334 THE PRINCIPLES OF SCIENCE. 



adopted the generalization that all substances could exist 

 in all three states. A certain number of gases, such as 

 oxygen, hydrogen, and nitrogen, have resisted all efforts 

 to liquefy them, and it now seems probable from the ex- 

 periments of Dr. Andrews that they are limiting excep- 

 tions. Dr. Andrews finds that above 88 Fahr. carbonic 

 acid cannot be liquefied by any pressure he could apply, 

 whereas below this temperature liquefaction is always 

 possible. By analogy it becomes highly probable that 

 even hydrogen might be liquefied if cooled to a sufficiently 

 low temperature. We must modify our previous views, 

 and either assert that below a certain critical temperature 

 every gas may be liquefied, or else we must assume that 

 a highly condensed gas is, when above the critical temper- 

 ature, undistinguishable from a liquid. At the same time 

 we receive an explanation of a remarkable exception pre- 

 sented by liquid carbonic acid to the general rule that 

 gases expand more by heat than liquids. This liquid 

 carbonic acid was found by Thilorier in 1835 to expand 

 more than four times as much as air ; but by the light of 

 Dr. Andrews' experiments we may learn to regard the 

 liquid as rather a highly condensed gas than an ordinary 

 liquid, and it is actually possible to reduce the gas to the 

 apparently liquid condition without any abrupt conden- 

 sation P. 



It is an empirical law of the planetary system that all 

 the bodies composing it revolve from west to east ; that 

 law is broken, as we have seen, in the cases of one planet 

 and several satellites, probably by the interference of an 

 accidental disturbing force. The law also fails to be 

 true of comets, which, taken as a whole, appear to move 

 according to no single uniform law. This exception, how- 

 ever, is one of limitation only, for in all probability comets, 

 although at present members of our system, have not 

 P Maxwell, 'Theory of Heat,' p. 123. 



