72 Notices respecting New Books. 
caloric thrown on the glass, instead of passing through’it, 
are absorbed by it. This result affords a method of sepa- 
rating the rays of caloric from the rays of light, when they 
accompany each other. Thus, if a burning candle be placed 
in the focus of the mirror, and a plate of glass interposed, 
a luminous image is formed on the ball of the thermome- 
ter in the opposite focus, from the light passing through 
the glass; but the calorific effect is greatly diminished, by 
the rays of caloric being arrested,—a fact which shows well 
the essential difference between radiant caloric and light. 
“* Some bodies are more disposed to absorb radiant calo- 
ric than others, and hence are much more heated by. it. 
Scheele observed, than when'a alass mirror is used instead 
of a metallic one, the heat is not reflected, but is absorbed 
and retained by the glass; and the resnit is similar, if a 
metallic mirror have its surface blackened. Pictet found, 
that when. the glass bulb of the thermometer is blackened, 
it is considerably more heated than when it is clean. But 
if the bulb be covered with tinfoil, the reverse happens, or 
the elevation of temperature is much less than when the 
glass bulb is opposed to the mirror. 
‘The power of reflecting the rays of caloric is of course 
the reverse of the absorbing power. Metals reflect most 
perfectly, hence the calorific effect is greatest in these ex- 
perimenis when metallic mirrors are employed; it is less 
with a glass mirror, and is scarcely sensible when the sur- 
face is blackened. It is, for the same reason, least when 
the ball of the thermometer has a metallic surface, is greater 
when of glass, and still greater when blackened. 
‘* An important difference exists among bodies in the power 
of radiating caloric, the quantity thrown from different 
<inds of surfaces at the same temperature being very dif- 
ferent. For the knowledge of this, we are indebted to 
Mr. Leslie. The apparatus he emploved te determine it is 
very simple. It is a canister of tinned iron in the form of 
a cube, the side being six or eight inches square; this is 
filled with hot water, a thermometer being inserted in it, to 
show the temperature during the continuance of the experi~ 
ment. The sides of the canister are variously prepared ; 
one for example is blackened, another is covered with paper, 
athird has a plate of olass applied to it, and the fourth is left 
ciean. When thus prepared, it is placed betore the concave 
surface of a mirror of tinaed iron, at the distance of three 
or four feet ; the ball of the differential thermometer being 
adjusted to the focus. All these surfaces being equally 
under the influence of the hot water in the eanister, are at 
the 
