a SS” 
ee = 
FROM THE HIGHER ATMOSPHERE. 471 
mersed jin 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 na- 
ked' ball will gain only 11 parts of heat, while the coated will 
receive: 20. parts , ; 9 10 
But the air is still an: essential vehicle:of didi 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 rarefyimg the medium. Thus, when the: 
air is rarefied about 200 times, the abductive power from the 
glass balls will be reduced from 6 to 14; while the peculiar 
discharge of heat at the naked surface is lalepititindd from 7 to 5, 
and that at the gilt surface from 1 to 2; the: naked ball now 
i 62 parts of heat, and’ the gilt one only 23. 
> ns 
ocThe: offsite ‘are baliemisatiad a differ ent gaseous. mmadium, Thus; 
dae 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 hae ane to 13 and 83.. 
As igesfinedoe oe ate or, still ‘ines one of ‘inert 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 ° 
