626 
ON RADIATION AND ABSORPTION. 
the impediment offered by this same vapour to the radiation from the earth which check 
the sudden drain of terrestrial heat, and thus renders our planet inhabitable. 
This poAver of electric absorption was illustrated by the action of two tuning-forks 
which sounded the same note. Both forks being mounted on their resonant stands, one 
of them was first sounded. The silent fork was then brought near the sounding one, 
and held near it for five seconds. The vibrations of the excited fork were then quenched, 
but the sound did not cease to be heard. In fact the silent fork had taken up the vibra¬ 
tions of its neighbour, and continued to sound after the latter had ceased to vibrate. 
Again, one fork being permitted to remain upon its stand, the other was dismounted 
and thrown into strong vibration. Detached from its stand, its sound was too feeble to 
be heard by the audience; but on bringing it near the mounted fork a mellow sound 
rose which filled the room. Thus the vibrations of the one fork were transmitted through 
the air and imparted to the other. To effect this transference it was necessary that the 
forks should be in perfect unison: the fixing upon either of them of a bit of wax not 
larger than a pea was sufficient to destroy the power of the forks to influence each other. 
Thus one sounding body absorbs the vibration of another sounding body with which 
it is in unison; and here we have in acoustics the representative of that great principle 
which in optics lies at the base of spectrum analysis, namely, that bodies absorb those 
rays which they can themselves emit. Thus green vapour of silver, if interposed in the 
path of a beam of white light, will absorb the green which it can itself emit. Thus 
also the incandescent vapour of sodium, itself intensely yellow, cuts clearly out the 
yellow band of the spectrum. And the same is true of aqueous vapour. Its periods of 
vibration synchronize with those of the rays, or more accurately ivaves, emitted by the 
warmed earth, and hence its power to intercept those waves by taking up their motion. 
But it is in dissonance with the luminous waves emitted by the sun, and hence those 
waves pass through large quantities of it with scarcely sensible absorption. 
This incompetence of aqueous vapours to absorb luminous rays is shared by all really 
transparent bodies; in fact, they are transparent in virtue of their incapacity to absorb 
luminous rays. Now, transparent bodies in a state of powder are always white, and in 
white bodies luminous rays have no powder. The light of the sun, for example, cannot 
warm white sugar, nor can it warm table salt, nor flour, nor a white dress; it cannot 
even melt snow. The most powerful luminous beam may be concentrated upon a sur¬ 
face covered with hoar frost without melting a single spicula of the frost crystals. How, 
then, it may be asked, does sunshine clear away the snow from the mountain heads ? 
Two or three days’ sunshine on the mountains suffices to obliterate the traces of a hea^y 
snow-fall: how can this occur if sunshine has no power to melt the snow crystals? It 
is not the luminous rays of the sun which perform this work, but a body of rays which, 
though possessing high calorific power, have no light in them. By a process of trans¬ 
mutation these dark rays may be converted into luminous ones, but as they come from 
the sun, and fall upon the mountain summits, they are utterly incompetent to excite 
vision. Every stream which channels the glaciers or tumbles down the valleys of the 
Alps is the direct product of this invisible radiation. To it also the glaciers owe their 
birth as well as their dissolution. For while the luminous rays of the sun falling on the 
tropical ocean penetrate the water to great depths without considerable absorption, the 
dark rays are in great part absorbed close to the surface of the ocean; they therefore 
heat the Avater at the surface, and are thus almost the sole excitants of evaporation. 
Not only, then, do those invisible solar rays, by the fusion of the ice, give birth to the 
rivers of Switzerland, but it is they that lift the material of these rivers from the sea and 
store it on the frozen summits of the mountains. 
Gathering up the rays emitted by a powerful electric lamp, and concentrating them 
upon a small focus, water, alcohol, or ether placed at the focus speedily boils, some of 
them, indeed, almost instantly. But they are not boiled by the luminous rays, though 
these produce an impression too dazzling to be borne upon the eye. Interposing in the 
path of the concentrated beam a glass cell containing pure distilled water, the light of 
the beam is not sensibly diminished, but it is no longer competent to boil or even heat 
water at the focus. Placing a piece of ice at the luminous focus it is not melted, though, 
if blackened wood be placed there, it is set on fire. The moment, however, the cell of 
water is withdrawn the ice melts—melts because the dark rays previously absorbed by 
the water of the cell are now absorbed by it. There are liquids of very low boiling 
points—bisulphide of carbon, for instance-—which, when placed at the focus where the 
