230 
PROFESSOR FORBES ON THE EXTINCTION OF THE SOLAR RAYS 
perature in terms of the time elapsed, from which he deduced the velocity of heating 
under similar circumstances. The result of these experiments is remarkable as an- 
ticipating the law usually attributed to De la Roche, that the facility of transmission 
through successive plates of glass continually increases with the number already 
passed through*. He thus finds 
Incident heat 
. 100 
Loss. 
Through one plate of glass . . 
84 
16 
Through two plates of glass 
69 
15 
Through three plates of glass . 
59 
10. 
We shall have occasion to return to this important experiment. 
14. In 1774 De Saussure employed an instrument which he called a Heliother- 
mometer, for measuring the force of the solar rays, particularly upon the top of 
mountains. It consisted of a wooden box lined with thick pieces of blackened cork, 
and covered with three separate superimposed glass plates which admitted the solar 
rays to a thermometer placed in contact with the cork : in this instrument the 
mercury rose to 70° Reaumur-}-. 
15. Sir John Leslie, in his Essay on Heat;};, proposed a modification of his diffe- 
rential thermometer with one bulb blackened, and the other clear, as a photometer ; 
since the excess of effect on the dark ball appearing only when heat is accompanied 
by light, might, he thought, be considered as a measure of the light. There is, how- 
ever, a twofold objection to consider this instrument as exact in its indications. 
1st. The quality of affecting dark surfaces more than pale ones, is a quality of heat 
often accompanying, but not inseparable from light. This has already been proved 
from Professor Powell’s experiments, which have been further confirmed by M. 
Melloni, who finds, for instance, that a black and a white surface absorb lamp heat 
in the ratio of 100 to 80‘5 ; and if the heat be transmitted through rock salt, the ratio 
is unchanged ; hut if alum be used instead of salt, though equally permeable to light, 
the ratio now becomes 100 : 42'9. But, secondly, an objection clearly foreseen by Lam- 
bert, is applicable to all measures of a radiant source by the statical effect on a thermo- 
meter. The condition of a body remaining at a higher temperature than the surround- 
ing medium is this:- — that it shall receive in a given time as much heat as it parts 
with : the more it receives the higher must be the temperature to which it must be 
raised in order to part (by the law of cooling) with an equal amount. The measure 
of intensity depending upon the stationary heat of the thermometer, clearly supposes 
that the cooling proceeds according to a constant law in all the experiments which 
are to be com pared §. 
* Pyrometrie, § 282. f Voyages dans les Alpes, § 932. X 8vo, Lond. 1804. 
§ The form which Lambert gave to his formula was this, — 
dy — ndt — 
y S 
where y is the excess of temperature marked by the thermometer, n the heat communicated directly in unit of 
