REPORT ON RADIANT HEAT. 
287 
far the same relation to the texture of surfaces which has been 
found in absorption of simple heat, may hold good in regard 
to the sun’s rays. But for surfaces of the same texture it has 
been incontrovertibly established that the effect in this case 
increases in proportion to the darkness of colour , or in propor¬ 
tion to the absorption , of light ; and it would seem most pro¬ 
bable that this relation is the only one which really holds good, 
the texture of the surface being probably quite indifferent ex¬ 
cept so far as it tends to the better absorption of the light. 
2. ) Among the earliest experiments on the subject, if not 
actually the first, were those of Mr. Boyle, on the different 
degrees of heat communicated by the sun to black, white, and 
red coloured surfaces. 
He caused a large block of black marble to be ground into the 
form of a spherical concave speculum, and found that the sun’s 
rays reflected from it were far from being too powerful for his 
eyes, as would have been the case had it been of any other 
colour; and although its size was considerable, yet he could 
not set a piece of wood on fire with it; whereas a far less spe¬ 
culum of the same form, made out of a more reflecting sub¬ 
stance, w r ould presently have made it inflame. 
It was remarked by Scheele, that the thermometer when filled 
with alcohol of a deep red colour, rose more rapidly when ex¬ 
posed to the sun’s rays than another filled with the same kind 
of spirit uncoloured ; but that the fluid rose equally in both 
when dipped together into the same vessel of warm water. 
(On Air and Fire, &c.) 
Dr. Franklin found that the hand when applied alternately 
to a black and to a white part of his dress in the sun, would 
feel a great difference in their warmth. 
He observed that black paper was sooner fired by exposure 
to the focus of a lens than white. 
His well known experiment of placing differently coloured 
pieces of cloth on the snow in the sun, and observing them sink 
deeper in proportion to the darkness of colour, was first sug¬ 
gested by Dr. Hooke. 
3. ) Cavallo observed that a thermometer with its bulb black¬ 
ened, stands higher than one which had its bulb clear when 
exposed to the light of the sun, or even of the clouds. {Phil. 
Trans. 1780.) 
Pictet made a similar observation, observing that when the 
two thermometers remained for some time in a dark place they 
acquired precisely the same height. He also found that when 
they had both been raised to a certain point, the clean one fell 
