312 M. V. Regnault on some Apparatus for determining 



the gas alone are measured at a known temperature T. The 

 bulb is then broken, and a scries of experiments made under 

 varying temperature and pressure. From that, the elastic force 

 of saturated vapour in gases for all cases in which liquid remains 

 on the surface of the mercury, and the laws of the elasticity and 

 expansion of the mixture of gases and of vapour at different degrees 

 of saturation, may be deduced. Lastly, the elastic force of the 

 vapour in the gas may be determined at the moment at which 

 dew commences to be deposited in the tube AE. A large 

 number of examples of these determinations will be found in 

 vol. xxvi. of the Memoires de V Academic 



It often happens that the fragments of the broken bulb hinder 

 the exact reading of the level of the mercury in the tube AE. 

 This inconvenience is easily avoided by allowing the tube A B to 

 terminate in a narrower part L, slightly spheroidal (fig. 16). 

 The bulb is lowered to this cavity, and retained there by a 

 small spiral of platinum; the remains of the bulb remain 

 then almost entirely on the spiral. 



We shall readily understand the advantages which this method 

 presents over those hitherto used for determining vapours ; for 

 it furnishes at the same time a great number of other elements, 

 a knowledge of which is necessary in order to know what use 

 can be made of the density from the point of view of our chemical 

 theories. It might be feared that it was only applicable to very 

 volatile substances; for the temperature of the bath cannot 

 much exceed 100°, even when the vessel is filled with a saline 

 solution. But I must say that, for slightly volatile substances, 

 which in general have high vapour-densities, it is especially 

 interesting to determine their vapour- densities under very feeble 

 pressures, because the limit density is thereby approximated to. 

 Now it is always possible to realize these favourable conditions 

 when the boiling-point does not exceed 200° under the ordinary 

 atmospheric pressure. 



Apparatus for determining the Vapour -density of Substances which 

 boil at High Temperatures. 



The veiy simple apparatus represented in figs. 10 and 11, 

 Plate II., may be used for a small number of imperfectly volatile 

 substances, the vapours of which do not readily alter in contact 

 with the air. Two vessels, A, B, cast in iron of the same thick- 

 ness, terminate in small tubes which are closed by bullets placed 

 above. The capacities V and V of the two flasks are gauged, by 

 ascertaining the weight of water which fills them. Mercury is 

 poured into one of them, A ; and in the other is placed the sub- 

 stance whose vapour-density is to be determined. The apparatus 

 being thus arranged, is placed in a muffle heated to a high tern- 



