1 7 2 HER OES OF INVENTION AND DISCO VER Y. 



in its temperature ; and, on the other hand, that exactly the 

 same quantity of heat was given out during the congelation of 

 the water. But this experiment, the result of which Dr. Black 

 eagerly longed for, only informed him how much heat was 

 absorbed by the ice during liquefaction, was retained by the 

 water while it remained fluid, and was again emitted by it in 

 the process of freezing. But his mind was deeply impressed 

 with the truth of the doctrine, by reflecting on the observations 

 that presented themselves when a frost or thaw happened to 

 prevail. The hills are not at once cleare'd of snow during the 

 sunshine of the brightest winter day, nor are the pools suddenly 

 covered with ice during a single frosty night. Much heat is 

 absorbed and fixed in the water during the melting of the snow, 

 and, on the other hand, while the water is changed into ice, 

 much heat is extricated. During a thaw, the thermometer sinks 

 when it is removed from the air and placed in the melting snow ; 

 during severe frost, it rises when plunged into freezing water. 

 In the first case the snow receives heat, and in the last the 

 water allows the heat to escape again. These were fair and 

 unquestionable inferences, and now they appear obvious and 

 easy. But although many ingenious and acute philosophers 

 had been engaged in the same investigations, and had employed 

 the same facts in their disquisitions, those obvious inferences 

 were entirely overlooked. It was reserved for Dr. Black to 

 remove the veil which hid this mystery of nature, and by this 

 important discovery to establish an era in the progress ot 

 chemical science — one of the brightest, perhaps, which has yet 

 occurred in its history." 



The theory of latent heat — as Dr. Black called it was ex- 

 plained by him to the members of a literary society on the 23rd 

 of April, 1762, and soon afterwards he laid before his students 



