94 J. W. Draper—Distribution of 
respecting the combination of those substances advantageously, 
since it gives accurate quantitative measures. 
By referring to my papers in the Philosophical Magazine 
. (Dec., 1843, July, 1844, Nov., 1845, Nov., 1857) it will be found 
that chlorine and hydrogen do not unite in the dark at any ordi- 
nary temperature or in any length of time; but if exposed to a 
feeble radiation such as that of a lamp they are strongly 
ected. The phenomena present two phases: 1st, for a brief 
period there is no recognizable chemical effect, a preliminary 
actinization, or as Professors Bunsen and Roscoe subsequently 
termed it, photo-chemical induction, taking place. It is mani- 
fested by an expansion and contraction of the mixture. 2d, 
the combination of the gases begins, it steadily increases, and 
soon acquires uniformity. In obtaining measures by the use of 
these gases we must, therefore, wait until this preliminary 
actinization is completed. That accomplished, the hydrochloric 
acid arising from the union of the gases is absorbed so quickly, 
that the movements of the index-liquid over the graduated 
scale give trustworthy indications. 
As regards the duration of the effect produced on the gases 
by this preliminary actinization, I found that it continued some 
time—several hours (Phil. Mag., July, 1844). Professors 
Bunsen and Roscoe, however, in their memoir read before the 
Royal Society, state that it is quite transient (Transactions R. 
Soc., 1856). 
This preliminary actinization completed, the quantity of 
hydrochloric acid produced measures the quantity of the acting 
radiation. This I proved by using a gas flame of standa 
height, and a measuring lens consisting of a double convex, 
five inches in diameter, sectors of which could be uncovered by 
the rotation of pasteboard screens upon its center, the quantity 
of hydrochloric acid produced in a given time being propor- 
tional to the area of the sector uncovered. The same was also 
roved by using a standard flame, and exposing the gases dur- 
ing different periods of time. The quantity of hydrochloric 
acid produced is proportional to the time. ie 
he following experiment illustrates the phenomena arising 
during the actinization of a mixture of chlorine and hydrogen, 
and substantiates several of the foregoing statements. : 
iverging rays of a lamp were made parallel by a suit- 
able combination of convex lenses. In the resulting beam a 
_chlor-hydrogen actinometer was placed, there being in front of 
it a metallic screen, so arranged that it could be easily remov 
or replaced, and thus permit the rays of the lamp to fall on 
the actinometer or intercept them. 
On removing the screen and allowing the rays to fall on the 
sensitive mixture in the actinometer, an expansion amounting 
