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XXI. On the Construction of a new Calorimeter for determining the Badiating Powers of 
Surfaces in Air ; and its Application to the Surfaces of various Mineral Substances. 
By W. Hopkins, JEsg.. M.A., F.B.S. 
Eeceived June 1, — Bead June 21, 1860. 
When my paper on the Conductivity of various Substances was presented to the Society, 
it was intimated to me on the part of the Council of the Society, that it might be 
advisable to determine absolute instead of relative conductivities, the latter being alone 
attempted in my previous experiments. It has been partly in consequence of this inti- 
mation, as well as from the desire to make my investigations the more complete, that I 
have given my attention to the construction of a calorimeter which might serve for this 
purpose. The present communication contains a description of this instrument, with 
the results which it has enabled me to obtain respecting the absolute quantities of heat 
which emanate from the surfaces of certain substances under given conditions. 
1. When a body is placed in atmospheric air (or any gas), the quantity of heat which 
is lost from its surface in a given time, when its temperature is higher than that of the 
surrounding medium, will be greater than if it were placed in a vacuum, other condi- 
tions remaining unaltered. In the latter case the heat escapes by simple radiation ; in 
the other case a portion of the heat also escapes in consequence of the contact of the air 
with the surface of the heated body. Dulong and Petit ascertained by a careful series 
of experiments, the laws according to which the mercury contained in the bulb of a 
thermometer cools, or those which govern the quantity of heat which escapes from the 
surface of the containing bulb, when placed in a vacuum, in air, or in several kinds of 
gases. These expeiiments were made with the glass bulb naked, and also when it was 
silvered, so that the laws of radiation which they established were strictly in reference 
only to surfaces of glass and those of silver. Certain laws were identical in both these 
cases, and hence it was concluded, though by a limited induction, that the same laws 
were applicable to all other surfaces. They did not, however, give the absolute quantity 
of heat which, under given circumstances, and in a given time, emanates from the surfaces 
of the glass or silver with which they experimented. The instrument which I have 
constructed gives very easily this absolute amount of heat, as I believe, with very 
approximate accuracy. 
The apparatus devised by Dulong and Petit is easily described with reference to its 
essential parts, and independently of the particular artifices which were required to 
secure its practical working. A hollow globe of thin copper, about a foot in diameter, 
3 E 2 
