298 FRAGMENTS OF SCIENCE. 
C. to 3,259 degrees C., we must conclude that the aug- 
mentation of temperature is applied to an increase of 
amplitude or width of swing, and not to the introduc- 
tion of quicker periods into the radiation. 
The part played by aqueous vapor in the economy of 
nature is far more wonderful than has been hitherto sup- 
posed. To nourish the vegetation of the earth the actinic 
and luminous rays of the sun must penetrate our atmos- 
phere; and to such rays aqueous vapor is eminently trans- 
parent. 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 
toward stellar space; and this is accomplished by the 
aqueous vapor 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 vapor is intercepted, 
in a special degree, by water, and, reciprocally, the radi- 
ation from water by aqueous vapor; for it follows from this 
that the very act of nocturnal refrigeration which produces 
the condensation of aqueous vapor 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, Pro- 
fessor 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 with- 
out success. Such a change of period, I agree with Dr. 
Miller in believing, occurs when the limelight is produced 
by an oxyhydrogen 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 refrangi'bility here effected differs from that of 
Professor Stokes; firstly, by its being in the opposite direc- 
tionthat is, from a lower refrangibility to a higher; and, 
