384 On the Theory of the ~ (Nov. 
Chemistry, and afterwards by M. Prevost, of Geneva, in his Essay 
on Radiant Heat; but, fearing to be very tedious, I have since 
given up the design, I. will say, however, that if to what is now 
generally known on the different modes in which heat is communi- 
cated from one body to another, be added the two following circum- 
stances, that substances become colder than the air before they 
attract dew, and that bright metals when exposed to a clear sky at 
night become colder than the air much less readily than other bo- 
dies, the whole of the appearances observed by M. Prevost may be 
easily accounted for.” 
Dr. Wells having under his inspection my treatise on radiant 
heat, the principles of which he has adopted, could not but have 
read my explanation of the curious phenomena observed by my 
relation, and which this last Gentleman has adopted. Since, then, 
in the above note, the author speaks of the explanation of these phe- 
nomena as still to seek for, it would seem that mine did not appear 
satisfactory to him. It is impossible for me to divine what fault he 
finds with it ; and I mention the subject here in order to be informed 
of this particular, and to draw the attention of philosophers to it. 
What embarrasses me most is, that my explanation is founded on 
the very same principles which the author announces would have 
been his own. Though this subject be known and explained in 
works within the reach of every man of science, I trust I shall be 
excused for dwelling upon ita little here. 
The phenomenon is this. Two masses of air of unequal tempe- 
yatures being separated from each other by a plate of glass, if we 
apply a leaf of metal on one of the: faces of the glass plate, the 
face opposite to this leaf attracts or repels humidity according as the 
metallic leaf is on the hot or the cold side respectively. The ex- 
planation consists in conceiving the naked side of the glass thus 
covered as a vessel (um poéle) destined to be dried, and the metal as 
ascreen: If the screen is put upon the hot side, the vessel cools, 
and humidity accumulates on the naked glass on the cold side. If 
the screen is on the cold side, it prevents the heat from being dissi- 
pated after traversing the glass, and consequently the vessel be- 
comes hot, and the humidity disappears from the. naked glass on the 
hot side.. I found this explanation on the property which metal has 
of reflecting seven or eight times more caloric than glass. (Du 
Calorique Rayonnant, §§ 195, et suiv.) If any attention be paid 
to this subject, it will be seen that the principles of this explanation 
differ in nothing from those. of Dr, Wells. But no doubt he em- 
ploys them in a different way from me; and I am really impatient 
to know in what this difference consists. 
V. I have nothing further to say on the principal object of this 
memoir, which was to reduce to very simple principles the answer 
to some objections against the equilibrium of heat. But I shall take 
an opportunity of making a remark on a set of experiments con- 
neeted with this theory, published by M. Ruhland in the Jour. de 
Phys. for Nov. 1813. A part of these experiments proves directly 
