232 Royal Society. 



the coefficient just found must remain unaltered when the chlorine 

 and hydrogen mixture is employed ; if on the contrary light is not 

 only lost by the optical extinction, b)it an amount of light vanishes 

 proportional to tlic chemical action, experiment must give a larger 

 value for tiie coefficient. 



In order to determine this important question we employed au 

 apparatus (fully described in the Paper), by means of which we 

 could expose columns of the sensitive gas of various lengths, to 

 a constant source of light. By determining the amount of action 

 effected in these columns of varying length, we are able to obtain the 

 value of the coefficient of extinction for the sensitive mixture. A 

 series of experiments showed that when the light had passed through 

 234 millimetres of the sensitive mixture of chlorine and hydrogen at 

 0° and 076, it was reduced to -^ of its original intensity. If, 

 instead of hydrogen, we had diluted the chlorine with some other 

 transparent but chemically inactive gas, the depth to which the rays 

 must penetrate in order to be reduced to -^ is according to the ex- 

 periments with pure chlorine, 34G millimetres. Hence it is seen 

 that for a given amount of chemical action effected in the mixture of 

 chlorine and hydrogen, an equivalent quantity of light is absorbed. 

 For we find that in the case of the standard chlorine and hydrogen 

 mixture, where, together with the optical absorption, a chemical action 

 of the hght occurs, the value of the coefficient of extinction is 

 0-00-127 r whereas in the chlorine mixture of similar dilution, where 

 the chemical action was absent, the coefficient is found to be 0-00289, 

 or very much smaller. 



It appeared of great interest to repeat these experiments with rays 

 from other luminous sources. For this purpose we employed the 

 diffuse light of morning reflected from the zenith of a cloudless 

 sky. The experiments gave a value of g of 4.5-6 mm. for chlorine. 

 That is, diffuse morning light was reduced to y\, by passing through 

 45-6 millimetres of chlorine. A series of experiments made with 

 our apparatus and the standard mixture gave a depth of 73-5 milli- 

 metres of chlorine and hydrogen before the light was reduced to -jL. 

 From this it is seen that morning zenith light is much more easily 

 absorbed by chlorine than lamp-light. Hence it was conjectured 

 that the diffuse solar light might differ in its properties in this respect 

 with the time of day or year. Experiment fully confirmed this sup- 

 position. Observations made with evening light proved that a depth 

 of 19-7 mm. of chlorine was sufficient to reduce the light to ^ of 

 its original amount, and that 57*4 mm. of the standard mixture was 

 required to effect the same end . 



The conclusion which we draw from all our observations is, that 

 the coefficients of extinction of pure chlorine for chemical rays from 

 various sources of light are very different. The depth to which 

 such light must penetrate chlorine at 0° and 0-76, in order to be 

 reduced to -^ of its original intensity, is — 



(1 .) For a flame of coal-gas 1 73-3 mm. 



(2.) Reflected zenith light, morning 45-6 mm. 



(3.^ Reflected zenith light, evening 19-7 mm. 



