Ultra-violet Light in different Gases. 513 



of soda-lime in an iron retort, the mixture having been previously 

 thoroughly dried. The gas was purified by passing it through 

 strong sulphuric acid and drying tubes containing pumice stone 

 moistened with strong sulphuric acid. 



Mean value of current when vessel filled with air = 98*8 

 » „ „ „ „ CH 4 = 841 



giving an absorption of 14'9 °/ in passing through 10 cms. of 

 marsh gas at atmospheric pressure. 



The same experiments, when repeated, after the whole appara- 

 tus had been thoroughly cleansed, and all the drying materials 

 renewed, gave an absorption of 16'3°/ which, considering the 

 difficulties of obtaining the gas pure, is in fairly close agreement 

 with the value previously determined, so that we may take the 

 mean value 15'6 / o as being approximately correct for the ab- 

 sorption. 



Ethylene. 



The ethylene was prepared by the usual method from absolute 

 alcohol and strong sulphuric acid, and was purified by passing- 

 it through a spiral tube immersed in a freezing mixture so as 

 to condense any ether or acetone which might be carried over, 

 through concentrated sulphuric acid, over potassium hydroxide 

 and finally through calcium chloride drying tubes. 



Mean value of current when vessel filled with air = 125*4 



giving an absorption of 22'3 °/ in the intensity of the light in 

 passing through a length of 10 cms. of gas. 



The experiments were repeated on another occasion when 

 an identical value for the coefficient of absorption was obtained. 



Acetylene. 



The acetylene was prepared by the action of water on calcium 

 carbide, and was purified by passing it through a silver nitrate 

 solution (to remove any phosphoretted hydrogen). The absorption 

 of ultra-violet light proved to be so great that practically no light 

 at all got through the vessel when it was filled with acetylene at 

 atmospheric pressure, the mean current in this case being only 04 

 arbitrary units as against IKS similar units when it was filled 

 with air. 



VOL. XII. PT. vi. 34 



