237 



When the apparatus is freshly filled with the requisite quantity of 

 water, the pure electrolytic gas is allowed to pass through, certain 

 necessary precautions being used, until a constant source of light, 

 such as a coal-gas flame burning under certain circumstances, pro- 

 duces in equal times always the same alteration of volume. This 

 constant maximum action is generally not reached until from eight 

 to ten litres of gas have passed through the instrument, and the 

 saturation has continued for from three to six days. As soon, how- 

 ever, as this maximum is attained, the instrument is ready for use, 

 and preserves this constant sensibility for many weeks, requiring 

 only a short saturation each day in order to fit it for accurate photo- 

 chemical measurements. 



On exposing the gas to the light, the quantity of hydrochloric 

 acid formed does not at once attain the maximum : a certain time 

 often elapses before any alteration of volume is perceptible ; a slight 

 action is, however, soon observed, and this gradually increases until 

 the permanent maximum is reached. This phenomenon, to which 

 we have given the name of photo-chemical induction, is one of great 

 interest and importance, and will be specially studied in our next 

 section. As the maximum action is not attained for several minutes 

 after the first exposure, the observations can only be made use of as 

 soon as the action for several successive minutes has become con- 

 stant. By a combination of several actual observations, the differ- 

 ences between the indications are found to be very slight. 



Found. Mean. Diff. 



13-23 .... 13-36 .... +0-13 

 13-50 .... 13-36 .... -0-14 

 13-35 .... 13-36 .... +0-01 



A spec-ial investigation was conducted for the purpose of deter- 

 mining the effect produced by the heat evolved from the slow com- 

 bustion of the chlorine and hydrogen. Experiment and calculation 

 gave the following as some of the more important results : 



1 . That the heat evolved in the insolation vessel from the com- 

 bustion of the gases exerts no perceptible influence on the indications 

 of the instrument. 



2. That the slight diminution in volume which occurs in the first 

 few seconds after exclusion of light, is entirely owing to a decrease of 

 temperature from a cessation of the combustion. 



