/ 



ON THE SOURCES A^D EFFECTS OF HEAT. 645 



which forms itself on the surface of polished iron or steel, serves as a test of 

 the degree of heat which is applied to it, the yellowish colour which it assume^ 

 indicating the first stage of tampering,- the violet the second, and the blue 

 the last; and if the heat be raised till the surface becomes grey, the steel 

 will be rendered perfectly soft. Th e density of metals is also a little increased 

 by tempering them, probably for the same reason as .water is more dense than 

 ice. In what manner the unequal distribution of the mutual actions of 

 the particles of bodies contributes to increase their hardness, cannot be very- 

 positively ascertained, although some conjectures might be formed which 

 would, perhaps, be in some measure explanatory of the fact^;'i)iit it is safer, 

 in the present state of our knowledge, to be contented \vitli tracing the 

 analogy between these effects in substances of different kinclg, and under 

 different circumstances, without attempting to understand completely the 

 immediate operation of the forces which are concerned. 



VOL. I. 4 L 



