212 Professor Hoscoe, [March 2, 



3. The quantity of hydrochloric acid thus formed is directly pro- 



portional to the intensity of the incident light, and serves, 

 therefore, as a measure of the chemical action produced. 



4. The chemical photometer is an instrument, by help of which 



the quantity of hydrochloric acid thus formed, can be 

 accurately measured. 



The chemical photometer consists essentially of three parts ; namely, 

 first, the apparatus in which the sensitive gas is generated ; secondly, 

 the apparatus in which the gas is exposed to the light ; and thirdly, 

 the apparatus in which the volume of hydrochloric acid produced in a 

 given time is read off. 



When very numerous precautions in the management of the photo- 

 meter are taken, it proves a most sensitive and reliable instrument. 

 Having thus obtained an instrument by which the chemical action of 

 light can be accurately measured, it only remains to graduate it. For 

 this purpose we require a standard of light, from which the determina- 

 tion is to proceed. For this comparative measurement, the possession 

 of a constant source of light is the first essential. This is obtained as 

 follows : — 



1. A flame of pure carbonic oxide gas, burning in the air and 



issuing from an opening of given size at a given rate, is em- 

 ployed as the standard flame. 



2. The unit amount of chemical action, is that effected by such a 



flame upon the sensitive mixture of chlorine and hydrogen 

 during one minute, at the distance of one metre. 



3. The quantity of chemically active light producing this action 



is called one chemical unit of light ; and ten thousand of 

 such units one chemical degree of light. 



4. The chemical photometer is graduated by observing how 



many of these chemical units of light correspond to one 

 division on the scale of the instrument. 



As an illustration of the mode in which this measurement of the 

 chemical action of light is employed, the speaker described the method 

 by which the chemical action produced by the direct solar rays lias 

 been determined.* For this purpose, it was necessary to admit a very 

 small, but a known, portion of direct sun-light into the dark room 

 in which the instrument was placed, and to allow the insolation vessel 

 to be bathed in the pencil of rays thus admitted. By help of Sil- 

 bermann's heliostate, the sun's image was reflected during the whole 

 day upon one spot, a small opening of known size, in the window 

 shutter of a dark room. The fraction of the total sun's rays thus 

 admitted and allowed to fall upon the chemical photometer can be 

 calculated, and the action thus effected, observed ; heiice the amount 



♦ The full memoir on this subject is to be found in Poggendcrff's Annal. Bd. 

 cviii. p. 193. In Abstract, Proceedings Royal Society, Vol. x. p. 39, 1859. 

 Photochemical Researches, Part 4, by R. Bunsen and H. E. Roscoe. 



