Mr Blackadder on the Formation of Dew . Wl 
moisture, is capable of dissolving the particles of a cloud that 
may be suspended in it, if they are both of the same tempera- 
ture, and if there be no extraneous supply of heat ? When the 
rays of the sun impinge on a cloud, it is possible that its par- 
ticles, or some of them, may be enabled to assume the form of 
vapour ; but if, during the sun’ s absence, clouds are dissolved 
by the air, a very remarkable degree of cold must be produced ; 
for it must take as much heat to convert a particle of water into 
vapour when it is suspended in the air, as in any other situa- 
tion. 
When air, capable of absorbing moisture, is in contact with a 
moist surface of the same temperature, and which has no means 
of acquiring heat, except from the air in contact with it, a par- 
ticle of water cannot be converted into vapour without a portion 
of heat being abstracted from the solid body, and whose tempe- 
rature is thereby always sensibly reduced. Neither the air nor 
the particle of moisture could, therefore, supply the requisite 
measure of heat, to convert the latter into vapour. If, then, a 
particle of moisture on the surface of a solid body cannot be con- 
verted into vapour, without heat being abstracted from the solid 
body, how is an aqueous particle suspended in the air to be dis- 
solved ? — from whence is the additional measure of heat to be 
derived ? If, again, dry air can convert the aqueous particles 
suspended in it, and having the same temperature, into vapour, 
how is it that evaporation is often very active, from the sur- 
face of wet solid bodies, at a time when the whole atmosphere 
is obscured by a thick fog ; and that whether the moist body 
be in the shade or otherwise, whether the sun be below or 
above the horizon ? It certainly would be the most obvious 
conclusion, that, in the absence of the sun’s rays, and as long 
as there is no extraneous supply of heat, aqueous particles, when 
suspended in the air, cannot assume the form of a pellucid 
vapour. 
As the cold produced by the refrigerating influence of evapo- 
ration is the cause of the primary formation of dew, a greater in- 
tensity of cold by the same means, causes that condensation and 
crystallisation of vapour termed hoar-frost ; for this is only a 
particular form of dew, and differs from it only inasmuch as 
