658 SCIENCE PROGRESS 



i. 300) noticed that when a mixture of hydrogen and chlorine 

 is exposed to light diminution in volume does not take place 

 immediately. There is no doubt that the cause of this phe- 

 nomenon was an actual delay in the interaction of the gases : 

 but neither of these authors attempted to prove that this was 

 the case by a conclusive experiment. 



The credit of having discovered that when a freshly prepared 

 mixture of the gases is exposed to light the chemical change 

 is more or less delayed belongs to Draper. He had previously 

 constructed a delicate instrument called the tithonometer {Phil. 

 Mag. 1843, 23, 401) for measuring the rate at which hydrogen 

 and chlorine interact and it was with this apparatus that his 

 experiments were performed. Concerning the period of delay 

 he says {Phil. Mag., 1844, 25, 9) : 



"A tithonometer exposed to the daylight is much too power- 

 fully affected to allow the successive stages of change to be 

 distinctly made out ; the preliminary tithonisation is accom- 

 plished so rapidly, that the indications of it are merged and 

 lost in the contraction which instantly follows. It is necessary, 

 therefore, that we should operate with a small lamp flame. 



" To such a flame I exposed a mixture of chlorine and 

 hydrogen and marked the number of seconds which elapsed 

 before contraction arising from the production of muriatic acid 

 took place. The first indication of movement occurred at the 

 close of 600 seconds. 



"The index then moved through the first degree in 480 seconds, 



„ ,, „ second „ 



„ „ „ third „ 



„ „ „ fourth „ 



,, ,, ,, nttn ,, 



,, ,, ,, sixtn ,, 



and continued to move with regularity at the same rate, 

 observations, therefore, prove that a very large amount of 

 radiant matter is absorbed before chemical combination takes 

 place ; and that in the case of chlorine and hydrogen the total 

 action is divisible into two periods : the first during which a 

 simple absorption is taking place without a chemical effect ; the 

 second during which absorption is attended with the production 

 of muriatic acid." Later on he proceeds : " I tithonised the 

 chlorine and hydrogen contained in the instrument and kept it 

 in the dark for ten hours. On exposure to the lamp rays it 

 moved after a few seconds, showing, therefore, that the change 

 which had been impressed on the chlorine was not lost. In the 

 former case 600 seconds had elapsed before any movement was 

 visible." 



