108 Dr. Draper's Further considerations 



Journal (Dec. 1843, p. 402) I have already stated that silent 

 combination would take place under these circumstances, but 

 more recently I have had instruments repeatedly destroyed by 

 explosions resulting in that way. 



It being understood that the indications of the photometer 

 and the tithonometer correspond, that they are affected by rays 

 of the same refrangibility, we are enabled to proceed to the 

 direct solution of the (|uestion and the determination of M. 

 Becquerel's hypothesis. 



A transparent medium, which absorbs to a greater or lesser 

 extent the more refrangible rays, being selected, it is required 

 to determine whether, when a given ray has passed through 

 it, the chemical effect diminishes as the intensity of the more 

 refrangible rays of light diminishes. According to M. Bec- 

 querel, the effect should be in direct proportion to the quan- 

 tity of light; if, for example, the ray lost one half of its blue 

 light, the chemical effect should diminish one half, &c. 



We require, therefore, two observations : first with the pho- 

 tometer, to ascertain how much of the light escapes the ab- 

 sorptive action of the medium under trial ; second, with the 

 tithonometer, to ascertain what quantity of the tithonic rays 

 escapes absorption. If the whole effect is due to light, the two 

 observations should give the same result. 



Before giving the results, which I have obtained in a tabular 

 form, in order that I may be clearly understood, I will give a 

 particular example. I took some naphtha, the colour of which 

 was slightly yellow, and placing it in the glass trough before 

 described, proceeded to determine its relation for the more 

 refrangible rays of light. This was done by adjusting the two 

 lamps of the photometer till they coincided, then interposing 

 the trough between one of the lamps and the end of the pho- 

 tometer. On looking through the tube a great diminution of 

 the intensity of the light on the corresponding semicircle of 

 paper was observed ; the other lamp was now removed until 

 the paper disc was uniformly illuminated; the distance of the 

 two lamps from the centre of the box was now measured, they 

 were respectively twelve and fifteen inches ; but the intensity 

 of the light is proportional to the square of the distance. 



(1.) For the light in the unobstructed beam 225 or 100. 

 (2.) For the light in the absorbed beam ... 144 or 64. 

 Supposing now that the value of the unobstructed beam be 

 represented by 100, and we calculate the amount of light which 

 passes through the naphtha, we find it is represented by 64. 

 Consequently, of every hundred rays of blue light which fell 

 upon this naphtha, sixty-four escaped absorption. 



The naphtha trough was now carried to the tithonometer; 

 first, it was determined how many seconds it took a given 



