PARTICULAR PHENOMENA EXHIBITED BY CHLORINE AND HYDROGEN. 79 



297. From this point our inquiries naturally branch in two directions. First, The 

 consideration of what happens to the substance which is thus in the act of being chan- 

 ged or tithonized. Second, What happens to the ray in undergoing its converse change, 

 or being detithonized. These I shall discuss in succession. 



298. Phenomena of the Tithonization of Chlorine and Hydrogen. The tithonome- 

 ter enables us to ascertain the leading phenomena in a very satisfactory manner. Its 

 sensitive material being (Ap., 838) the very mixture the properties of which we are 

 considering, to determine the changes, and the rapidity of the changes which take place 

 in that mixture, we are only required to place a tithonometer in the rays of a lamp, 

 remove all external sources of disturbance, such as the action of radiant heat, &c. and 

 note the results. 



299. To carry out these views, I have employed the arrangement represented in Jig. 

 124. A is an argand lamp, which, during the period of observation, burns with uni- 

 formity. In front of this, and at a distance of about two inches, an arrangement of 

 double convex lenses, B, is placed. Beyond, at a distance of 7 inches, is a second 

 convex lens, 3-5 inches focus. Between B and C, a metallic screen, E, is arranged, 

 so that it can be easily removed or replaced, according as it is desired, to allow the 

 rays of the lamp to fall upon the tithonometer, or to cut them off. 



300. The mode of action of the lenses of this arrangement is, to give a uniform 

 disc of light, M, on the sentient tube of the tithonometer. When a piece of white pa- 

 per is placed so as to receive this, in front of the instrument, there is a circular disc, 

 which is equally luminous all over. If this condition be not exactly fulfilled, the lamp 

 or the lenses are to be moved and adjusted until the illumination is sensibly the same. 

 We have then the sentient tube of the tithonometer plunged in an area of light which 

 remains uniform in intensity during the period of our researches. 



301. In this invariable disc of light we have to expose the mixture of chlorine and 

 hydrogen, and mark on the scale of the tithonometer the progress of its union. This 

 we do by noticing how many seconds elapse before the contraction arising from the 

 production of muriatic acid begins, and then how many seconds elapse as the liquid in 

 the index tube passes over each division. 



302. In a particular experiment of this kind, the following numerical determinations 

 were obtained : 



303. On removing the screen E, and allowing the rays to fall on the sensitive mix- 

 ture, first of all an expansion, amounting to half a degree upon the scale, was observed. 

 In sixty seconds this expansion ceased. 



304. The mixture now remained stationary, no apparent change going on in it. 

 At length, after the close of 270 seconds, it was beginning to contract, and muriatic 

 acid to form. 



305. At the end of 45 seconds more, a contraction of half a degree had been accom- 

 plished ; the dimensions of the mixture were therefore now the same as when the ex- 

 periment first began ; this half degree of contraction compensating for the half de- 

 gree of expansion. 



306. The number of seconds which elapsed as the liquid descended over the scale, 



