146 ACTION OF HEAT. 



But we must just notice one or two of those ef- 

 fects of heat which are not so obviously connected 

 with the display of light ; for though we should 

 continue to write, and read, and observe till the light 

 of our own eyes were extingished, we should be no 

 more near tl^e end of the beautiful subject of light 

 than we are at this moment. 



The most general and most active property of 

 heat is that of overcoming the cohesion of the parts 

 of substances ; and thus softening them, and ex- 

 panding them into more bulk or space. It acts with 

 very different degrees both of rapidity and of energy 

 in different substances ; but it is probable that there is 

 no substance that could not be melted, and after that 

 changed into air or vapour, by a sufficient degree of 

 heat applied under 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 re 

 duced 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. Mar- 

 ble, 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 sprinkled on it. But marble 

 and chalk, even when in powder, have been artifi- 

 cially 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 sub- 

 stance 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 sensible 



