374 On the Influence of Metals upon Radiant Heat. 
heat diffusely reflected, from the unreflected heat, disappears 
when the incidence is very oblique. Jt is thus placed beyond 
doubt, that the properties of the reflected rays are dependent on the 
inclination at which they meet the reflecting surface. 
The principal results of this investigation may be stated to be 
as follows :— 
1. Metals, as gold, silver and platinum, when in thin layers, 
are to be regarded as diathermanous bodies, which permit a por- 
tion of the calorific rays to pass through them; which portion 
naturally becomes less as the thickness of the layer increases. 
In thus transmitting the calorific rays, certain metals, as gold 
and silver, exercise an elective absorption, similar to that of co- 
loured transparent bodies upon light. Others, on the contrary, 
like platinum, act in the same manner upon all rays, and are 
therefore to be regarded as analogous to colourless bodies in the 
case of light. 
2. In the case of diffuse reflexion, also, certain metals, such 
as gold, silver, mercury, copper and brass, similar to coloured 
and opaque bodies as regards light, exercise an elective absorp- 
tion upon the calorific rays, in consequence of which the pro- 
perties of the latter are altered. Others, on the contrary, for 
example, platinum, iron, tin, zine, lead, alloy of lead and tin, 
German silver, reflect all kinds of calorific rays in the same pro- 
portion, exactly as colourless opaque bodies do with regard to 
light. 
The properties which distinguish calorific rays reflected from 
metals, from unreflected heat, are so far dependent on the source 
of heat, that differences, for example, which exhibit themselves in a 
striking manner when solar heat is made use of, are diminished 
in the case of a Locatelli lamp, and completely disappear when 
the source of heat is a metallic cylinder not heated to redness. 
The surface has the power either of causing the differences to 
appear in their maximum degree, or to disappear totally, accord- 
ing as the surface produces a diffuse or a regular refiexion. 
The same is true of the change of the angle of incidence. 
In the case of a rough metallic surface, as the angle of the rays 
‘with the surface diminishes, the reflexion passes gradually from 
the diffuse to the regular, and at the same time the differences 
between the reflected and unreflected heat also gradually become 
‘léss, until finally both have exactly the same character. 
