262 THE CHLOR-HYDKOGEN PHOTOMETER. [MEMOIR XVIII. 



The photometer being charged, place it in a proper 

 position. At a given instant remove its cap, and the 

 liquid descends. When the time marked by a seconds 

 watch has elapsed, drop the cap on the sentient tube. 

 The liquid simultaneously pauses in its descent, but does 

 not entirely stop, for a little uncondensed hydrochloric 

 acid still exists, which is slowly disappearing in the sen- 

 tient tube. Now, incline the instrument for a moment 

 to one side, so that the liquid may run up to the end #, 

 but not so much as to let any gas escape. Restore it to 

 its position and read off on the scale. It is then ready 

 for a second trial. 



The difference between continuous and interrupted ob- 

 servation is this, that in the latter we pause to wash out 

 the hydrochloric acid, and though this is effected by the 

 simplest of all possible methods, continuous observations 

 are always to be preferred when they can be obtained. 



I have extended this Memoir to so great a length that 

 many points on which remarks might have been made 

 must be passed over. It is scarcely necessary to say 

 that the sentient tube must be uniformly and perfectly 

 clean. As a general rule also, the first observation may 

 be cast aside, for reasons which I will presently give. 

 Further, it is to be remarked, as it is an essential prin- 

 ciple, that during different changes of volume of the gas 

 its exposed surface must never vary in extent, the liquid 

 is not to be suffered to rise above the blackened portion 

 at d. If the measures of the different parts be such as 

 have been here given, this cannot take place, for the 

 liquid will fall below the fiftieth division before its 

 other surface rises above d. 



The same original volume of gas in a d will last for 

 a long time, as we keep replenishing it as often as the 

 fiftieth division is reached. 



