PRESENT FUNDAMENTAL CONCEPTIONS OF PHYSICS. 499 



produced a more rapid evaporation of the fluid, but no increase of its 

 temperature above the boiling point. What becomes of the great 

 amount of heat which is supplied to the melting and boiling substances? 

 We shall answer this question by a historical digression; this will guide 

 us to the solution of the present problem. 



A century and a halt ago Reaumur and Celsius were acquainted with 

 the fact of the constancy of the molting and boiling points of ice and 

 water. Obtrusively as these phenomena courted notice, they were yet 

 only used by the manufacturers of thermometers as very desirable for 

 their purpose, but were not further heeded. Black, whom we previously 

 eulogized as the discoverer of specific heat, comprehended the impor- 

 tance of this natural phenomenon, and endeavored to solve it (1759-'G4). 

 It was he who put forward the following proposition generally known 

 to-day, and which he verified by many experiments. As often and as 

 long as any substance melts or boils (evaporates) so long all heat sup- 

 plied to it disappears, for the latter then no longer acts on the thermo- 

 meters which may be immersed in these melting or boiling (evapo- 

 rating) substances. As soon however as the vapors of any substance 

 become again fluid, or as soon as a fluid substance congeals, the heat 

 which, as Black proved, apparently disappeared during the boiling or 

 melting of these substances reappears; it then acts again on the ther- 

 mometers which are immersed in either of the substances which are 

 transforming themselves from vapor into fluid or which are again con- 

 gealing. This returning heat amounts, as shown by Black, to exactly 

 as much as that which was apparently lost during the melting or 

 boiling. What becomes of that heat concealed during the melting or 

 boiling of a substance, up to that time, when by a reversion of the 

 process it reappears ? 



We know that Black assumed a highly elastic caloric without weight, 

 which penetrated a substance and heated it. For the melting and boil- 

 ing (evaporating) of substances was required, according to Black, much 

 more of the caloric which, by means of its elasticity, expands the mole- 

 cules, and which, by virtue of a dominating attraction between the ponder- 

 able substances and the caloric, combines with the molecules of the pon- 

 derable matter. Black, for this reason, called the heat used in melting 

 and boiling (evaporating), — " latent" heat. By reversing the process the 

 previously " latent" heat became again sensible ; the latter had the prop- 

 erty of acting on the sense of feeling as well as on the thermometer. 



The discovery by Black of "latent" heat (as well as of specific heat) 

 is most wonderful, because something hidden was brought to light by 

 ingenuity and genius, and the indestructibility of heat was actually 

 proved ; in consequence the way for our modern heat theory was pre- 

 pared.* We know that Black's theory in relation to the material 



* [This is not quite an accurate expression of the fact, and therefore deserves com- 

 ment. Were there such a thing as " caloric," it would certainly be " indestructible," 

 and it was precisely this fallacious conception which led the illustrious Black, in his 



