MR. W. M. WILLIAMS ON AN APPARATUS FOR V.-Description of an Apparatus for Collecting Gases over Water or Mercury. BY W. M. WILLIAMS. This apparatus is intended to supersede the cumbrous and expen- sive pneumatic trough and its appendages. It is constructed on the principle of a common wash-bottle a flask or tube for generating the gas being fitted to the tube corresponding to that to which in the wash-bottle the mouth is applied. It may be constructed either with a three-necked bottle as in Fig. 1 or a common wide-mouthed bottle of sufficient size as in Fig. 2. In both figures A represents the bottle used as a receiver; B the tube to which the generating flask is attached which ter- FIG. 1. minates in the upper part of the receiver and may be called the gas-tube ; C is the tube through which the water is forced; this tube which dips to the bottom of the vessel we may call the water-or mercury -tube.Besides these there is in the appa- ratus represented in Fig. 2 a short straight tube D fitted into the cork of the receiver COLLECTING GASES OVER VATER OR MERCURY. PIG. 2. its lower part terminating like the gas-tube in the upper part of the receiver. This tube which is corked or stoppered may be called the Jilling-tube. The mode of using the apparatus is very simple and obvious. Suppose we wish to prepare oxygen gas and de- monstrate its properties. The receiver is filled with water and the flask or tube con-taining the mixture of chlorate of potash and oxide of manganese fitted by a cork to the gas-tube and heat applied; as the gas is generated it passes through the gas-tube into the receiver displacing the water by forcing it up the water-tube.The use of the filling- tube D is to get rid of the atmospheric air which comes off at first; this is done by taking out the stopper and refilling the receiver with water after the first portion of the gas has been evolved. In the three-necked apparatus Fig. 1 the middle stopper is used for this purpose. This may be used for demonstration without removing the gas- and water-tubes (on which account the water-tube should be kept as close to the side of the receiver as possible) by simply taking out the middle stopper which should be large enough to admit the spoon &c.required in the experiments. In the apparatus Fig. 2 the cork and all the tubes must be moved together; and when the gas is to be used immediately it will be found convenient to slip a piece of plate-glass over the neck of the receiver with the left hand at the moment the cork holding the tubes is removed with the right. When the gas is to be kept for any length of time of course a stopper will be requred. A number of bottles may be thus filled with gas by connecting them as a series of receivers the water-tube of the first being attached by a caoutchoucitube to the gas-tube of the second and so on. The first only need be filled with water or mercury which successively fills all the rest asitthe gas displaces it. In constructing the apparatus care should be taken not to make the gas-tube too long in order that it may easily bear the weight of the generating flask ;also to bcnd it at such an angle as to place the flask at a convenicrit height for thc spirit-lamp the retort-stand as well as the retort and pneumatic trough being thus dispensed with.46 APPARATUS FOR COLLECTING GASES OVER WATER ETC. The water- or mercury-tube should not be carried too high before bending in order to avoid unnecessary pressure from the column of fluid to be raised. The outer end of the water- or mercury-tube should dip into the fluid contained in the vessel which receives the water or mercury driven out in order to prevent regurgitation of air which would otherwise take place when the gas in the flask and receiver is cooling after the generation of gas has ceased.One of the advantages of this apparatus is the facility with which varying quantities of gas may be collected in the same receiver as the generation of the gas may be stopped at any stage when the receiver is a fourth a third or half full and this small quantity of gas experimented upon without disturbing the apparatus ;the receiver may then be filled again more gas made and so on any number of times. By taking advantage of this a mixture of gases in any proportions may be easily made. I have fitted up on this prin- ciple a small bottle for exploding the mixture of oxygen and hydrogen gases. For demonstrating the properties of oxygen on a large scale a globe might be fitted up on this principle a layer of sand being kept at the bottom to prevent cracking by ignited iron &c.The difficulty of keeping sand at the bottom of a globe which is to be inverted when filled is by this arrangement obviated. For collecting gases over mercury an apparatus fitted up on this principle enables us to attain the minitnum of expenditure of mer-cury ; since the quantity required is only equal to the volume of the vessel receiving the gas and with this any number of such vessels may be filled by connecting them as a series. The precautionsbefore referred to for preventing undne pressure of course require especial attention in using mercury. This pressure however may be reduced to nothing after the mercury-tube is .once filled by keeping the level of the fluid the same both in the vessel in which the gas is being received and that into which the mercury driven out is being col-lected; for when the mercury-tube is filled and its outside end dips into this vessel it acts as a syphon. In order to do this some additional mercury will of course be required. If however care be taken to make all the fittings tight this will not be necessary except in a very high apparatus.