266 
SECOND REPORT- 1832. 
consists in a thermo-electric combination, susceptible of excita¬ 
tion from the feeblest conceivable application of heat, and con¬ 
nected with a delicate galvanometer, which gives a measure of 
the effect produced, and consequently of the heat . 
The pile is in a case coated with the smoke of a flame when 
used for radiant heat, but left naked when for heat of tempera¬ 
ture, on account (as they observe) of the bad conducting quality 
of this coating. 
They applied this instrument to the examination of the dif¬ 
ferent reflecting, absorbing, and radiating powers of surfaces. 
They confirmed in general the results of Leslie and others 
already mentioned. They found that polish augments the re¬ 
flecting power much less than usually supposed. Non-metallic 
substances possess scarcely any reflecting power, whatever be 
the state of their surfaces. 
They examined the absorptive power of different substances, 
taking laminae of equal thickness and similarly fixed, &c.: these 
having been heated for a few minutes in the rays of the 
sun, were placed in pairs on apertures at the opposite sides 
of the thermo-multiplier, and in this way the order of their 
absorptive powers was considered to be obtained by the degree 
of heat they respectively radiated ; and the results were, that 
the effect increased by blackness of colour and with roughness of 
surfaces. Also the following surfaces were in this order,—silk, 
wool, cotton, flax, hemp, (all white,) which is the inverse of 
their conducting powers. In like manner, with metals of nearly 
the same colour and polish, the order was—copper, silver, gold, 
steel, iron, tin, lead, exactly in the inverse order of the con¬ 
ducting powers ;—the same with several -woods and minerals. 
On these experiments I must remark, that the heat acquired 
from the suns rays is so obviously dependent on colour, that it 
is astonishing that any experimenter should adopt this as 
affording any ground for making conclusions respecting the 
comparative absorbing or radiating powers for lieat in general. 
The later results, when the surfaces were all of th esame colour , 
are extremely important. Supposing they all acquired the 
same degree of solar heat which was thus converted into heat 
of temperature, and then radiated from the surfaces as simple 
heat, the real conclusion established is, that the radiating 
powers of surfaces for simple heat are in the inverse order of 
their conducting powers. 
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