ON RADIATION AND ABSORPTION. 
627 
whole radiation, dark and bright, of the electric lamp is converged, cannot be caused to 
boil, can hardly be warmed. Water, for instance, requires a temperature of 212 *ahr. 
to boil it, bisulphide of carbon requires only 118 0, 4 ; still the former is boiled in a time 
insufficient to warm the latter. This arises from the fact, that while water powerfully 
absorbs the dark calorific rays and allows the luminous ones free transmission, the bi¬ 
sulphide of carbon is transparent to both classes of rays, and hence is warmed by neither 
of them. Thus, also, when it was stated that sugar could not be warmed by the tight 
of the sun, the invisible solar rays were meant to be excluded, for when the total radia¬ 
tion of the sun is converged upon white sugar it is immediately burnt up, the agent ot 
its combustion being, however, the dark radiation. 
It is possible to filter the composite radiation from the sun or from the electric light, 
so as to detach almost completely the visible from the invisible rays. It has been already 
stated that bisulphide of carbon is transparent to both classes of rays; now iodine, a sub¬ 
stance which dissolves freely in the bisulphide, is eminently transparent to the mv isib e 
ravs alone. Hence, a combination of these two substances furnishes us with a ray-filter, 
which, while it pitilessly cuts off the bright rays, allows the dark ones free transmission. 
At the dark focus we can boil water or alcohol, but we cannot warm bisulphide or bi¬ 
chloride of carbon. Bromine also, notwithstanding its volatility, beam exposure at the 
focus without being heated. Sulphur also bears the temperature of the focus foi a con¬ 
siderable time without ignition. Common phosphorus, a combustible so quick that the 
warmth of the fingers when in contact with it suffices to provoke combustion, bears for 
twenty or thirty seconds without ignition the action of radiant heat at a ^ocus where i 
the fraction of a second, platinized platinum is raised to a white heat. The phosphorus 
is in a great degree transparent to radiant heat. The red iodide of mercury strewn on 
paper and exposed at the focus has its colour discharged where the ratable ™ag e s of 
the carbon points fall upon it, but owing to the transparency of the iodide to radiant 
heat it requires some exposure to produce the thermograph. This red substance is lai 
less absorbent of radiant heat than white paper, and hence it is sometimes easier to 
obtain a thermograph of the carbon points by exposing to the radiation from the lamp 
the L/c of the paper on which the iodide is strewn, than by exposing the face covered 
with the iodide, Ris often, indeed, more easy to burn a thermograph through the papei 
than to discharge the colour of the iodide. Hence, white paper may be protected irom 
radiant heat bv being; covered with a substance like the iodide of meicury. 
We are here 7 Sally reminded of the experiments of. Franklin, which coasted in 
placing cloths of various colours upon snow, and observing the depth to which they 
sank in the snow when exposed to direct sunshine. Franklin concluded that the lighter 
the colour of the body the less is its power of absorption. The generalizations founded 
on this experiment are, for the most part, fallacious. Results long ago obtained, esta¬ 
blishing the vast influence of chemical constitution on radiant heat, led the speakei to 
contrast iodine an element, with alum, a body of highly complex character. Both sub¬ 
stances were in’powder, the one being dark, the other white. Exposed to the radiation from 
various sources Le white powder proved itself in all cases the most powerful absorber. 
The dark powder of amorphous phosphorus was also compared with the hydrated oxide 
of zinc but the white powder was the best absorber. Bodies of the same coloui com- 
pared^together showed similar differences. The red oxide of lead, for example was con¬ 
trasted with the red iodide of mercury, and the oxide proved the most poweiful absorber. 
So also the white chloride of silver was compared with the white carbonate of lead, the 
lead salt proved by far the most powerful absorber. In this way it was proved that as 
regards the absorption of radiant heat, white in some cases exceeds black, black in some 
cases exceeds white, and the other colours are equally capricious; all evidently depend¬ 
ing on the chemical constitution of the substances. Here, as m other cases moreover, 
radiation and absorption go hand in hand, the substance which absorbs heat most power- 
fully radicitinsr the same h6&t most copiously# _ , . „ „ 
In the case of Franklin’s white cloth exposed on snow to.sunshine, there>ui noreaso 
why it should sink at all; there is, on the contrary, reason to conclude that it must rise 
rpEtivelv to the snow surrounding it. For, as regards tbe luminous lays of the sun, 
Ihey warm the cloth or to melt the snow. Whatever effect is pro- 
Led is therefore due to the dark solar rays Now snow absorbs ^loX llss than the 
greediness than any other substance; hence the white cloth, wh ,“* p ab ®“ rb ^Lnd owL to 
snow, really defends the snow underneath it from the action of the sun, and owing to 
