249 v 
Condensations of Humidity on Solid Surfaces. 
particular!}' if metallic, and a very slight though momentary in- 
crease of its quantity, is readily discernible ; but it is far other- 
wise in the case of rough and unpolished surfaces, whether ve- 
getable or mineral. On these moisture is often deposited, and 
on other occasions evaporated, without our being able to detect 
the change by ocular inspection *. 
5. Glass and lead, bulk for bulk, have nearly the same capacity 
for heat, and which is about one-half that of water. Glass also 
is a bad conductor of heat ; and, among metals, lead is the worst 
conductor, platinum alone, perhaps, excepted. When exposed 
on a clear evening after sunset, glass is sooner dewed than me- 
tals ; and lead is the soonest dewed of metals, at least of all 
those that can be readily procured for experiment. 
This greater facility of being dewed possessed by glass, has 
been attributed to its greater radiating power, and, by others* 
apparently to a greater attraction which glass has for water ; air 
at the same time being understood to be admitted into closer 
physical contact with glass than with polished metals. A know- 
ledge, however, of the small capacity and low conducting power 
of glass, seems to be quite sufficient to enable us to account for 
the difference found to subsist between it and metals, in regard 
to the disposition to acquire moisture, when similarly exposed in 
circumstances favourable to that operation. 
The principle of radiation has also been introduced to explain 
the occasional condensation of moisture on the glass of a cham- 
ber window, as modified by the operation of an inside and an 
outside shutter. But to account satisfactorily on this prin- 
ciple for the peculiar forms which the moisture is sometimes 
found to assume would seem to be rather a difficult task. 
If, on the other hand, we take into consideration the well 
known physical properties and mechanical operation of the wood 
and the glass, in connection with those of the two bodies of air, 
• In the course of these remarks on the relations of polished metals to aqueous 
vapour, experiments made by myself have been little adverted to, in order to pre- 
vent objections as much as possible. I may mention, however, that, in 1812, I 
had several thermometers constructed, the sentient parts of which were polished 
metals, and these plates could be covered with gold and silver leaf, copper-foil, tin- 
foil, and mercury ; and it is by means of such instruments alone, perhaps, that ac» 
curate experiments of this kind can be made, 
