104 PHYSICS. 
finally it assumes a constant temperature when it receives as much heat as 
it gives off. In this manner the excess of stationary temperature of the 
calorimeter above the surrounding medium may be determined for the 
various kinds of gases, and as in equal times equal volumes of gas pass 
through the apparatus, it is evident that the values of the specific heat of 
various gaseous bodies must, for equal quantities, be in direct proportion to 
ihe above-mentioned excess of temperature. Then taking the specific 
heat of air as unity, the proportional values for other gases may readily be 
determined. The philosophers above-mentioned, referred the specific heat 
of air, and consequently that of other gases, to water. As De la Roche and 
Berard have determined the capacity for heat of gases at a constant 
pressure, Laplace has determined the same for constant volumes. 
g. Transmission of Heat. 
Heat is transmitted partly by radiation, partly by conduction. Heated 
bodies send off heat on all sides, as it were heat rays, which traverse the 
air. If we imagine a source of heat at any point, then the intensity of heat 
at different points will be inverselv as the squares of the distances. Ata 
distance =1 the intensity = 1 x 1= 1’; at a distance = 2 the intensity = 
+x i= (5) =}, &c. Inradiant heat, however, there is no uniform heating 
of the strata of the air, for although near a fire we may experience a 
piercing heat, this becomes immediately stopped on interposing a screen. 
Placing two large spherical or parabolic concave reflectors of polished brass 
(pl. 19, fig. 35) at a distance of sixteen or eighteen feet apart, and putting 
in the focus of one a piece of tinder, and in that of the other a red hot iron 
ball, the tinder will become inflamed. If, instead of the red hot ball, one 
merely hot, at a temperature of about 300° for instance, be employed, and 
instead of the tinder a thermometer, the latter will quickly rise. If a vessel 
containing hot water be placed in one focus, the ordinary thermometer will 
not exhibit any appreciable change of temperature; we are not to suppose 
from this, however, that the vessel of water radiates no heat. The truth of 
the matter is, that while a radiation does take place, the ordinary thermo- 
meters are not sufficiently sensitive to exhibit it, for which reason it 
becomes necessary to employ a more delicate instrument. Such thermo- 
meters are: 
1. The differential thermometer of Rumford ( pl. 19, fig. 36), consisting 
of two glass bulbs, a and 6, connected by a bent glass tube. In this a drop 
of alcohol or sulpburic acid serves as an index, upon which the air presses 
from both sides. The position of this index or drop of fluid, when both 
bulbs are of the same temperature, is taken as the zero of the scale, which is 
placed on the horizontal part of the tube. If one bulb be heated more than 
the other, the drop, by the expansion of the contained air, will be driven 
towards the colder one; and the distance to which it is driven will be in 
proportion to the difference of temperature of the two bulbs. 
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