432 Answer to Prevost’s Queries respecting the [Dec, 
ARTICLE X. 
Answer to Mr. Prevost’s Queries respecting the Explanation of 
Mr. B. Prevost’s Experiments on Dew. By Wm. Charles Wells, 
M.D. F.R.S. 
(To Dr. Thomson.) 
SIR, 
Havine seen in the last number of your Journal an indireet 
application to me by the acute and learned Mr. Prevost, of Geneva, 
T request permission to inform that Gentleman, through the same 
channel, that the explanation which he has given, in his work on 
Radiant Heat, of Mr. Benedict Prevost’s observations on dew, is 
regarded by me as being neither referrible to the whole of them, nor 
altogether satisfactory with respect even to those to which it applies. 
In the first place, he takes no notice whatever of a whole class of 
Mr. B. Prevost’s observations ; those, namely, which relate to what 
happened, when glass vessels partly filled with various substances 
were exposed by him to the influence of the causes of dew. In 
these experiments the lower parts of the vessels remained dry, 
though other parts of them, which were above the level of the con- 
tained substances, were covered with dew. The author adds, that 
the distance between the upper surface of the contained substance, 
and the part of the vessel at which dew began to appear, varied 
according to the nature of the substance; it being greater, for 
example, in a vessel containing mercury, than in another of the 
same size containing water. 
In the second place, Mr. Prevost, of Geneva, suppeses dew to 
form in circumstances, in which, J venture to say, it cannot occur. 
If a thin plate of a bright metal be fastened to a pane of glass in a 
window of a room, the air in which is warmer than that without, 
dew, according to his representation, will be deposited on the out- 
side of this piece of glass, as the metal covering its inside is a 
screen against the heat, which is radiated towards it by the walls and 
contents of the warm chamber. Now it is manifest, that the utmost 
effect which can be preduced in this way will not occasion the out- 
side of the glass to be as cold as the external atmosphere ; for the 
metal will admit into itself some part, however small, of the heat 
which is radiated to it, and will communicate this to the glass, along 
with that. which it acquires at the same time, by conduction, from 
the contiguous warm air. But, if the outside of the glass be warmer 
than the air, dew will not form upon it ; since, according to my 
experience, bodies will not receive dew unless they be colder than 
the air. 
‘EF think, Sir, that I need say nothing more in justification of the 
intention, which | formerly entertained, of offering an explanation 
of Mr. B. Prevgst’s observations on dew, though, one had already 
been given of a part of them by Mr. Prevost, of Geneva. Much 
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