546 THE PKINCIPLES OF SCIENCE. [CHAP. 



of inversion; for instance, Thomson's speculations on the 

 relation between pressure and the melting-point. But 

 many other illustrations could be adduced. The usual 

 agent by which we melt 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 mixtures. The affinity of salt for water causes it 

 to melt ice, and we may thus reduce the temperature to 

 Fahrenheit's zero. Calcium chloride has so much higher 

 an attraction for water that a temperature of 45 C. may 

 be attained by its use. Even the solution of a certain 

 alloy of lead, tin, and bismuth in mercury, may be made 

 to reduce the temperature through 27 C. All the other 

 modes of producing cold are inversions of more familiar 

 uses of heat. Carry'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 that a current of steam at 100 C. can 

 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 100, 

 it will continue, on account of its affinity for water, to con- 

 dense steam when above 100 in temperature. It will con- 

 dense the steam until heated to the point at which the ten- 

 sion of its vapour is equal to that of the atmosphere, that 

 is, its own boiling-point. 1 Again, since heat melts ice, we 

 might expect to produce heat by the inverse change from 

 water into ice. 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 instantly solidifies and rises in temperature to o C. 

 The effect is still better displayed in the lecture-room 

 experiment of the suspended crystallisation of a solution 

 of sodium sulphate, in which a sudden rise of temperature 

 of 15 or 20 C. is often manifested. 



The science of electricity is full of most interesting cases 



1 Proceedings of the Manchester Philosophical Society, Feb. 1870. 



