the Densities of Gases and Vapours. 341 



liquid to be determined almost exactly fills a glass bulb, closed 

 at the lamp, and the weight of which is known. The case 

 EFGH being detached, the screw c (fig. 13) is loosened, and 

 the tube A B removed. The bulb is placed in the tube A B, 

 which is replaced in its original position and dried (as has been 

 already described, p. 339), by exhausting and allowing dry air to 

 enter. Lastly, the vacuum being continued, the stopcock 11 is 

 placed in the position 1 (fig 8) ; the mercury completely fills the 

 tube AB, and the stopcock r is closed when the mercury has 

 reached it. The bath is replaced and filled with water, and its 

 temperature raised by means of a lamp placed beneath it until 

 the liquid by its expansion breaks the bulb. The mercury 

 remains raised in the capillary tube A b, and hence neither the 

 liquid nor its vapour come in contact with the stopcock r. 



The water in the bath is gradually raised to various tempera- 

 tures, and, by means of arrangements which I have already 

 described, the volume is measured which the vapour occupies at 

 different temperatures and under different pressures. The pres- 

 sures may be made to vary from the lowest limits up to eight or 

 ten atmospheres. We may thus determine at once, and within 

 very extensive limits, — 



1st. The laws of the compressibility of vapour at various tem- 

 peratures. 



2nd. Its coefficient of expansion at various degrees of the ther- 

 mometric scale, the pressure remaining constant at different 

 temperatures, while the volume of the vapour alone varies, the 

 pressure varying within considerable limits. 



3rd. The increase of the elasticity of the vapour in conse- 

 quence of the increase of temperature, the volume remaining 

 constant, and the original pressure varying within considerable 

 limits. 



4th. The real density of the vapour at different temperatures 

 and pressures. 



5 th. The limit density of the vapour, which is that to which 

 the real density constantly approximates when the pressure is 

 diminished and the temperature raised. 



The apparatus further gives the elastic forces of saturated 

 vapours for various temperatures j for it is merely necessary to 

 keep the pressure so that, while the temperatures gradually 

 increase, liquid remains condensed on the surface of the mercury, 

 though the vapour occupies part of the space of the tube AB. 



Lastly, it may be used to measure the clastic forces of vapour, 

 either saturated or not, in air, or in other gases, at various tem- 

 peratures and pressures. For this purpose, the bulb having been 

 introduced into the tube A B, air or the gas (dry) is allowed to 

 enter by the stopcock r. The elastic force and the volume of 



