230 Prof. Draper's Description of the Tithonometer. 



adjusted, an entire condensation will take place, the sentient tube 

 ad filling completely. In practice this precision is not however 

 obtained, and if a bubble as large as a peppercorn be left, the 

 operator will be abundantly satisfied with the sensitiveness of his 

 instrument. Commonly, at first, a large residue of hydrogen 

 gas, occupying perhaps an inch or more, will be left. It is to be 

 understood that even this large surplus will disappear in a few 

 hours by absorbing chlorine. But this is not to be waited, for ; as 

 soon as no further rise takes place in a minute or two, the siphon 

 is to be inclined on one side, and the residue turned out into the 

 open leg. 



Now, recurring to what has been said on the equilibrium, it is 

 plain that this excess of hydrogen arises from a want of chlorine 

 in the tithonometric liquid. A proper quantity must therefore 

 be furnished by proceeding as follows. 



The sentient tube being filled with the liquid by inclination, 

 connect the polar wires with p q, as before. These may be call- 

 ed generating wires. Allow the liquid to rise in b c, until the 

 third platina wire z, which may be called the adjusting wire, is 

 covered an eighth of an inch deep. Then remove the negative 

 wire from the cup p into the cup r, and now the conditions for 

 saturating the liquid are complete ; hydrogen escaping away from 

 the surface of the liquid at z, and chlorine continually accumu- 

 lating and dissolving between x and d. This having been car- 

 ried on for a short time, the gas in a d is to be turned out by in- 

 clination and the instrument recharged. That a proper quantity 

 is evolved, is easily ascertained by allowing total condensation to 

 take place, and observing that only a small bubble is left at a. 



It will occasionally happen in this preliminary adjustment, that 

 an excess of chlorine may arise from continuing the process too 

 long. This is easily discovered by its greenish-yellow tint, and 

 is to be removed by inclining the instrument and turning it out. 



Thus adjusted, every thing is ready to obtain measures of any 

 effect, there being two different methods by which this can be 

 done, — 1st, by continuous observation; 2d, by interrupted obser- 

 vation. 



Of the method of continuous observation. — This is best describ- 

 ed by resorting to an example. Suppose, therefore, it is required 

 to verify table I, or, in other words, to prove that the effect on the 

 tithonometer is proportional to its time of exposure. 



