TO 
PROFESSOR TYKDALL ON THE ABSORPTION AND 
A close inspection of Melloni’s Table* reveals, I think, the tendency of solid bodies 
also to become more transparent to heat as their composition becomes more simple. 
After rock-salt itself comes the element sulphur, and after it fluor-spar. But the case 
of lampblack will here occur to many, who regard it as the most powerful absorber and 
radiator yet discovered. No doubt the grouping of the atoms of an elementary sub- 
stance may make it tantamount to a compound, and no doubt this is actually the case 
with lampblack ; another eminent example of this kind is referred to in this paper in 
the section on ozone. Leslie, however, found water to be a better radiator than lamp- 
black, and Wells found several substances which were more capable of being chilled by 
nocturnal radiation. On reflection, moreover, the following considerations arise. The 
lampblack of commerce and the soot of a lamp or candle, that is to say, the substances 
which have been hitherto employed in experiments on radiant heat, are copiously mixed 
with hydrocarbons, which are the most powerful absorbers and radiators in Nature. It 
might fairly be questioned whether the reputed experiments with lampblack really dealt 
with lampblack at all. But even the impure substance is to some extent transparent to 
radiant heat. 
I have plates of black glass, rendered so by the solution of carbon in the glass while 
in a state of fusion, and which, though they are impervious to the rays of the most 
intense electric light, allow of a copious transmission of the rays of obscure heat. 
Melloxi’s beautiful experiments on glass of this character are well known ; indeed mine 
are but repetitions of his. Another of Melloni’s experiments which I have recently 
verified is the following. A plate of transparent rock-salt was placed over a smoky 
camphine lamp, and soot was deposited on its surface until it intercepted every ray of a 
brilliant jet of gas. The plate was placed between a source of heat possessing a tempe- 
rature of 100° C., and a thermo-electric pile, a polished screen being placed between the 
salt and the soui’ce. As long as the screen remained, the needle of the galvanometer 
connected nuth the pile stood at zero ; but the moment the screen was removed the 
needle promptly advanced, showing the instantaneous transmission across the layer of 
soot of a portion of the heat incident upon the salt. The actual numbers obtained in 
this experiment are these : — The deflection produced by the heat transmitted through 
the soot was 52° ; which is equal to 90 units. The deflection produced when the layer 
of soot had been carefully removed, so as to leave both surfaces of the salt smooth and 
transparent, was 71°, which is equal to 300 units. The quantity transmitted through 
the soot is therefore to the total quantity as 
90 : 300, 
or as 
30 : 100 ; 
that is to say, the lampblack, which was perfectly opake to the light of a gas-jet, was 
transparent to fully 30 per cent, of the incident heat. On consulting Melloxi’s Table, 
I was gratified to find that he made the transmission by a plate similarly prepared 
* ‘ La Thermochrose/ p. 164. 
