246 THE CHLOK-HYDKOGEN PHOTOMETER. [MEMOIR XVIII. 



er, they can never be perfectly satisfactory, nor bear any 

 comparison with thermometric results. 



Impressed with the importance of possessing for the 

 study of the properties of the chemical rays some means 

 of accurate measurement, I have resorted in vain to many 

 contrivances; and, after much labor, have obtained at 

 last the instrument which it is the object of this Memoir 

 to describe. 



This photometer consists essentially of a mixture of 

 equal measures of chlorine and hydrogen gases, evolved 

 from and confined by a fluid which absorbs neither. 

 This mixture is kept in a graduated tube, so arranged 

 that the gaseous surface exposed to the rays never varies 

 in extent, notwithstanding the contraction that may be 

 going on in its volume, and the hydrochloric acid result- 

 ing from its union is removed by rapid absorption. 



The theoretical conditions of the instrument are, there- 

 fore, sufficiently simple ; but, when we come to put them 

 into practice, obstacles appearing at first sight insur- 

 mountable are met with. The means of obtaining chlo- 

 rine are all troublesome ; no liquid is known which will 

 perfectly confine it; it is a matter of great difficulty to 

 mix it in the true proportion with hydrogen, and have 

 no excess of either. Nor is it at all an easy affair to 

 obtain pure hydrogen speedily, and both these gases 

 diffuse with rapidity through water into air. 



Without dwelling further on the long catalogue of 

 difficulties thus to be encountered, I shall first give an 

 account of the capabilities of the instrument in the form 

 now described, which will show to what an extent all 

 those difficulties are already overcome. In a course of 

 experiments on the union of chlorine and hydrogen, 

 some of which were read at the last meeting of the Brit- 

 ish Association, I found that the sensitiveness of their 

 mixture had been greatly underrated. The statement 



