250 THE CHLOR-HYDROGEN PHOTOMETER. [MEMOIR XVIII. 



the aperture in the plate by the current established in 

 the cylinder. 



As much radiant heat is emitted by this flame, in 

 order to diminish its action and also to increase the 

 chemical effect I adopt the following arrangement: Let 

 A B (Fig. 40) be the lamp ; the rays emitted by it are 

 received on a convex lens, D, four inches and three quar- 

 ters in diameter, that which I use being the large lens of 

 a lucernal microscope. This, placed at a distance of 

 twenty-one inches from the lamp, gives an image of the 

 flame at a distance of thirteen inches, which is received 

 on the sentient tube F ; between it and the lens there is 

 a screen, E. 



Things being thus arranged, and the lamp lighted so 

 as to give a flame about three inches and a half long, 

 the experiments may be proceeded with. It is conven- 

 ient always to work with the flame at a constant height, 

 which may be determined by a mark on the glass cylin- 

 der. At a given instant, by a seconds watch, the screen 

 E is removed, and immediately the liquid begins to de- 

 scend. When the first minute is elapsed the position on 

 the scale is read off and registered ; at the close of the 

 second minute the same is done, and so on with the 

 third, etc. And now, if these numbers be compared, 

 casting aside the first, they will be found equal to one 

 another, as the following table of experiments, made at 

 different times and with different instruments, shows. 



From this it will be perceived that, taking the first 

 experiment as an example, if at the end of 30 s the pho- 

 tometer has moved 7.00, at the end of 60 s it has moved 

 8.00 more; at the end of 90 s , 7.50 more; at the end of 

 120 s , 7.75 more; the numbers set down in the vertical 

 column representing the amount of motion for each 30 s . 

 And, when it is recollected that the readings are all 

 made with the instrument in motion, the differences 



