on the Existence of a Fourth Imponderable. 109 



beam of light, coming from an Argand lamp, burning steadily, 

 to move the index through one division. 



(3.) For the tithonic ray of the unobstructed beam 3P. 



The trough was then interposed in the column of light, and 

 the number of seconds required to make the index move 

 through one division determined. 



(4.) For the tithonic ray in the absorbed beam ... 65^. 



But as the lamp might have varied in intensity, or the ti- 

 thonometer in sensitiveness, the first observation was repeated; 

 it gave 



(5.) F'or the tithonic ray of the unobstructed beam 3P. 



This process of repetition was uniformly resorted to, and 

 the mean of the two taken. It may be proper to remark, that 

 there was rarely any perceptible difference. 



It follows, that a ray which could effect the union of a given 

 quantity of chlorine and hydrogen in 31 seconds, required 

 after passing through the naphtha Q5 seconds. 



Calculating on these principles how many of the tithonic 

 rays passed through the naphtha, as before, we find, 



(6.) For the titljenic rays in the unobstructed beam 100. 

 (7.) For the tithonic rays in the absorbed beam ... 48. 



Now, comparing this result (6.) and (7.) with the result (1.) 

 and (2.) for light, we find that the absorptive action of naphtha 

 of a slightly yellow tint is very much greater for the tithonic 

 rays than for the luminous rays. 



Consequently it follows, that it is not the light comprehended 

 between the extreme blue and extreme violet rays which brings 

 about the union of chlorine and hydrogen, but another and 

 invisible class of rays, which is absorbed by naphtha under a 

 different law for that for its action on light. 



The foregoing example shows the mode by which I have 

 obtained the results of the following table : — 

 Table showing the absorptive action of certain media for the 

 luminous and tithonic rays. 



