16i HISTORY OF TIIEHMOTICS. 



mercury, its density will be doubled, tlic air being compressed into 

 one half the space. If the pressure be increased threefold, the densi- 

 ty is also trebled ; and so on. The same law was soon afterwards (ii: 

 1676) proved experimentally by Mariotte. And this law of the air's 

 elasticity, that the density is as the pressure, is sometimes called the 

 Boylcan Laiv, and sometimes the Law of Boyle and Mariotte. 



Air retains its aerial character permanently ; but there are other 

 aerial substances which appear as such, and then disappear or change 

 into some other condition. Such are termed va2)ors. And the dis- 

 covery of their true relation to air was the result of a long course of 

 researches and speculations. 



[2nd Ed.] [It was found by M. Cagniard dc la Tour (in 1823), that 

 at a certain temperature, a liquid, under sufficient pressure, becomes 

 clear transparent vapor or gas, having the same bulk as the liquid. 

 This condition Dr. Faraday calls the Cagniard de la Tour state, (the 

 Tourian state ?) It was also discovered by Dr. Faraday that carbonic- 

 acid gas, and many other gases, which were long conceived to be per- 

 manently elastic, are really reducible to a liquid state by pressure. 1 

 And in 1835, M. Thilorier found the means of reducing liquid carbo- 

 nic acid to a solid form, by means of the cold produced in evaporation 

 More recently Dr. Faraday has added several substances usually gaseous 

 to the list of those which could previously be shown in the liquid 

 state, and has reduced others, including ammonia, nitrous oxide, and 

 sulphuretted hydrogen, to a solid consistency. 2 After these discoveries, 

 we may, I think, reasonably doubt whether all bodies are not capable 

 of existing in the three consistencies of solid, liquid, and air. 



We may note that the law of Bovle and Mariotte is not exactly 



./ j * 



true near the limit at which the air passes to the liquid state in such 

 cases as that just spoken of. The diminution of bulk is then more 

 rapid than the increase of pressure. 



The transition of fluids from a liquid to an airy consistence appears 

 to be accompanied by other curious phenomena. See Prof. Forbcs's 

 papers on the Color of Steam under certain circumstances, and on 

 the Colors of the Atmosphere, in the Edin. Trans, vol. xiv.] 



Phil. Trans. 1823. Ib. Ft. i. 1845. 



