218 
Relation of Moisture in Air 
[April, 
Degrees Fahrenheit. 
' ' — N 
— 30 — 20 — 10 0 10 20 32 40 50 60 70 80 90 100 212 
o*oog2 0 0163 0*0270 0*0434 0*0684 0*1078 o*i8ii 0*2476 0*3608 0*5179 0*7329 1*0232 1*4097 1*9179 2*9922 
' 1 • 
Inches of Mercury Column. 
From which it will be seen that the elastic force falls off 
rapidly with the temperature. At any given temperature 
vapour, possessing the above value of elastic force, exists 
in the atmosphere as a part of its volume, whenever there is 
water present to supply it, and such an atmosphere is said to 
be saturated. When there is not sufficient water at hand 
to supply the vapour due to the temperature, what there is 
of vapour in the air is slightly superheated, as it accepts 
the temperature of the air and not that of its tension, but 
this effedt is so small that it may be negledted. The air is 
then said to be dry ; the usual way of estimating such dry- 
ness is by naming the percentage of vapour present to that 
which would fully saturate the air at a given temperature. 
Dry air seeks moisture from any source, and the difference of 
elastic forces, between that of the vapour in the air at anytime, 
and that of saturation of the same air, represents the avidity 
for moisture of such dry air as a species of partial vacuum. 
Now, the quantity of moisture as vapour accompanying 
a given quantity of air is neither increased nor diminished in 
the same air by heating or cooling it (of course, in the 
latter case to the temperature when the air is saturated, 
below which point the moisture condenses). Hence it does 
not matter, so far as moisture is concerned, at what degree 
of heat we introduce the air for warming a room ; if only 
the final temperature of the room be 70° or 8o°, then the 
drying effedt of the air of that room upon the persons 
occupying it, or its furniture, or its materials of construc- 
tion, is that due to air of 70° or 8o°, which air shall contain 
only the normal moisture of supply. In other words, our 
hot air furnaces which supply air at 150° to 200° do not 
(except, perhaps, close to the mouths of supply, before the 
heat is distributed) dry up wood-work or absorb any more 
moisture from the persons occupying a room any more 
than do currents at 8o° or go°, provided the general tempe- 
rature of the room is the same in both cases. But a yet 
more important truism can be stated, which is that the 
drying effedt of air of a ventilated room at 70° or 8o° which 
has received no increase of humidity in the hot air of ven- 
tilation from out of doors — air that has been warmed arti- 
ficially from zero, let us say— the drying effedt of this 
heated air upon a person occupying it is very nearly the 
