106 PHYSICS. 
mitted. That an absorption of heat rays must occur is shown by the 
heating of a body placed in one focus of the above-mentioned system of 
concave reflection, whenever a heated body is placed in the other. 
Although this power of absorption is common to all bodies, it is yet not the 
same in all, the less dense the body the greater being its absorbing power. 
Of the reflecting power of bodies we have an illustration in the above- 
mentioned concave metallic reflector, the mirrors themselves experiencing 
no elevation of temperature when a heated body is placed in one of their 
foci. The powers of absorbing and reflecting heat in bodies may be 
considered as complementary to each other; both taken together explain 
what becomes of the heat reaching any body. Thus, a body reflects what 
it cannot absorb, and the greater the absorption the less the reflection, and 
vice versa. The angle of reflection of heat rays is equal to the angle of 
incidence. From the surface of plates not well polished, rays are dispersed 
irregularly, or diffused in all directions ; and the same is the case with heat 
rays. Of this we may readily become convinced by directing a small beam 
of light against the wall of a dark room. By presenting the thermo-electric 
pile towards the light spot, a deviation of the needle will be observed in 
whatever part of the room it may be placed ; it returns immediately to 0, 
however, whenever the aperture admitting the beam of light is closed. 
There is of course no other heat present than that diffused by the beam of 
heht. 
Solid bodies may transmit heat rays just as transparent do light. These 
are called by Melloni diathermanous, and those which intercept heat, 
athermanous. Melloni, in his experiments on the passage of heat rays, 
employed the apparatus represented in pl. 19, fig.37. As sources of heat he 
employed: 1. a Locatelli lamp; 2.a spiral of platinum wire kept red hot by 
the flame of alcohol ; 3. a blackened copper plate, / (fig. 39), heated to 752°F. 
by an alcohol lamp; 4. a hollow cube of brass plate ( fig. 40) filled with hot 
water, maintained at an equal temperature by alamp. ‘These sources of 
heat were successively placed upon the stand e. The screen, 0, composed 
of two brass plates, and turning on a hinge, could be brought between the 
source of heat and the thermo-electric pile, to keep from the latter any heat 
rays. The plate of the body to be investigated was placed at r. If the 
source of heat be placed at such a distance that the needle experiences 
a certain deflection (30°), and a plate be interposed at 7, it was found 
that the needle returns more or less to its original position; and also that 
plates of equal thickness and transparency do not transmit an equal number 
of heat rays, and even that some bodies transmit heat better than others of 
much greater transparency. The thickness of the plates employed 
averaged from one third to two thirds of a line. Plates of rock salt 
were found to be most diathermanous (92%), and plates of ice the least 
(63). 
It was also found that the diflerence of radiants involved a difference in 
the number of rays transmitted through the same plate. In the lamp of 
Locatelli the transmission was greatest, in the brass plate the least, although 
the original deflection (30°) was the same in all. Plates of rock salt 
280 
