increase of amplitude or width of swing, and not to the introduction
of quicker periods into the radiation.

The part played by aqueous vapour in the economy of nature is far more
wonderful than has been hitherto supposed. To nourish the vegetation
of the earth the actinic and luminous rays of the sun must penetrate
our atmosphere; and to such rays aqueous vapour is eminently
transparent. The violet and the ultra-violet rays pass through it
with freedom. To protect vegetation from destructive chills the
terrestrial rays must be checked in their transit towards stellar
space; and this is accomplished by the aqueous vapour diffused through
the air. This substance is the great moderator of the earth's
temperature, bringing its extremes into proximity, and obviating
contrasts between day and night which would render life insupportable.
But we can advance beyond this general statement, now that we know the
radiation from aqueous vapour is intercepted, in a special degree, by
water, and, reciprocally, the radiation from water by aqueous vapour;
for it follows from this that the very act of nocturnal refrigeration
which produces the condensation of aqueous vapour at the surface of
the earth--giving, as it were, a varnish of water to that
surface--imparts to terrestrial radiation that particular character
which disqualifies it from passing through the earth's atmosphere and
losing itself in space.

And here we come to a question in molecular physics which at the
present moment occupies attention. By allowing the violet and
ultra-violet rays of the spectrum to fall upon sulphate of quinine and
other substances Professor Stokes has changed the periods of those
rays. Attempts have been made to produce a similar result at the
other end of the spectrum--to convert the ultra-red periods into
periods competent to excite vision--but hitherto without success. Such
a change of period, I agree with Dr. Miller in believing, occurs when
the limelight is produced by an oxy-hydrogen flame. In this common
experiment there is an actual breaking up of long periods into short
ones--a true rendering of unvisual periods visual. The change of
refrangibility here effected differs from that of Professor Stokes;
firstly, by its being in the opposite direction--that is, from a lower
refrangibility to a higher; and, secondly, in the circumstance that
the lime is heated by the collision of the molecules of aqueous
vapour, before their heat has