make up nearly the same amount at all temperatures; but the amount is
somewhat greater at the higher temperatures. As a damp sponge becomes wet
when subjected to pressure, so warm vapor becomes hot when forced into less
bulk, but in neither case does the quantity of moisture or the quantity of
heat sustain any alteration. Common air becomes so hot by compression that
tinder may be inflamed by it, as is seen in the instrument for producing
instantaneous light by suddenly forcing air into a syringe.

173. _Q._--What law is followed by surcharged steam on the application of
heat?

_A._--The same as that followed by air, in which the increments in volume
are very nearly in the same proportion as the increments in temperature;
and the increment in volume for each degree of increased temperature is
1/490th part of the volume at 32°. A volume of air which, at the
temperature of 32°, occupies 100 cubic feet, will at 212° fill a space of
136.73 cubic feet. The volume which air or steam--out of contact with
water--of a given temperature acquires by being heated to a higher
temperature, the pressure remaining the same, may be found by the following
rule:--To each of the temperatures before and after expansion, add the
constant number 458: divide the greater sum by the less, and multiply the
quotient by the volume at the lower temperature; the product will give the
expanded volume.

174. _Q._--If the relative volumes of steam and water are known, is it
possible to tell the quantity of water which should be supplied to a
boiler, when the quantity of steam expended is specified?

_A._--Yes; at the atmospheric pressure, about a cubic inch of water has to
be supplied to the boiler for every cubic foot of steam abstracted; at