as connected with the Theory of Substitutions. 349 
I filled them with a solution of chlorine in recently boiled water, 
and inverted them in small glass bottles containing the same 
solution, as shown in fig. 1. With these bulbs the following ex- 
periments were made. 
I. An aqueous solution of chlorine does not decompose in the 
dark. 
One of the bulbs was shut up in a dark closet, and kept there 
for a week,—being examined from time to time. No decompo- 
sition was perceptible, for no gas collected in the upper part of 
the bulb. 
II. An aqueous solution of chlorine decomposes in the light. 
One of the bulbs was placed in a beam of the sun reflected into 
the room by a heliostat. For sixteen minutes no change was 
perceptible ; then small bubbles of gas made their appearance ; 
they increased in quantity for a time, but finally the speed of de- 
composition became uniform. On analysis by explosion with 
hydrogen, after washing out any chlorine contained in it, this 
gas was found to contain 97 per cent. of oxygen. 
Ill. The rapidity of this decomposition depends on the quan- 
tity of the rays, and on the temperature. 
In various repetitions of these experiments, on different days, 
I soon convinced myself that the rate of evolution of the oxygen 
depended on the quantity of the rays. Among other proofs I 
may mention this :—After ascertaining the rate of decomposition 
in the reflected beam, if the bulb be set in the direct sunshine, 
the bubbles increase in number; the total quantity of oxygen 
evolved becoming greater in the same space of time, an effect ob- 
viously due to the difference of intensity of the reflected and in- 
cident beams. When a certain point is gained, apparently no 
further increase of effect takes place on increasing the brillianey 
of the light, as I found by employing a convex lens. 
With respect to the influence of temperature. If while one of 
the bulbs is actively evolving gas in the sun-rays, it be warmed 
by the application of a spirit lamp, the amount of gas thrown off 
becomes very much greater. A difference of a few degrees pro- 
duces a striking effect. As an illustration of this, I placed in the 
sunshine two bulbs which were nearly alike, except that one of 
them was painted black with India ink on that portion which 
was farthest from the sun. The rays coming through the trans- 
parent part had access to the solution, and then impinging on the 
Vol. xix, No. 2.—July-Sept. 1845. 45 
