MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 245 



MEMOIR XVIII. 



DESCRIPTION OF THE CHLOR-HYDROGEN PHOTOMETER. 



From the American Journal of Science and Art, Vol. XL VI. ; Philosophical Mag- 

 azine, December, 1843. 



CONTENTS : Properties of a mixture of chlorine and hydrogen. It is 

 acted upon by lamplight, an electric spark at a distance, etc. The 

 gases unite in proportion to the amount of light. Mode of measuring 

 out known quantities of radiations. The maximum action is in the 

 indigo space. Construction of the instrument. The gases are evolved 

 by electricity and combined by light. Theoretical conditions of equi- 

 librium. Preliminary adjustment. Method of continuous observa- 

 tion. Method of interrupted observation. Remarkable contraction 

 and expansion. 



I HAVE invented an instrument for measuring the 

 force of the chemical rays found at a maximum in the 

 indigo space, and which from that point gradually fade 

 away to each end of the spectrum. The sensitiveness, 

 speed of action, and exactitude of this instrument will 

 bring it to rank as a means of physical research with the 

 thermo-multiplier of Melloni. 



The methods hitherto available in optics for measuring 

 intensities of light by a relative illumination of spaces 

 or contrast of shadows are admitted to be inexact. The 

 great desideratum in that science is a photometer which 

 can mark down effects by movements over a graduated 

 scale. With those optical contrivances may be classed 

 the methods hitherto adopted for determining the force 

 of the chemical rays by stains on daguerreotype plates 

 or the darkening of sensitive papers. As deductions 

 drawn in this way depend on the opinion of the observ- 



