ACTION OF HEAT. 



proper circumstances. Some of those substances 

 which we call simple, because we have not been 

 able to find more than one ingredient in them, 

 cannot be melted into liquids in the open air. The 

 diamond is one of these ; but though the diamond 

 cannot be melted it can be burnt, or reduced 

 wholly to vapour ; and there is no doubt that, if 

 sufficient heat were applied to it under sufficient 

 pressure, it might be made as liquid as water. 

 Marble, or limestone, or chalk, or shells, when 

 burnt in the open air, give out the very same kind 

 of air into which the diamond is converted by 

 burning , and the lime (for it is lime in them all) 

 remains, and falls to powder when water is sprin- 

 kled on it. But marble and chalk, even when in 

 powder, have been artificially melted by heat under 

 pressure, and have been so completely melted that 

 in cooling they formed into crystals of the very 

 same figure as those which the same compound of 

 lime naturally assumes in the rock. Nor is there 

 any doubt, that any substance whatever might be 

 melted by a similar mode of treatment. 



Heat is thus the grand instrument in perhaps all 

 the operations of nature ; for our not being sen- 

 sible of it is no proof that it is not there, any 

 more than our ignorance of any other truth is a 

 contradiction of that. The different susceptibilities 

 of different substances to heat, are the means by 

 which almost every change is performed, not only 

 in nature but in the arts ; and even when we cut 

 wood with a knife, or grind iron upon a stone, it 

 is by no means improbable that we effect our pur- 

 pose chiefly, if not wholly, through the instru- 

 mentality of heat. When we work hard, the tool 

 gets heated in its whole substance ; and when a 



