268 Scientific Intelligence. 
pear, and even, conversely, to bring about a more abundant transtnission 
of the heat of the lamp. 
I, (1.) In the passage of radiant heat through rock-salt covered with 
be an elective absorption nesta by Melloni) takes place without 
diffusion. A diffusive action (supp by Forbes) never takes place in 
consequence of the rough surface of the layer of soot, but sometimes in 
een = a tarnishing of the rock-salt plate in the process of cov- 
ering wit 
2.) In Bier case of transmission through thin rita of metal Jaid upon 
glass the first process takes place without the las 
(3 e presence of an elective absorption shetiad daring transmission » 
is most certainly pi gts by spent whether the heat before and 
after its passage through the substance in question retains unchanged or 
varies its capacity to pass through othe (clear) diathermanous bodies 
“ a smooth surface. 
b. 
a consists of parallel and the other of "diffusive rays, the latter passes 
mos a a the substance tested, this substance is diffusive. In 
is ethod is pointed out for the co pil oke with each other 
whic 
with an capillaked or nas ohh ihe diffusion exerted upon them is in 
general increased. This increase, with the change in inclination, in the 
first place becomes larger with the generally diffusive property of the 
screen, but then again grows less to such a degree that in very rough 
and sufficiently cloudy plates, just as in the case of clear ones no differ- 
ence can be detected in the behavior of rays which are transmitted at 
different angles of inclination, 
b. A diffusion produced by reflection from rough surfaces diminishes, 
on the contrary, ee = more obliquely incident rays, and passes 20 nally 
in the Hiteror of the su ubstance, the Panek ean qua ality of the 
surface produces a “coloration” of the transmitted hea 
(3.) It follows, sachet that in the case of the pe and cloudy 
media in question, a sess on must be made between the action of the 
which which is always present, and that of the elective absorption 
occu 
Fused common = produced a diffusion but no heat-co -coloration. _ 
5} Another piece of rock-salt was found to be ebemicaly ' -— et 
chanically i impure, — exercised both a diffusive action and 
absorption. Circumstances of this kind explain the varying obeer 7 
made in different etguibiaaiies with rock-salt.—Pogg. Ann., CX%, ay 
