PHYSICAL AND LITERARY. 151 



aggregate than on the nature of the mixt; 

 I was ready to conclude, that all fluids what- 

 ever, would, in evaporating, produce cold. 

 But I have found a feeming exception. 

 When the ball of the thermometer is 

 moiflened with any of the foffil acids, a 

 confiderable degree of heat is produced. It 

 is however to be doubted, if this affords an 

 exception. We know that thefe acids attrad: 

 water from the air ; and alfo that thefe acids, 

 mixed with water, always produce heat : it 

 may therefore be fuppofed, that the heat pro- 

 duced, by moiftening the ball of the ther- 

 mometer with thefe acids, is to be imputed 

 rather to their mixing with the water of the 

 air, than to their evaporation fingly. This 

 perhaps cannot be poiitively determined, till 

 the evaporation of thefe acids, in a very per- 

 fe6t vacuum, is examined ; which I have not 

 yet had an opportunity of doing. In the 

 mean time, I have made an experiment 

 which I think is to the purpofe. To one 

 part of ftrong acid of vitriol, I added two 

 parts of fpring-water. When this mixture, 

 which produces a great degree of heat, was 

 returned to the temperature of the air, I ufed 

 it for moiftening the ball of the thermome- 

 ter. 



