PHYSICS OF THE EIGHTEENTH CENTURY. 291 



i Ib. of steam contains as much latent heat as will raise 5^ Ibs. of 

 water 180 F.; that 990, or 180 x 5J, is as much as will raise i Ib. of 

 water i: 990 is, therefore, the latent heat of steam. 



Black was able to determine specific heats with accuracy by a method 

 which has never been surpassed for directness and simplicity. He 

 procured large blocks of pure ice, and after having ground one side 

 of a block perfectly level, he formed a cavity in the mass, as shown at 

 A, Fig. 138. A lid was formed by giving a plane surface to another 

 piece of ice, B c ; and it would thus be impossible for any heat to 

 reach the cavity from the outside ; for supposing that any air of a tem- 

 perature above 32 should have penetrated between the block of ice 

 and its cover, the excess of heat of this air would be expended in con- 

 verting some of the ice into water. When a determination of the 

 specific heat of a body was about to be made, the body was raised to 

 some accurately determined temperature, and in a perfectly dry state 

 was quickly introduced into the cavity A, which had been previously 

 wiped with a dry but ice-cold cloth. The lid was then applied, and 

 the apparatus was allowed to remain for several hours, when the warm 

 body having parted with its excess of heat, the cavity contained a 

 quantity of water at 32, the latent heat of which represented the 

 amount of heat which the body had parted with in falling from its 

 original temperature to 32, and therefore its specific heat could be 

 found by determining the weight of the cold water. Black accom- 

 plished this by wiping up and absorbing the whole of the water by a 

 dry ice-cold piece of linen which had been previously weighed. The 

 increase of its weight was of course the weight of water it had ab- 

 sorbed. 



The researches of Black may be said to have created the science 

 of heat. There had before been abundance of speculations on the 

 nature of heat, and observations were not wanting of the changes it 

 produces ; but Black first introduced the means of measuring heat as 

 a quantity, and from that period heat measurements have contributed 

 some most important data to many branches of science. The mode 

 of measuring heat which has just been explained is quite independent 

 of all theoretical views as to its nature. We have already seen that 

 Bacon guessed that heat might be an insensible motion of the small 

 particles of bodies (page 136); but at the end of last century the pre- 

 valent conception of heat was that of a kind of subtle imponderable 

 fluid, which was capable of combining with substances in greater or 

 less proportion. It was supposed to enter into " the pores " of bodies, 

 which received it in greater or less proportion, and from which it could 

 even be, as it seemed, squeezed out by compression. This supposi- 

 titious fluid received the name of caloric, and the notion and the name 

 survived until a comparatively recent time, and are found in most 

 scientific treatises published during the first half of the nineteenth 

 century. 



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