258 On Dr. IFelli's Essay on Dew. [April, 



the metal. If this interpretation be admitted, his statement, as far 

 as it reaches, seems to me quite correct, except where it is said, 

 that, when the metal is on the warmer, or inner, side, the humidity 

 is deposited more copiously either on the metal, o?- on the glassy 

 surface opposite to it ; since, according to Mr. P., it may appear 

 on both at the same time. 



My rcmaiks on Dr. Y.'s explanation will chiefly refer to 

 what takes place on the colder or outer side of the window. This 

 learned author has said, that, when the metal is placed upon the 

 warmer or inner side of the glass, the humidity is deposited more 

 copiously on its outer surface, but has not mentioned where the less 

 humidity occurs. He has compared, therefore, a thing that is 

 known with something that is unknown, as far as his account of 

 facts is concerned. This deficiency is to be supplied from Mr. P., 

 who informs us, that the less humidity occurs on glass similarly 

 exposed with the former, but having no metal attached to its inner 

 surface. The appearance, consequently, on the bare glass, from 

 being influenced by fewer circumstances, and being that with which 

 the otlier is compared, is to be regarded as the standard fact. But 

 surely we must be able to explain a standard fact, before we can 

 fully account for any deflection from it. No sufficient reasoi>, 

 therefore, could, in my opinion, have possibly been ofi^isred by Dr. 

 Y. for the greater quantity of moisture on the outer surface of glass, 

 to the inside of which a metal was affixed, while he was ignorant, 

 why moisture appeared in ani/ quantity on bare glass similarly 

 placed with the former, which 1 must conclude he was, from the 

 reason given by him for the formation of dew upon vegetables, the 

 relations of these substances and of glass to that fluid ijcing the 

 same, or nearly the same. 



But it is manifest to me, for another reason, that Dr. Y. could 

 not have given an adequate explanation of the facts related by him 

 from Mr P., which is, that he was unacquainted with certain con- 

 ditions necessary for tlie appearance of those, which occur on the 

 outside of the window, for they are not described by Mr. P., and 

 Dr. Y. seems never to have made any experiments himself upon 

 this subject. The conditions are, 1. The exposure of the window 

 to a considerable extent of the heavens : 'J. Stillness of the air : 

 3". Clearness of the sky. 



Again : — The heat of the air in the room is a given condition ; 

 but whence is the radiant cold, to use an expression rendered neces- 

 sary by Dr. Y.'s mode of accounting for the appearances, which 

 enables the outer surface of the glass to condense the watery vapour 

 of the atmosphere ? It cannot be furnished by the neighbouring 

 air, since this body must be unable to give more than what is pos- 

 sessed by itself, and unless the glass becomes colder than the air, 

 the latter will not deposit moisture on the former. If it be now 

 said, that the outside of the window-glass becomes colder than the 

 air by receiving cold, which is radiated to it by the heavens, to 

 employ still an expression authorised by Dr. Y,. it will be replied, 



