190 



CHLORINE AND HYDROGEN UNITE IN PROPORTION TO THE 



be purchased in the shops. I have, however, contented myself with using a common 

 Argand study-lamp, supporting the perforated plate, d d, at a proper altitude by a retort 

 stand. It will be easily understood that the great increase of light arises from the cir- 

 cumstance that the flame is drawn violently through the aperture in the plate by the 

 current established in the cylinder. 



851. As much radiant heat is emitted by this flame, in order to diminish its action, 

 and also to increase the tithonic effect, 1 adopt the following arrangement: Let A B 

 (fig. 104) be the lamp: the rays emitted by it are received on a convex lens, D, four 

 inches and three quarters in diameter, that which I use being the large lens of a lucer- 

 nal 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 sen- 

 tient tube of the tithonometer, F ; between the tithonometer and the lens there is a 

 screen, E. 



852. 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 convenient 

 always to work with the flame at a constant height, which may be determined by a 

 mark on the glass cylinder. At a given instant, by a seconds watch, the screen E is 

 removed, and immediately the tithonometer begins to descend. When the first minute 

 is elapsed, the position on the scale is read off and registered : at the close of the sec- 

 ond minute the same is done, and so on with the third. &c. And now, if those num- 

 bers 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 instru- 

 ments, shows : 



TABLE I. 



Showing that, when the radiant source is constant, the amount of movement in the tithonometer is directly proportional to the time* 



of exposure. 



From this it will be perceived that, taking the first experiment as an example, if 

 at the end of 30 seconds the tithonometer has moved 7'00, at the end of 60" it has 

 moved 8-00 more ; at the end of 90", 7-50 more ; at the end of 120", 7-75 more : the 

 numbers set down in the vertical column representing the amount of motion for each 

 thirty seconds. And when it is recollected that the readings are all made with the 

 instrument in motion, the differences between the numbers do not greatly exceed the 

 possible errors of observation. It may be remarked that the third and fourth experi- 

 ments were made with a different lamp. 



853. Though a certain amount of radiant heat from a source so highly incandescent 

 as that here used will pass the lens, its effects can never be mistaken for those of the 

 tithonir rays. This is easily understood when we remember that the effect of such 



