when the same vessel was clothed with an equal thickness of raw silk,
water at the same heat and under the same process required 1,264 seconds
before it reached the same decrease of temperature. It was also found
by Sir Humphry Davy that even metals became non-conductors when their
cohesion was destroyed by reducing them to the gaseous state.

It is now generally admitted that, heat being motion, anything, which,
by the cohesion of particles, preserves the continuity of the molecular
chain along which the motion is conveyed, must augment calorific
transmission. On the other hand, when there is a division or
disintegration of atoms, such as exists in sawdust, powdered charcoal,
furs, and felt, the particles composing such bodies are separated from
each other by spaces of air, which the instructed among us well know are
good non-conductors of heat. The motion has, therefore, to pass from
each particle of matter to the air, and again from the air to the
particle adjacent to it. Hence, it will be readily seen, that in
substances composed of separate or divided particles, the thermal
bridge, so to speak, is broken, and the passage of heat is obstructed
by innumerable barriers of confined air. The correctness of
these assumptions has been so abundantly proved by experimental
demonstrations, that every mind that is tolerably informed on the
subject must be relieved of every shade of doubt respecting the greatly
superior non-conducting powers which bodies consisting of separate atoms
possess over those of a solid concrete nature.

The next matter of interest connected with the subject under notice is
its relation to the philosophy of radiation. It has long been known that
the emission of heat from a polished metallic surface is very slight,
but from a surface of porcelain, paper, or charcoal, heat is discharged
profusely. Even many of the best non-conductors are powerful radiators,