EMPIRICAL KNOWLEDGE, EXPLANATION, &c. 183 



adduced. The usual agent by which we melt or liquefy 

 a substance is heat ; but if we can melt a substance 

 without heat, then we may expect the negative of heat 

 as an effect. This is the foundation of all freezing mix- 

 tures. The affinity of salt for water causes it to melt 

 snow or ice, and may thus reduce the temperature 

 to Fahrenheit's zero. Calcium chloride has so much 

 higher an attraction for water that a temperature of 

 50 Fahr. may thus be attained. Even the solution of 

 a certain alloy of lead, tin, and bismuth in mercury, may 

 be made to reduce the temperature from 63 to 14 Fahr. 

 All the other modes of producing cold are inversions of 

 more familiar uses of heat. Carre's freezing machine is 

 an inverted distilling apparatus, the distillation being 

 occasioned by chemical affinity instead of heat. Another 

 kind of freezing machine is the exact inverse of the 

 steam engine. 



A very paradoxical effect is due to another inversion. 

 It is hard to believe at the first moment that a current 

 of steam at 212 could raise a body of liquid to a higher 

 temperature than the steam itself possesses. But Mr. 

 Spence has pointed out that if the boiling-point of a saline 

 solution be above 212, it will continue, on account of its 

 affinity for water, to condense steam when above 212 

 in temperature. It will condense the steam until heated 

 to the point at which the tension of its vapour is equal 

 to that of the atmosphere, that is, its own boiling-points. 



Since heat, again, melts ice, we might expect to produce 

 heat by the inverse change from water into ice. Now this 

 is accomplished in the phenomenon of suspended freezing. 

 Water may be cooled in a clean glass vessel many degrees 

 below the freezing-point, and yet retained in the liquid 

 condition. But if disturbed, and especially if brought 

 into contact with a small particle of ice, it immediately 



s ' Proceedings of the Manchester Philosophical Society,' Feb. 1870. 



