76 Notices respecting New Books. 
is, that itis much dependent on its contiguity to it. They 
therefore, Mr. Leslie conceives, establish the conclusion, 
that these skreens, in every case, arrest the radiant caloric, 
ud that where any effect 1s produced on the thermometer, 
this is to be ascribed to the interposed skreen acquiring 
heat, and being thus enabled to display the same action as 
a similar radiating surface would do at the same teimpera- 
ture. Accordingly, when a skreen 1s employed which is 
not much disposed to receive radiant caloric on the one 
hand, or to radiate it on the other, as one of metal, no effect 
is produced; or if the skreen is such, that its temperature 
cannot be raised, as is the case for example with a plate of 
ice, there is also no effect; but, if the skreen be of a sub- 
stance disposed both to atsorb and radiate caloric, as in the 
case with. glass or paper, then a certain effect will be pro- 
duced, the side next to the hot body arresting the calorific 
radiation and having its temperature raised, and the other 
radiating proportional to this rise of temperature,—and this, 
of course, will be greater the nigher the skreen is to the 
heated body. 
«¢ Now this effect of these interposed skreens, Mr. Leslie 
further conceives, can only be explained on the supposition 
that the airis the vehicle of the communication, as already 
explained, the skreen arresting the chain of pulsations, and 
acquiring in its turn to a certain extent the power of trans- 
mitting these pulsations with the accompanying discharges 
of caloric from the other surface; and on this assumption 
in a great measure rests his hypothesis. 
<« It is one which does not appear necessarily to follow, 
and it is perhaps equally conceivable on the hypothesis of the 
exiStence of rays of caloric, that these may be arrested by 
the skreen, its lemperature may be raised, and corresponding 
rays be projected to a certain extent from its other surface : 
it must, in fact, be supposed, that the interposed skreen 
receives. caloric at the one surface, and communicates it 
from the other, whether the caloric be supposed to be pro- 
pagated by pulsations in the atmosphere, or by actual pro- 
jection of calorific particles; and in either hypothesis, those 
most disposed to receive it, and again to discharge it, will 
be those which will admit of the greatest heating effect be- 
ing produced on the thermometer. 
“There is also some obscurity with regard to the principle 
of Mr. Leslie’s theory; for admitting, that a chain of vibra- 
tions, such as he supposes, may be established in an elastic 
medium from a heated surface, it is not very obvious how 
each pulsation should be accompanied with a diseharge “me 
the 
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