The present invention relates to a gas compressor. Diverse gas compressors are known in the art. As a result of the inherent technical configuration thereof, the known gas compressors are particularly- unsatisfactory when the gases to be compressed have a high degree of humidity. Conversely, it is inevitable in determined applications that gases to be compressed contain much moisture, for which an adequate device has not been proposed heretofore. Such applications are for instance the (temporary) storage of gases prior to (re)use, such as nitrogen as byproduct of a combustion device, such as a heating installation, in glass horticulture, where the gas is pumped into the glasshouse to enhance plant growth. The gas can then also be recovered from the glasshouse, wherein much moisture is also present in the gas. Such gas compressors are known, and usually comprise a cylindrical reservoir having therein a practically sealing plunger or piston which moves in the cylinder, and with which the volume in the cylinder can be reduced during a compression stroke, wherein the pressure in the cylinder will be increased, or the volume can be increased, wherein the pressure in the cylinder will be decreased. Because the cylinder is further sealed but comprises valve connections with which gas can be vented when the pressure is increased, or gas can be admitted when the pressure is decreased, there is a pumping action with which a liquid or gas can be displaced. The plunger or piston can be moved by coupling thereof to a rotation motor, via a crankshaft and plunger rod, wherein the rotation motor is driven electrically, pneumatically, hydraulically or by means of combustion. It is known of these types of compressor that excess liquid in the space intended for the gas, for instance as a result of condensation, seriously impedes the proper operation of these compressors. Furthermore, because liquid is not compressible, direct damage can be caused, while the liquid can also cause indirect damage, such as corrosion or excess wear, because the lubrication is disrupted. Compressors of this type are therefore not readily usable for practically saturated gases, unless the condition of this gas is improved, for instance by reducing the moisture content. Only when the dewpoint is decreased is the compressor able to process the gas. Such problems occur for instance in large heating installations combined with heat storage such as are applied in glass horticulture. Water is heated in large heat storage tanks during burning of gas. The residual product C02 from this combustion is required for carbon dioxide fertilization of the crops. The heat is extracted from the heat storage tanks for the rest of the day to heat the glasshouses. The changes in temperature cause a change in the volume of the total water content. In order to allow this volume change to be accommodated, an expansion space is integrated in most cases into the heat storage tank. In order to prevent corrosion an inert gas is maintained at the top of this expansion space of the heat storage tank. Because the pressure may not change too much, a compressor of the type according to the invention will be well able to temporarily compress this inert gas in a pressure vessel to be used for this purpose in the case of a pressure increase during heating of the water in the heat storage tank. Because this inert gas is almost wholly saturated with water vapour, the dewpoint will be exceeded almost immediately and much condensation will be released into the compressor. In contrast to the known compressors, the compressor according to the invention is particularly suitable for this task, because the released condensation can be discharged in controlled manner and causes no disruption whatever in the operation of the compressor. When the temperature in the heat storage tank falls and the volume decreases, the pressure will also drop. The inert gas can now be vented in simple manner from the pressure vessel, where a higher pressure prevails, to the heat storage tank. The gas pressure in the heat storage tank can hereby be held practically constant. The present invention has for its object to provide an adequate gas compressor and to fulfil a long-felt need. The gas compressor according to the present invention is defined in the appended claims, in particular claim 1, wherein the dependent claims relate to non-limitative preferred embodiments. It is precisely through the use of liquid to place under pressure and thus compress the gas that an adequate gas compressor is provided in surprising and effective manner which, even if the gas for compressing has a high degree of humidity, can realize compression of the gas in effective manner. The compressor according to the invention is not embodied with plungers or pistons as in the above mentioned known compressors. The change in volume in the compressor is obtained according to the invention by changing the liquid level in the cylinder. As with a compressor embodied with plunger or piston, the gas flow is controlled by means of valves and conduits. Making direct use of a liquid as drive means for the compression or suction stroke has the advantage that, aside from the valves, no moving parts, which could be damaged in the case of excess moisture in the gas, are used in the cylinder. The invention further preferably provides pump means which are preferably of the centrifugal pump type which is can operate intermittently or continuously. The liquid reservoir preferably contains sufficient liquid to allow the hydraulic process to proceed properly. A preferred objective will be a closed liquid reservoir to be provided with a gas space such as the usual expansion tank. This liquid reservoir will herein have to be dimensioned such that a pressure prevails which is an average of the highest delivery pressure and the lowest suction pressure in the compression reservoir, and wherein this average varies minimally during the liquid movements. A pre-pressure to be realized for this purpose in the gas space is necessary to obtain the required elasticity. The advantage hereof is that the pump power can be kept relatively low since only half the pressure has to be overcome with this pump. An expansion tank separated by a membrane can be chosen in simple manner as liquid reservoir. A liquid reservoir can of course be embodied as an open reservoir. In this case the pump will have to be able to overcome the full pressure difference. In an embodiment according to claim 2, a replenishment feed is provided. It is thereby possible to replenish the gas compressor where necessary. Gas in particular can be compressed and liquid can here also be taken up again in the compressed gas and discharged, or there may just be leakage phenomena which make replenishment necessary. In one embodiment, the gas compressor according to the invention can have the feature that the feed and discharge means comprise non-return valves which are oriented in opposing directions. An almost automatically functioning system can thus be provided. When the liquid is pumped out of the compression reservoir an underpressure is created, and the non-return valve in respect of feed of gas for compressing can be opened under the influence of this underpressure. Vice versa, it is also the case that if liquid is pumped into the compression reservoir, a pressure of the gas for compressing is reached at some point where the non¬ return valve in the discharge means is opened. It is possible here for the non-return valve to be a controllable valve which can be opened on the basis of measurements of underpressure and overpressure in the compression reservoir. Because diverse valves which must all also be controlled are also necessary in the conduit system, the embodiment of non-return valves as (for instance electronic) controllable valves hardly results in an additional load. It is also possible in one embodiment for the compression reservoir to comprise at least two individual chambers. If these operate in opposite phase, a practically continuous process of gas compression can be realized, wherein one of the chambers at a time is engaged in compression of gas and the other is being filled at the same moment with gas to be compressed thereafter, following which this task allocation is reversed. The chambers are preferably connected here to joint feed and discharge means to also achieve the least possible complexity of the system. In a further embodiment, the conduit system, the liquid reservoir and the pump means are formed as a standard mains system for the provision of water by a public utility or by a private company. The liquid reservoir and the pump means can herein form part of this standard mains system, although if no provisions are made to collect liquid from the compression reservoir or one of the chambers thereof, for instance in a liquid reservoir forming part of the gas compressor, this liquid (for instance water) must be discharged via the sewage system or in other manner. A liquid is preferably applied which is the same as the liquid released during cooling and/or compressing of the gas. In many cases this will be water, whereby in the further description water will be referred to as condensation from the gas or as liquid necessary for the hydraulic operation. Detection means can be arranged in order to precisely determine the stroke length of the liquid in the compression reservoir. Using detection means, for instance level detectors, the maximum height and the lowest point in the compression reservoir are detected, whereupon the liquid direction must be changed in each case. Detection means are preferably also present in the liquid reservoir to monitor the minimum and/or maximum level. When the level is too high, excess liquid is discharged, and too low a level will result in replenishment of the liquid shortfall. These corrections of the level preferably take place automatically, optionally controlled from detection means. The present invention will be further elucidated on the basis of the embodiments shown in the drawings, to which the present invention is not however limited, but wherein the same or similar parts and components are designated with the same reference numerals, and in which: fig. 1 and fig. 2 show an embodiment of a gas compressor in different situations thereof; and fig. 3 shows a second embodiment of a gas compressor according to the present invention. In fig. 1 a gas compressor 1 in a first embodiment of the present invention is shown in a first situation thereof. In fig. 2 the same gas compressor 1 is shown in a second situation. Gas compressor 1 comprises a per se known expansion tank 2 having water 10 therein as embodiment of the liquid. Expansion tank 2 is connected to an expansion reservoir 3 via a conduit system 18 which incorporates a pump 4. Expansion reservoir 3 comprises an inlet 14 and an outlet 15 with respective non-return valves 16 and 17 oriented in opposing directions. Controllable valves 5, 6, 7 and 8 are incorporated in conduit system 18 to enable adjustment of a desired throughfeed direction of water 10 from expansion tank 2 to compression reservoir 3 (as shown in fig. 1) or vice versa (as shown in fig. 2) . The flow of water 10 in the throughfeed direction set with valves 5, 6, 7 and 8 is brought about with pump 4, which in the embodiment shown here can be continuously operating. In the embodiment shown here the gas compressor 1 comprises a further controllable valve 9 which is arranged in a replenishment line which serves to have a desired amount of liquid available in the gas compressor and to replenish this where necessary. The operation of gas compressor 1 shown in fig. 1 and 2 is as follows: In the situation shown in fig. 1, water 10 is pumped out of expansion tank 2 to compression reservoir 3, wherein valves 5 and 8 are closed and valves 6 and 7 are opened. Controllable valve 9 is closed. Prior to pumping of water 10 to the compression reservoir, the level of water 10 therein has fallen such that an underpressure is created in the compression reservoir. As a result of this underpressure, non-return valve 16 is opened for feed of gas for compressing from feed 14 to compression reservoir 3. Gas compressor 1 is then brought into the situation shown in fig. 1 by suitable control of controllable valves 5, 6, 7 and 8. The water level herein rises in compression reservoir 3, whereby the gas previously admitted into compression reservoir 3 is compressed. A pressure of the gas is herein reached at a given moment where non-return valve 17 in the outlet is pressed open and the compressed gas can escape. It is noted that non-return valves 16, 17 can also take the form of controllable valves which can be under the same control (not shown) as the other controllable valves 5, 6, 7 and 8, and also controllable valve 9. It is also possible to embody outlet 15 in some other manner, for instance by connecting thereto a container or tank which is connected to outlet 15 without non-return valve and is filled with gas for compressing via inlet 14, whereafter the level of water 10 in the compression reservoir will begin to rise with gas compressor 1 in the situation shown in fig. 1. When the level of water 10 has risen in compression reservoir 3 such that the gas has been compressed to a desired extent, valves 5, 6, 7 and 8 can be reversed after the thus compressed gas has been removed from compression reservoir 3 via outlet 15 and after non¬ return valve 17, which forms part of outlet 15, has been opened. In an embodiment with a tank or container connected to an outlet 15, the tank or container filled with the compressed gas (under pressure) can be uncoupled from outlet 15 and stored. As already noted, the situation of gas compressor 1 shown in fig. 2 is then realized by switching the controllable valves 5, 6, 7 and 8 in suitable manner. Controllable valve 9 is here also closed. The water 10 is pumped out of compression reservoir 3 to expansion tank 2 by pump 4. As the level of liquid 10 in compression reservoir 3 falls, an underpressure is also obtained at a given moment which causes the non-return valve 16 to be opened and fresh gas for compressing to be supplied. Sensors 19 are arranged in compression reservoir 3. In the embodiment shown here these sensors are provided in the form of floats which can regulate the operation of the gas compressor together with the level of water 10 in compression reservoir 3. When the liquid level reaches for instance the height of the upper sensor 19, the situation of the gas compressor shown in fig. 2 can be set. When the level of the water 10 in compression reservoir 3 reaches the lower sensor 19, the situation shown in fig. 1 should then preferably be set. Fig. 3 shows a further embodiment of a gas compressor 20 according to the present invention. Gas compressor 20 comprises a liquid reservoir in the form of an open vessel 11 which, via a conduit system 15 again having pump 4 therein, is connected to two compression chambers 12, 13 as components of an assembly to be designated in its entirety as compression reservoir 3. Both of the compression chambers 12 and 13 are connected via associated non-return valves 16 and 17 to a joint inlet 14 and outlet 15 in the same manner as shown in fig. 1 and 2. The throughfeed of water 10 with one of the two compression chambers 12 and 13 at a time is controlled with controllable valves in conduit system 15. Pump 4 is in continuous operation and drives water 10 out of vessel 11 to one of the two compression chambers 12 and 13 at a time so as to raise the level of the water therein. At a given moment the relevant controllable valves associated with emptying of the other of the compression chambers 12 and 13 are opened, wherein the water therefrom flows back to open vessel 11. The relevant compression chamber 12 and 13 which is emptying at that moment must herein be filled again with gas for compressing from inlet 14. For this purpose the non¬ return valve 16 takes a form which can be triggered very easily, while conversely the non-return valves 17 must only open the passage to outlet 15 at a high pressure. By giving the non-return valves 16 an easily triggered form a relevant compression chamber 12 and 13 from which water 10 is being emptied can easily be refilled with gas for compressing from inlet 14. A continuously operating system is thus provided, and the two compression chambers 12 and 13 run in opposite phase through the process as described with reference to figures 1 and 2. It will be apparent after examination of the foregoing description and perusal of the accompanying drawings representing the described embodiments that diverse alternative and additional possibilities will occur to the skilled person. It is thus possible, as already noted above, for the non-return valves 16 and 17 to be replaced by controllable valves. Use can be made of an expansion tank 2 or an open vessel 11, and diverse options are known to the skilled person for pump 4 as embodiment of the pump means. More than two compression chambers can also be provided or, in an embodiment with only or precisely two compression chambers, these can also be controlled in phase in order to increase the output and efficiency of gas compressor 1 or 20.