Laws of Heat.—Radiation of Cold. 9 
‘the same when deduced from different premises. Thus the 
weight of an atom of hydrogen as deduced from its relations 
to oxygen in water, is 125, oxygen being taken for unity ; 
and the same number is obtained by deducing its weight 
from its relations to nitrogen in ammonia. A theory which 
brings us to the same conclusion by different routes, and 
whose calculations often coincide with the results of chemi- 
¢al analysis to the place of thousandths in decimals, must be 
founded in truth. 
The laws of Hear were so thoroughly investigated, and 
so faithfully expounded, by Black, Scheele, Crawford, Rum- 
ford, Lavoisier, by Leslie, Dalton and Prevost, that in this 
departmentlittle has been done, within a few years past, but 
to establish the same laws by more accurate and rigorous ex-- 
periments. In this way several of the French chemists have 
labored very successfully. They have availed themselves of 
stances, shown how advantageously that mathematical knows 
ledge, for which the men of science of that nation are so 
distinguished, may be applied in the investigation of the 
s even of chemical phenomena, affording as it does an in- 
ing researches on heat, have been recently afforded by 
i leme , by Dulong and Petit, and 
by Berard and Delaroche. | 
Upon the supposition that heat is the only positive princi- 
ple, and that cold is merely the necessary result of its ab- 
sence, as darkness is the absence of light, it has been found 
difficult to account for the apparent radiation of cold between 
two parallel concave reflectors. The case is this: iftwo cons 
cave metallic mirrors (suppose of burnished brass, or tin, or 
silvered copper,) be placed parallel to each other, and in the 
us of one of the mirrors there be placed a thermometer, and 
in the focus of the other a pan of coals or a red-hot cannon ball, 
the rays of heat proceeding from the focus to the adjacent mir- 
ror, will go out parallel from that to the other mirror, and be 
reflected by it to the thermometer and cause it to rise, 
YOL. XII.—No. 1. 2 
