Notices respecting New Books. 75 
and concentrated in its focus, and each pulsation being ac- 
companied, according to the hypothesis, with a discharge 
of the caloric by which the expansion exciting it had been 
produced; the whole is transported with the velocity of 
these undulations, and the calorific effect is obtained where 
they are concentrated on a solid substance. The degree of 
heat excited will, of course, he greater as the temperature 
of the surface communicating it is greater. And the diver- 
sity in the effect trom different kinds of surface at the same 
temperature, Mr. Leslie explains by the hypothesis, that 
they admit of a more or less perfect contact of the atmo- 
spheric air; those with which the air comes into closest 
contact, and this, of course, is supposed to be the case with 
the blackened surface, communicating the’ largest quantity 
of caloric in a given time; and for a similar reason, the 
game surfaces will be those most disposed to receive caloric, 
and will therefore be those most heated by this kind of 
communication. 
*¢ This hypothesis rests principally on certain facts observed 
by Mr. Leslie with regard to the effect of skreens interposed 
between the hot body and the mirror on the calorific radia- 
tion. It had been observed, that when a plate of glass is 
interposed, the effect on the thermometer in the focus is 
greatly diminished. Mr. Leslie found that this is much 
dependent on the distance at which the glass plate is placed 
from the heated body. In the apparatus with the single 
reflecting mirror, if the plate of glass be placed at about 
two inches from the blackened surface of the canister, a 
rise in the thermometer is produced equal to about one-fifth 
of what would he produced by the same surface, the glass 
heing withdrawn ; if further removed from the heated sur- 
face, the effect on the thermometer diminishes; and when 
it is removed ahout a foot, it does not amount to one- 
thirtieth of what it is in the first position. Mr. Leslie fur- 
ther found that the effect was very different with skreens of 
different kinds; with one of paper interposed, it did not 
differ much from that with the glass, but if a metallic skreen 
was used, though extremely thin, as for example gold leaf, 
the effect on the thermometer was completely mtercepted. 
“* These results cannot be explained on the supposition 
that these skreens operate hy intercepting more or less the 
calorific radiation, some doing so completely, others more 
impertectly ; for, were this the case, the action of those 
which allow a certain degree of heating effect to be pro- 
_ Auced on the thermometer ought to be the same at whatever 
Aistance it i: placed from the heated surface, while the fact 
is, 
