of Vapours to Mariottc and Gay-Lussac's Law. 285 



dependence of temperature and density, i. e. of temperature 

 and volume. Cahours's investigations do not justify a wider 

 conclusion than the general one that these bodies exhibit a 

 vapour-density more widely different from the theoretical one the 

 nearer they are to their condensation. Bineau then furnished a 

 few numbers concerning the relation between all three quantities, 

 P, V, and a + t, for the three above-named acids. However, 

 these few numbers demonstrate only the absolute fact of a de- 

 parture of vapours from the laws of the ideal gaseous condition. 



Regnault showed later* for aqueous vapour, that at low tem- 

 peratures (from 30° to 55°) it does not conform to the laws of 

 gases until the tension amounts to about *8 of the maximum 

 tension corresponding to the particular temperature. 



More detailed investigations respecting the same vapour were 

 instituted by Fairbairn and Tatef . These physicists determined 

 the specific volume of perfectly saturated vapour for temperatures 

 from 136° to 199° and from 243° to 288° Fahr., and, further, 

 deduced the coefficient of dilatation for vapour heated some de- 

 grees above the latter temperature. Their method consists in 

 heating different quantities of water to the same degree in two 

 communicating globes ; a change in the levels of the mercury 

 enclosed in them indicates the moment when the smaller mass 

 of water is changed entirely into vapour, and so a less ten- 

 sion commences to be exerted. But in this mode of operating 

 there lurk many sources of error. My own experiments have 

 above all things assured me of this, that it is by no means at the 

 same instant when the temperature that has been reached requires 

 theoretically a certain density that the vapour will indicate the 

 corresponding pressure, but a certain time is requisite for the 

 manifestation of this condition. I have found generally that the 

 vapour does not pass instantaneously even from a superheated 

 state into another degree of superheating as soon as the external 

 circumstances are produced. Much more slowly will the forma- 

 tion of stable conditions proceed at the limit of the saturated 

 state. 



Besides this incorrectness in the method of Fairbairn and 

 Tate, it appears also, from the arrangement of their bath, to be 

 scarcely possible that the temperatures prevailing in the globes 

 should be sharply defined. 



Hirn also has investigated the case of aqueous vapour {. He 

 has calculated the volume of the (superheated) vapour under 

 pressures of 1, 3*5, 4, and 5 atmospheres, and at a few different 

 temperatures for each. Thus the degree of the dilatation of super- 

 heated aqueous vapour is maintained under different circum- 



* Mem. Acad. Scien. vol. xxvi. p. 700. 

 f Phil. Mag. S. 4. 1861, vol. xxi. p. 230. 

 % Thcorie Mecanique de la Chaleur. 



