FROM THE HIGHER ATMOSPHERE. 471 



mersed in a cold liquid, while the vitreous surface projects 7 

 parts, and the metallic surface only 1 part. The same dif- 

 ferences depending on the nature of the superficial boundary, 

 are observed to take place in the heating as well as in the 

 cooling of bodies. Thus, if the silver balls be filled with wa- 

 ter colder than the temperature of the room, they will acquire 

 heat in the same proportions as they before lost it. The nar- 

 ked ball will gain only 11 parts of heat, while the coated will 

 receive 20 parts. 



But the air is still an essential vehicle of these various im- 

 pressions of heat or cold. An absolute vacuum is unattain- 

 able in Nature ; but the dispersive effects are always diminish- 

 ed, though slowly, by rarefying the medium. Thus, when the 

 air is rarefied about 200 times-, the abductive power from the 

 glass balls will be reduced from 6 to 1^; while the peculiar 

 discharge of heat at the naked surface is depressed from 7 to 5, 

 and that at the gilt surface from 1 to f ; the naked ball now 

 emitting 6^ parts of heat, and the gilt one only 2\. 



The effects are changed in a different gaseous medium. Thus, 

 the same balls, with a vitreous and a metallic surface, would 

 discharge 31 and 25 parts of heat, if immersed in hydrogen 

 gas ; both of them now losing. 24 parts by the powerful abduc- 

 tion of this gas. But if the medium be rarefied about 200 

 times, the quantities of heat emitted, from the naked and the 

 gilt ball, will be reduced to IS and 8f. 



As a surface of glass, or, still better, one of linen^ paper, or 

 vegetable pigment, projects heat the most copiously; so those 

 surfaces likewise intercept the impressions most effectually. 

 But a bright metallic surface detains only the tenth part of these 

 impressions, and reflects all the rest. Hence the power of a me- 

 tallic speculum, contrasted with that of a glass mirror, in con- 

 centrating 



