CONDITIONS OF CHEMICAL CHANGE 85 



in the anode limb towards the cathode : if this precaution be 

 not taken, the hydrogen evolved at the cathode is contaminated 

 with a large proportion of chlorine. 



The gases enter the insolation chamber P of the actinometer 

 and after passing through the index tube m escape through the 

 water contained in the reservoir H. The insolation vessel P 

 is immersed in a bath of water provided with a glass window 

 through which it can be illuminated. The water-bath is kept 

 at a constant temperature by means of a delicate thermo- 

 regulator. 



When P is exposed to light, the hydrogen and chlorine it 



lT-X~N 



'hiiIhu^ lm 



contains are converted into hydrogen chloride, which dissolves 

 very rapidly in the water present. The consequent reduction 

 in volume is measured by the movement of the water in the 

 index tube m (to which a scale is attached) towards the insolation 

 vessel. In order to keep the pressure in-// constant, the tube n 

 is connected with a large bottle filled with air which is placed 

 in the same water-bath as the insolation vessel. 



The graduated tube Q is used to add measured quantities 

 of liquid to the insolation vessel P. The liquid is drawn 

 through the tube to the left of the three-way tap e into Q ; and 

 e is then turned so as to communicate with the insolation 

 vessel only and a measured quantity of liquid is forced by 

 pressure exerted through the tap / into the insolation vessel. 



