344 On some Apparatus for determining the Densities of Gases, 



the base of this cylinder, at a distance of a decimetre from the 

 end A of the tubes A B. The other end of the cylinder is open ; 

 it corresponds to about the middle of the tube C E. This cylin- 

 der may be made to move rapidly about its axis by means of a 

 rackwork, one of whose toothed wheels, F, is mounted at the 

 end of the cylinder E F. A gas-furnace, like those used for 

 organic analysis, serves to heat the cylinder to a strong red heat. 



To make an experiment, the apparatus being arranged, a por- 

 tion of the substance whose vapour-density is to be determined 

 is introduced into each of the similar iron tubes, and the screw 

 B put in its place. The air which fills the three tubes is first 

 completely expelled; for this purpose pure dry hydrogen is 

 allowed to enter by means of a capillary silver tube, which is 

 introduced by the tube CD, until its open end touches the 

 closed end A of the tube A B. When the air is expelled the 

 silver tube is withdrawn, and by means of an india-rubber tube, 

 C D, is connected with an apparatus which disengages hydrogen. 

 In fine, the three iron tubes, including that which serves as gas- 

 thermometer, remain during the rest of the experiment in con- 

 nexion with an apparatus for disengaging hydrogen under the 

 atmospheric pressure. 



The gas-furnace is lighted, and its temperature raised as 

 rapidly as possible ; the substances converted into vapour expel 

 the gas ; the excess of vapour condenses in the half of the tube 

 B C which is not contained in the heated metallic cylinder E F. 

 In order to spread the heat uniformly over the three iron tubes, 

 the heated metallic cylinder which surrounds the fixed system 

 of tubes is continuously and rapidly turned. 



The experiment is concluded by closing the stopcock of the 

 gas-thermometer, extinguishing the lamps, and cooling the 

 apparatus. The temperature of the gas-thermometer is deter- 

 mined by the method I have mentioned. 



To ascertain the weight of vapour which filled the two other 

 tubes at the moment of maximum temperature, the screws B 

 are unfastened, and the substance which is condensed in the 

 tube A B is determined by chemical methods. 



The operation is much simpler when it is not attempted to 

 determine the temperature by a gas-thermometer, but simply to 

 seek the ratio of the density of the vapour of the substance to 

 that of mercurial vapour, the two vapours being under the same 

 circumstances of temperature and of pressure. It is simply 

 necessary then to place mercury in one of the tubes A B, and to 

 weigh the mercury which remains in the space A B after the 

 experiment. 



The volumes of the three tubes have been determined pre- 

 viously. 



