Condensation of' Humidity on Solid Surfaces. 241 
may at least go some length in inducing the advocates of that 
theory to reconsider the grounds upon which its exclusive influ- 
ence is supposed to be established. In aiming to do so, at least 
an attempt at brevity is, on the present occasion, indispensably 
requisite. 
Though, in general, polished metals, when exposed after sunset, 
are, cateris paribus, the last and the least dewed of all other bodies, 
they may acquire moisture in three several ways : 1$£, Acting 
mechanically i?n preventing aqueous vapour from being dispersed 
in the air, at a time when the latter is not saturated with mois- 
ture, and when both the air and the metal are of the same tem- 
perature. 2 d, Acting mechanically, in merely receiving or in- 
tercepting particles of condensed vapour in their descent, after 
the air has become super-saturated with moisture, and at a time 
when the temperatures of the metal, and of the contiguous air, 
are equal. 3d, Not acting as a simply mechanical agent, but as 
& cold body attracting moisture from damp air, of a somewhat 
higher temperature. 
Of the first, we have various familiar examples, — thus, if, 
when the weather is both warm and dry, we approach the fin- 
ger to a highly polished metal of the same temperature with the 
air, aqueous vapour is instantly observed to be condensed on the 
metallic surface, — or, if we breathe opposite to, and at some in- 
terval from a metallic or glass mirror, the polished surface is in- 
stantly more or less obscured, though the mirror be of the 
same temperature with the air, and the latter far from a state 
of saturation. The breath is completely saturated with mois- 
ture, and warmer than the air ; but, though we expire with 
the utmost force of the respiratory organs, against the ambient 
air, which has the same temperature as the mirror, we shall 
not be able to discover the slightest obscuration, in the form of 
a haze or fog ; for this only takes place when the temperature 
of the air has been reduced from 50° to 60° below that of the 
human body. Hence, the mirror acts mechanically in prevent- 
ing the diffusion of the aqueous vapour. Pieces of unpolished 
metals, and other rough, solid, and non-absorbent substances, 
produce the same effect ; though, from the optical property 
of their surfaces, the effect is less, if at all discernible. The 
same effect is also produced, by bodies possessed of a hygro- 
