On Light, 21 
imprisoned by the powers of life; and is followed, after some 
time, by the relatively less elastic gases, whose evolution consti- 
tutes putrefaction. 
2. This light is a constituent chemical principle of some bodies, 
particularly of marine fishes, from which it may be separated by 
a peculiar process, retained and rendered permanent for some 
time. A solution of one part of sulphate of magnesia, in eight of 
Ww ater. is the most convenient menstruum for extracting, retaining 
and increasing the britlancy of this light. Sulphate and muriate 
of soda, also answer in a proper state of dilution with water. 
When any of the saline solutions is too concentrated, the light 
disappears, but instantly bursts forth again from ae deck 
ness, by dilution with water. I have frequently made this curious 
experiment with the light procured from whiting. Common 
water, lime-water, fermentec liquors, acids even very dilute, al- 
kaline leys, and many other bodies, permanently extinguish this 
spontaneous light. Boiling water destroys it, but congelation 
merely suspends its exhibition ; for it reappears on liquefaction, 
A gentle heat increases the vividness of the phenomenon, but 
lessens its duration. 
We shall conclude the subject of Light with the following im- 
portant practical fact and practical problem, 
1. Gount Rumford has shown that the quantity of light emitted 
by a given portion of inflammable matter in combustion, is pro- 
portional in some high ratio to the elevation of temperature ; 
and that a lamp having many wicks very near each other, so as 
mutually to increase their heat, burns with infinitely more bril- 
liancy than the Argand’s lamps in common use. 
2. To measure the proportional intensities of two or more 
lights. Place them. a few inches asunder, and at the distance of - 
a few feet or yards from a screen of white paper, or’a white wall. 
On holding a small card, near the wall, two shadows will be pro- 
jected on it, the darker one by the interception of the brighter 
jight, and the lighter shadow by the interception of the duller 
light. Bring the fainter light nearer to the card, or remove the 
brighter one further from it, till both shadows acquire the same 
intensity; which the eye can judge of with great precision, par- 
ticularly from the conterminous shadows atthe angles. Measure 
now the distances of the two lights from the wall or screen, syuare 
them, and you have the ratio of illumination. Thus, if an Argand 
flame, and a caudle, stand at the distances of 10 feet and 4 feet, 
respectively, when their shadows are equally deep, we have 10+ 
and 4%, or 100 and 16, or 6} and 1, for their relative quantities 
of light.* 
II. An- 
