210 ABSORPTIVE ACTION OF MEDIA. 



rect proportion to the quantity of light ; if, for example, the ray lost one half of its hlue 

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



964. We require, therefore, two observations : first, with the photometer, to ascer- 

 tain how much of the light escapes the absorptive 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. 



965. 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 de- 

 scribed, 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 inter 

 posing the trough between one of the lamps and the end of the photometer. On look- 

 ing through the tube, a great diminution of the intensity of the light on the correspond- 

 ing 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 repre- 

 sented 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 beam of light, coming from an Argand lamp, burn- 

 ing steadily, to move the index through one division. 



(3.) For the tithonic ray of the unobstructed beam 31'. 



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 tithonometer in sensitiveness, 

 the first operation was repeated ; it gave, 



(5.) For the tithonic ray of the unobstructed beam 31 s . 



This process of repetition was uniformly resorted to, and the mean of the two ta- 

 ken. It may be proper to remark, that there was rarely any perceptible difference. 



966. 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, 65 seconds. 



Calculating on these principles how many of the tithonic rays passed through the 

 naphtha as before, we find, 



(6.) For the tithonic 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 



