MEMOIII XVIII.] THE CHLOU-HYDKOGEN PHOTOMETER 263 



The experimenter cannot help remarking that, on sud- 

 denly exposing the sentient tube to a bright light, the 

 liquid for an instant rises on the scale, and on dropping 

 the cap in an instant falls. This important phenomenon, 

 which is strikingly seen under the action of an electric 

 spark, I shall consider hereafter. 



In conclusion, as to comparing the photometric indi- 

 cation at different times, if the gases have the same con- 

 stitution the observations will compare; and if they 

 have not, the value can from time to time be ascertained 

 by exposure to a lamp of constant intensity. To this 

 method 1 commonly resort. 



From the space occupied in this description the reader 

 might be disposed to infer that this photometer is a 

 very complicated instrument and difficult to use. He 

 would form, however, an erroneous opinion. The pre- 

 liminary adjustment can be made in five minutes, and 

 with it an extensive series of measures obtained. These 

 long details have been entered into that the theory of 

 the instrument may be known, and optical artists con- 

 struct it without difficulty. Though surprisingly sensi- 

 tive to the action of the indigo ray, it is as manageable 

 by a careful experimenter as a common differential ther- 

 mometer. 



UNIVERSITY OF NEW YORK, Sept. 26, 1843. 



NOTE. (From Haider's Magazine, No. 328.) Pro- 

 fessors Bunsen and Roscoe, in their photo-chemical re- 

 searches, made at the University of Heidelberg, and 

 communicated to the Royal Society of London, 1856, 

 say : " The first and only attempt which has been 

 made to refer the chemical action of light to a stand- 

 ard measure is to be found in the researches of Draper. 



