"I supposed air spaces were necessary for warmth and dryness," said
Jill.

"So they are. But there are air spaces in a woolen blanket, in a
brickbat and in common mortar, as well as in sawdust, ashes and
powdered charcoal, quite enough to serve as non-conductors of heat and
of moisture too, if properly protected. One of the best and most
available materials at present known for this purpose is 'mineral
wool,' a product of iron 'slag.' If the open spaces between the studs
and rafters of a wooden building (or in a brick building between the
furrings) are filled with this substance, or anything else equally
good, if there is anything else--of course sawdust or other
inflammable material would not answer except for an ice-house or a
water-tank--'fire-bugs' would find it difficult to follow their
profession with any success, and the insurance companies would build
more elegant offices and declare larger dividends than ever before.
Houses might be burned possibly, but the inmates would have ample time
to fold their nightgowns, pack their trunks, take up the carpets and
count the spoons before vacating the premises."

"How much will that sort of stuffing cost?"

"For a wooden dwelling house of medium size a few hundred dollars would
cover the first outlay, and the saving in worry would be worth twice as
much every year."

"Now to consider the relative merits of brick and wood, for I see Jack
is going to sleep again: The chief excellence of wood has already been
mentioned. It is cheap, so cheap that any man who can earn a dollar a
day and live on fifty cents, may at the end of a year, have a house of