A CENTURY OF CHEMISTRY. 95 



yielded to the combined effects of high pressure and 

 low temperature, and had been obtained in liquid or 

 solid form. Andrews, Mendelejeff, Pictet, Caille- 

 tet, Wroblewsld, Olszewski, and many others have 

 contributed to the striking series of experiments. 



By a long series of researches, extending through 

 the century, it has been made clear that all ponder- 

 able matter may be thought of as essentially of the 

 same nature, irrespective of what its state solid, 

 liquid, vaporous, or gaseous may be. The differ- 

 ences of state are conceived of as due to the way in 

 which the relations of the component particles are 

 affected by the greater or less relative activity of the 

 attractive molecular forces and the dispersive ther- 

 mal motions. As every one knows, water may occur 

 as a solid, a liquid, a vapour, or a gas (saturated 

 steam above 720.6 C.). " Above 30.92 C. carbonic 

 acid is a true gas; no pressure will .then liquefy it; 

 but at 30.92 C. a pressure of 77 atmospheres, and 

 below 30.92 C. progressively smaller pressure will 

 condense it; at and below that temperature (An- 

 drews' Critical Temperature) gaseous carbonic acid 

 is a ' vapour/ condensable by pressure alone." * It 

 may also be procured as a solid. Endless examples 

 might be given, for the idea of necessary permanence 

 of state has now disappeared, and theoretically no 

 case is more striking than another, though technical 

 difficulties have enhanced the interest of some par- 

 ticular instances. 



It was about the beginning of the century that 

 Northmore and others liquefied sulphurous acid gas 

 by pressure, but progressive research on the subject 

 began with the work of Faraday and Davy in 1823. 

 They used the method of " enclosing materials from 

 * Article " Gas," by Daniell, Chambers's Encyclopaedia. 



