344 PROFESSOR TYNDALL’S CONTRIBUTIONS TO MOLECULAR PHYSICS. 
Thus also as regards lampblack : the blackness of the substance is due to the accord 
which reigns between the oscillating periods of its atoms and those of the waves 
embraced within the limits of the visible spectrum. The substance which is thus im- 
pervious to the luminous rays is moreover the very one from which the whitest light of 
our lamps is derived. It can absorb all the rays of the visible spectrum, it can also 
emit them. But though in a far less degree than iodine, lampblack is also to some 
extent transparent to the longer undulations. Melloni was the first to prove this ; and 
in an experiment described in a former memoir, I myself found that 30 per cent, of the 
radiation from an obscure source found its way through a layer of lampblack, which 
cut off totally the light of the most brilliant jet of gas. I shall have occasion to show 
that, for certain sources of heat of long period, between 40 and 50 per cent, of the 
entire radiation is transmitted by a layer of lampblack which is perfectly opaque to our 
most brilliant artificial lights. Hence, in the case of lampblack, while accord exists 
between the periods of its atoms and those of the light-exciting waves, discord, to a con- 
siderable extent, exists between the periods of the same atoms and those of the extra- 
red undulations. 
$ 7. 
The power of varying at will the temperature of the platinum spiral, renders it pecu- 
liarly suitable for the examination of the influence of temperature on the transmission 
of radiant heat. To obtain sources of different temperatures, Melloni resorted to 
lamps, to spirals heated to incandescence by the flame of alcohol, to copper laminae 
heated by flame, and to the surfaces of vessels containing boiling water. No conclu- 
sions regarding temperature can, as will afterwards be shown, be drawn from such 
experiments ; but by means of the platinum spiral we can go through all those changes 
of temperature, retaining throughout the same vibrating atoms , and we can therefore 
investigate how the alteration of the rate of vibration affects the rate of absorption. 
The following series of experiments were executed on the 9 th of October, with a pla- 
tinum spiral raised to barely visible redness, and vapours at a tension of 0*5 of an inch. 
Table XII. — Radiation of heat through Vapours. Source of heat, platinum spiral 
barely visible in the dark. 
Name of vapour. 
Deflection. 
Absorption 
Bisulphide of Carbon . 
. . 7*5 
6-5' 
Bisulphide of Carbon . 
. . 7-45 
6-4. 
Chloroform .... 
. . 10-5 
9-T 
Chloroform .... 
. . 10-5 
9-1. 
Iodide of Methyl . 
. . 14-5 ■ 
12-5' 
Iodide of Methyl . 
. . 14-5 
12*5. 
Iodide of Ethyl . . . 
. . 24-2 
20-9' 
Iodide of Ethyl . . . 
. . 24-5 
21-1 
