82 Mr Blackadder on Circumstances connected with the 
metal, not perfectly continuous, and much thinner than itself, it 
must greatly influence the latter, both in regard to the admis- 
sion and discharge of heat, and that more especially in the cir- 
cumstances necessarily connected with the experiments in ques- 
tion. Experiments made with gilt or silvered paper, therefore, 
cannot, with any pretensions to accuracy, be brought forward, 
as if they were equivalent to experiments made with thin sheets 
of polished metals, — and any reasoning that may have been 
built upon them under such an impression, must go for nothing. 
I may observe, that when paper, to which a thin layer of 
gold, silver, or other metal has been made to adhere, is thorough- 
ly embued with varnish, the phenomena presented by the con- 
densation of vapour are obviously modified ; but still, as we had 
reason to anticipate, they are not the same as when a thin sheet 
of metal has alone been employed *. 
2. For the purpose of ascertaining the degree of cold sup- 
posed to be produced by the radiation of heat, and, on other oc- 
casions, the amount of heat produced by direct solar radiation, 
it has most commonly been the practice to surround the bulb of 
* Hygroscopic substances of an animal or vegetable origin, cannot be entirely 
deprived of moisture, by a degree of heat short of that which is sufficient to pro- 
duce a change in their chemical condition. When, therefore, a hollow ball of 
polished metal or of glass, containing a heated fluid, is observed to cool more 
quickly when covered with muslin, and suspended in the air, than when the balls 
have been left naked, is it sufficiently evident that vaporization has no influence in 
expediting the discharge of heat ? When, again, a heated ball of metal is observed 
to part with its heat more quickly, when its surface has been covered with succes- 
sive layers of gold-beater’s leaf, than when only one layer has been applied, is it 
demonstrable that evaporation is in no degree operative ? Perhaps it is not too 
much to take for granted, that no two hygroscopic substances absorb equal quan- 
tities of moisture in equal times ; and, admitting this to be the case, we may con- 
clude that they also part with moisture with different degrees of facility. May not 
the different degrees of velocity, therefore, with which heat is observed to escape 
from a polished metallic ball, according as its surface is covered with muslin-paper, 
gold-beater’s leaf, glue, with or without pigment, &c., depend on some other cir- 
cumstance than merely a difference in the mechanical form or structure of the sur- 
faces ? It is certain, that hygroscopic substances, when in that state commonly 
considered dry, are still far from being wholly deprived of moisture. If, when the 
atmosphere contains a moderate degree of humidity, the temperature of a hy- 
groscopic substance be raised considerably above that of the air, the substance will 
