110 Letter to Dr. Wells respecting Dew. (Fux. 
_ Respecting the Explanation—I have said, ‘* If water is depo- 
sited on the side in which the air is coldest, it ought to be deposited 
still more abundantly on that part of the glass which has been co- 
vered with a metallic plate, because that plate acts the part of a 
screen.” Dr. Wells perhaps admits this explanation; but, in con- 
sequence of the principle which he has discovered, he ought, I con- 
ceive, to go further, and to say, “if water is deposited on the 
external side of the glass, we must conclude that this side is colder 
than the air which touches it. Yet it receives a portion of heat by 
conduction, through the thickness of the glass. Therefore the 
refrigerant cause is sufficient to cool the external surface of the 
glass more than it is heated by conduction.” And if we ask of Dr. 
Wells what is this refrigerating cause ? he will answer, I conceive 
the radiation of the glass. 
I shall think myself happy if I have succeeded in making my 
explanation agree with that of a philosopher who unites the powers 
of reasoning with the qualities which distinguish the good observer. 
2. But I shall carry my pretensions“a'little further. Ihave ob- 
-served that several philosophers of the first rank appear partial to a 
system opposite to that of emission. They are disposed to consider 
the action of caloric (and of light) as the result of undulation ; 
and some of them, considering cold as equally positive with heat, 
admit frigorific rays. 'My reasons for being of a different opinion 
are founded chiefly on general physics ; for in other respects the 
phenomena of radiation and the moveable equilibrium may be re- 
conciled to their conceptions. ‘This has some resemblance to the 
dispute about phlogiston. A great number of phenomena may be 
explained according to either system, merely by a change of expres- 
sion. Iam not, however, nor ought I to be, indifferent respecting 
the choice. The emission of heat, I repeat, appears to me clearly 
indicated, and alone conformable to the general principles of 
physics. But we may, I conceive, admit the opposite language : 
and if we do so, probably the explanations of Dr. Young would 
agree with mine. If this be the case (I cannot decide from the 
short extracts in the letter which serves me as a guide), nothing fur- 
ther remains than to complete it by adding the explanation which 
your discovery furnishes. 
3. Permit me here to add a remark which supports my explana- 
tion. I have shown (in a memoir inserted in the Journal de Phy- 
sique for February, 1811, vol. xxii. p. 181) that glass in all pro- 
bability transmits heat in two ways, immediately and mediately. 
The caloric transmitted immediately, like light, contributes ob- 
viously to the evaporation of the water which has a tendency to be 
deposited on the external surface of the glass. This caloric is en- 
p- 249 of my Calor. Rayonn, as denying that proposition. There must be a mis~ 
take in the quotation, Nor am I aware of having any where denied a fact with 
which I was not concerned, 
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