OF NATURAL PHILOSOPHY. 321 



liquid under ordinary circumstances, owe their li- 

 quidity to heat, and would freeze or become solid if 

 their heat could be sufficiently reduced. In many 

 we see this to be the case in ordinary winters ; for 

 some, severe frosts are requisite ; others freeze only 

 with the most intense artificial colds; and some 

 have hitherto resisted all our endeavours; yet the 

 number of these last is few, and they will probably 

 cease to be exceptions as our means of producing 

 cold become enlarged. 



(358.) A similar analogy leads us to conclude that 

 all aeriform fluids are merely liquids kept in the 

 state of vapour by heat. Many of them have been 

 actually condensed into the liquid state by cold ac- 

 companied with violent pressure ; and as our means 

 of applying these causes of condensation have im- 

 proved, more and more refractory ones have succes- 

 sively yielded. Hence we are fairly entitled to 

 extend our conclusion to those which we have not 

 yet been able to succeed with ; and thus we are led 

 to regard it as a general fact, that the liquid and 

 aeriform or vaporous states are entirely dependent 

 on heat ; that were it not for this cause, there 

 would be nothing but solids in nature ; and that, on 

 the other hand, nothing but a sufficient intensity of 

 heat is requisite to destroy the cohesion of every 

 substance, and reduce all bodies, first to liquids, and 

 then into vapour. 



(359.) But solids, themselves, by the abstraction 

 of heat shrink in dimension, and at the same time 

 become harder, and more brittle ; yielding less to 

 pressure, and permitting less separation between 

 their parts by tension. These facts, coupled with 



