288 
SECOND REPORT— 
fir • 
much faster than the coated one. (Sur le Feu , ch. iv. Thom¬ 
son, i. 126.) This last statement is so contrary to all other 
experiments, that we must suppose some mistake. 
De Saussure received the sun’s rays into a box lined with 
charred cork, containing a thermometer with a glass front; it 
rose in a few minutes to 221°, when the temperature of the air 
was 75°. ( Voyages , ii. 982.) 
Professor Robison in a similar experiment employed three 
vessels of flint glass within each other at ^rd of an inch distance, 
set on a base of charred cork, and placed on down in a paste¬ 
board cylinder; the thermometer within, in clear sunshine rose 
to 280°, and once to 237°. (Black’s Lect. i. 547. Thomson, 
i. 327.) 
Sir H. Davy took several small disks of copper of equal 
weight, size, and figure, on one side painted respectively white, 
yellow, red, green, blue, and black. A mixture of oil and wax, 
which became liquid at a temperature of 76° Fahr., was at¬ 
tached to the other surface of each disk; and on exposing the 
coloured surfaces together to the sun’s rays, the length of 
time elapsed before the mixture on each began to be affected, 
was in the order in which they are above enumerated. (Bed- 
does’s Medical Contributions , p. 44.) 
4. ) The experiments of Sir E. Home (Phil. Trans. 1821, 
Part I.) are particularly deserving of attention, as exhibiting 
what might at first sight be considered an exception to the 
above remarks; a greater effect being produced in some in¬ 
stances on a white, than on a black surface. A more attentive 
examination, however, will show us that these experiments 
prove thus much : The heat occasioned by the rays of the 
sun when received directly, or when in some degree intercepted, 
as by thin white cloth, on the skin, is greater than that com¬ 
municated by conduction to the same skin, through a black 
cloth in contact with it, which is itself, in the first instance, 
heated by absorbing the rays. 
He observes also that a white skin is scorched, and that of 
a negro is not, in 10 minutes, by the direct rays of the sun; that 
is, as before, the outer coat of the skin allows some of the direct 
rays to passthrough and affect the sentient substance beneath; 
whereas in the case of the black, the ra 3 r s are absorbed and 
converted into heat of temperature, which diffuses itself equally 
and does not produce the effect of scorching. 
5. ) The most singular facts connected with the absorption of 
the sun’s rays, are those exhibited by the substances called 
“ phosphori" or “ pyrophori”. (Thomson’s Chew. i. 17.) 
