BY JAMES TOLSON, ESQ. 65 
Loss or Heat By Contact or Air.—‘‘ The loss of heat by con- 
tact of cold air is independent of the nature of the surface, so that 
cast iron, stone, wood, etc., etc., lose the same amount of heat 
under the same conditions of temperature, but the form of the 
body affects the results considerably, so that a plane, a sphere, and 
a cylinder will lose different amounts of heat per square foot in the 
same time.’’—Box, op. cit. 
“Tt has been already shown that a body placed in vacuo and 
surrounded by an enclosure at a lower temperature than itself will 
gradually lose heat. If the body be surrounded not by vacuum, 
but by a gas, it will lose heat more rapidly than in vacuo, and the 
difference between its velocity of cooling in the two cases is due to 
the presence of the gas. Thus the w hole velocity of cooling of a 
body in air or gas is due partly to radiation and partly to 
gas.’’—Stewart, op. cit. 
“The velocity of cooling due to the sole contact of a gas is 
entirely independent of the nature of the body. Thus, as far as 
gas or air is concerned, a silvered thermometer will cool just as 
rapidly as a blackened one ; but as far as radiation is concerned 
it will cool less rapidly than the blackened one. 
“The velocity of cooling due solely to the contact of a gas is 
proportional to the excess of temperature raised to the power of 
1.233. From this law also we see the difference between cooling 
in vacuo and the cooling due to a gas. 
“In vacuo the effect for the same excess of temperature of the 
thermometer above the enclosure varies also with the temperature 
of the enclosure, whereas in the case of a gas it depends only on 
the excess of temperature. The cooling power of a given gas and 
fora given excess of temperature depends zof on the density but 
on the pressure of the gas.” —Stewart, op. cit. 
‘For the small differences of temperature (between the air and 
the body) we may admit that the loss of heat is simply proportional 
to that difference; but with great differences of temperature, 
Dulong has shown ‘that the loss of heat increases in a much higher 
ratio than that difference, so that, for instance, when a body is 
450° F. above the temperature of the air, the loss per degree is 
double the loss with a small difference of say 20° F.’’—Box, op. cit. 
““The researches of Dulong show that the loss by contact of 
cold air is independent of the absolute temperature of the heated 
body, differing in this respect from radiant heat. He also found 
that the heat lost increases more rapidly than the simple ratio of 
the excess of temperature. Putting his formula with the constants 
given by Pictet into such a form as to give a va//o for the different 
temperatures we have the rule 
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