204 
Relation of M oisture in A ir 
[April, 
5*46 grains of moisture should accompany each cubic foot 
of air in winter. One more step in calculation, and I have 
done. A cubic foot of air at 70° weighs 0*074 lb., and if it 
has been elevated in temperature from 34 0 or 36°, then 
0*635 unit of heat will have been expended in warming the 
air. Again, if the amount of moisture present in this 
cubic foot of air has been increased from 1*44 grains to 
5*46 grains = 4*02 grains, then 0*612 unit of heat will have 
been expended in evaporation of water to supply the moisture 
to vaporise the air. The quantity of heat for warming 
very nearly corresponds with the quantity of heat for 
vaporisation ! The tension of vapour of the external air 
at 34 0 , with 25 0 dew-point, is 0*155 inch of mercury ; that 
at 70°, with 59 0 dew-point, is 0*515 inch of mercury. It 
follows that the pressure tending to diffuse the aqueous 
vapour from the “ hydrated ” room to the external air 
would be 0*365 inch of mercury. The vapour itself, within 
the room at the same time, possesses but i-qSth the tension 
of that of the air present, and hence, as it is endeavouring 
to escape under the pressure of about 25 lbs. per square 
foot (which corresponds to the 0*365 inch of mercury 
column), it becomes evident that it would diffuse through 
cracks, outlets, and even through the passages for supply of 
fresh air, with great rapidity, and that this ratio of satura- 
tion is practically impossible to maintain in any ventilated 
room, or even in any room whatever, as ordinarily enclosed 
and built. 
These estimates and considerations show fairly what 
would result from the attempt to produce an artificial sum- 
mer climate in the houses of our Northern States in winter ; 
but while the futility of the effort in its complete accom- 
plishment is made evident by them, it by no means follows 
that some degree of hydration of warmed air is not the 
requisite of health or of comfort, and the question recurs — • 
What proportionate hydration is needed for these ends ? 
§. It is with some reluctance that I refer to the “ Theory 
of Ventilation.” The past eighty years have witnessed the 
growth of chemical science, which, after passing through 
numerous stages of development, as witnessed by the dif- 
ferent nomenclatures, has finally reached the point that only 
the chemists of continual study can comprehend it, and at 
which point he who knows most about it is the least satis- 
fied with its present condition. 
But thirty or forty years since chemistry was supposed to 
have unlocked the mysteries of matter, and, by the extension 
