LIFE of Dr BLACK. 107 



** Does not the heat produced in the flacking of quicklime, arife 

 " from the fudden fixing a part of this fluid (water) into a folid 

 i( ftate ? Is not this the cafe in the fetting of gypfum ?" In the 

 fame way he accounts for the intenfe cold produced by ice and ni- 

 trous acid, applies the principle again to all faline folutions, and 

 afcribes the flow granulation of fait to the re-appearance of heat, 

 " Is not ice," he fays, " cryftallized water ? and does it not al- 

 *' ways feel cold, becaufe it melts on our handling it ? This i3 

 " fimilar to the folution of" fait in water." Thefe notes, from 

 circumftances intermixed, appear to have been written as early 

 as 1756. In a following note-book, which, from circumftances 

 alfo, does not appear to be of a later date than the year 1757, 

 there are feveral queries, of the fame nature with thofe above 

 mentioned, particularly with refpect to the curious obfervation 

 of Fahrenheit, that water, if not difturbed, will cool below 

 32 degrees without freezing, but the moment it is difturbed, it 

 raifes the thermometer to 32 degrees, and freezes. " Is not this," 

 he fays, " the heat that is unneceffary to ice ?" In the fame 

 note-book conjectures are carried dill farther, and applied to the 

 production of vapour. A way is mentioned to eftimate the 

 quantity of heat which is employed and difappears in the for- 

 mation of vapour, and cannot afterwards be difcovered by the 

 thermometer. " Place a phial," he fays, " with water, clofe- 

 " corked in a ftove ; open it fuddenly, and fee how much of it 

 u is converted into vapour, while the water comes down to the 

 " boiling point." Obfervations to the fame effect are continued 

 through fix fmall note books ; in different places of which, it is 

 obferved, that animal heat originates in the lungs, and that heat 

 always accompanies the production of fixed air. 



From thefe notes, it appears, that the experiments in which 

 Black evinced his doctrine of latent heat to the fatisfaction of 

 his pupils, were not made at random, or without previous appre- 

 henfion of the fact, as it appeared in the ordinary courfe of na- 

 ture 



