters (temperature, pressure, density) and chemical (cell concentration, nutrient amount, pH) that can differ from tube to tube because of the different exposure to solar light and wind.

With regard to the system junctions between the on line degassers and the tubes, but also between the tubes themselves, these are"free joints", in the sense that whilst guaranteeing that the liquid will remain contained within they allow the sliding of the tubes to compensate for thermal expansion and contraction.

Principal Degasser (DP, figure 5) This consists of a cylindrical tower on a vertical axis, the construction material being plastic or metal.

In the principal degasser the culture liquid enters in A. From here the liquid passes into a distributor B, its shape having been studied so as to optimize the distribution of the liquid itself (without however promoting stress) maximizing the surface apt to free oxygen. The two septa C complete the dispersion of the liquid, that then collects on the bottom. With the aim of further increasing the surface of air-liquid exchange, a toroidal distributor (D) disperses very fine air bubbles (input in H) into the culture medium. The septum E has the function of creating a calm region in the liquid corresponding to the outlet (F) of the degasser, avoiding the possibility of carrying along air bubbles that would give rise to cavitation noise phenomena of the circulation pump (CA1, attached 3).

The reintegration of the nutrient enriched water occurs in G, also in this case the turbulence, produced by the liquid falling from above, favors the degassing of the culture medium.

The apparatus is supplied with an"overflow" (I), in the event of the circulation pump being obstructed, breather pipe (L), of aeolian energy (like those used on the chimneys of fireplaces), that maintains a slight underpressure within the apparatus itself, therefore favoring the rushing out of gas eliminated from the culture liquid and avoiding the introduction of air from outside, and of a discharge pipe (M) to empty the circuit during the rest periods when maintenance etc. is carried out.

It is noted that a microorganism culture in a closed room or environment can be compared with a cardiac system; indeed to reproduce a natural ové-espace environment, it is necessary to establish an osmotic system in which the closed space conditions, in particular the pressure, must not greatly exceed the conditions of the natural open environment. According to the invention the pressure of a natural open room and that of an artifical closed environment must be substantially near and compatible. In other words, algae that grow naturally on the water surface and in deep water are to be cultivated in a closed room at, respectively, low and high pressures. Among the further advantages of the invention, the following are to be stressed: * algae grown in our closed circuit are easily dried with the possible help of vacuum, but without any heating (contrary to naturally grown algae that need drying temperatures, even up to 200°C, that cause an, at least, partial degradation; * as our algae are contamination free (e. g., without polluants and pesticides) we can proceed on an industrial scale to the extraction of their components with the same safety of a laboratory extraction; for instance extraction of cyanoids particularly of precious phyco-cyanones from our blue green algae gives extremely pure products.

The invention as described, for clarity reasons with reference to the embodiments represented in the accompanying drawings, is suspecptible to those variants and modifications that being within the reach of skilled persons falls obviously and naturally within the scope of the following claims.