THE 

 LONDON, EDINBURGH and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[THIRD SERIES.] 



DECEMBER 1843. 



XLIX. Description of the Tithonometer, an instrument for 

 measuring the Chemical Force of the Indigo- tithonic Hays. 

 By John W. Draper, M.D., Professor of Chemistry in 

 the University of New York*. 



T HAVE invented an instrument for measuring the chemical 

 force of the tithonic rays which are 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 

 M. Melloni. 



The means which have hitherto been found available in op- 

 tics 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 tithonic rays 

 by stains on Daguerrotype plates or the darkening of sensitive 

 papers. As deductions, drawn in this way, depend on the 

 opinion of the observer, they can never be perfectly satisfac- 

 tory, nor bear any comparison with thermometric results. 



Impressed with the importance of possessing for the study 

 of the properties of the tithonic rays some means of accurate 

 measurement, I have resorted in vain to many contrivances ; 

 and, after much labour, have obtained at last the instrument 

 which it is the object of this paper to describe. 



The tithonometer 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 



* Communicated by the Author. 



Phil. Mag. S. 3. Vol. 23. No. 154. Dec. 1843. 2 D 



