MEMOIR XVIII.] THE CHLOR-HYDROGEN PHOTOMETER. 261 



Thus adjusted, everything is ready to obtain measures 

 of any effect, there being two different methods by 

 which this can be done: 1st, by continuous observa- 

 tion ; 2d, by interrupted observation. 



Of the method of continuous observation, This is best 

 described by resorting to an example. Suppose, there- 

 fore, it is required to verify Table L, or, in other words, 

 to prove that the effect on the photometer is propor- 

 tional to its time of exposure. 



Put on the cap of the sentient tube a d, connect the 

 polar wires with p, q, and raise the liquid to zero. 



Place the photometer so that its sentient tube will re- 

 ceive the rays properly. 



At a given instant, marked by a seconds watch, re- 

 move the cap A D, and the index-liquid at once begins 

 to descend. At the end of the first minute read off the 

 division over which it is passing. Suppose it is 7. At 

 the end of the second do the same: it should be 14 ; at 

 the end of the third, 21, etc. This may be done until 

 the fiftieth division is reached, which is the terminus of 

 the scale. 



Recharge the tube by a momentary application of the 

 polar wires; but it is convenient first to remove any 

 excess of hydrochloric -acid gas in the sentient tube by 

 allowing it time for condensation ; or, if that be inadmis- 

 sible, by inclining a little to one side, so as to give an 

 extensive liquid contact. 



Of the method of interrupted observation. It frequent- 

 ly happens that observations cannot be had during a 

 continuous descent, as when changes have to be made in 

 parts of apparatus or arrangements. We have then to 

 resort to interrupted observations. 



This method requires that the gas and liquid should 

 be well adjusted, so that no change can arise in volume 

 when extensive contact is made by inclination. 



