Ultra-violet Light in different Gases. 515 



giving an absorption of 50 °/ in passing through 10 cms. of the 

 gas at 28 mms. pressure equivalent to a length of 0"36 cms. at 

 atmospheric pressure, giving practically the same coefficient of 

 absorption as the previous value (1"92 as against 1'87). 



Sulphuretted Hydrogen. 



The sulphuretted hydrogen was prepared from pure ferrous 

 sulphide and hydrochloric acid, and was well washed by passing 

 it through water and thoroughly dried before allowing it to pass 

 into the apparatus. 



The mean values of the currents obtained on two separate 

 occasions were (1) when the vessel was filled with air 97*8 and 

 100"8, (2) when the vessel was filled with HS. 2 8 - 5 and 10 - 5, giving 

 absorptions of 91 3 °/ and 89 '6 °/ respectively or a mean value of 

 90'5 °/ for the absorption of the light in passing through a 10 cm. 

 length of sulphuretted hydrogen at atmospheric pressure. 



Aqueous Vapour. 



The absorption of ultra-violet light in water vapour was also 

 roughly determined, a modified form of apparatus being employed 

 for this purpose. The best method appeared to be to find the 

 absorption in air saturated with moisture at about 50 — 60° and 

 for this purpose it was necessary to enclose the vessel V in an air 

 jacket heated by a flame underneath. By this means the whole 

 of the vessel V, including the quartz windows, was heated up to a 

 uniform temperature, and no water condensed on the quartz, as 

 was otherwise the case. A few cubic centimetres of water were 

 then introduced into V at a temperature of 17° and the intensity 

 of the toansmitted light measured ; the temperature was now 

 raised until it became constant at 58° when the intensity of the 

 light was again measured. 



Mean value of current when vessel filled with air saturated 

 with moisture at 17° = 217. 



Mean value of current when vessel filled with air saturated 

 with moisture at 58° = 11 6. 



Now the tension of aqueous vapour at 58° C. is 129 mms., while 

 at 17° C. it is only 14 mms. Neglecting the absorption at 17° we 

 find that the absorption = 54 °/ in passing through 6 cms. of 

 aqueous vapour at 129 mms. pressure or an equivalent of 1 cm. at 

 atmospheric pressure. Of course this is only a rough approximation 

 but it is sufficient to show the order of the absorption in aqueous 

 vapour. 



34—2 



