Professor Draper's Description of the Tithonometer. 411 



vessel is the zinc, which is a cylinder one inch diameter, two 

 inches long and two-tenths thick; it is amalgamated. The 

 whole is contained in a cup, two inches in diameter, and two 

 deep, which also receives the dilute sulphuric acid. 



The force of this battery is abundantly sufficient both for 

 preparing the fluid originally and for carrying on the titho- 

 nometric operations ; it can decompose muriatic acid with ra- 

 pidity, and will last with ordinary care for a long time. 



Before passing to the mode of using the tithonometer, it is 

 absolutely necessary to understand certain theoretical condi- 

 tions of its equilibrium ; to these in the next place I shall 

 revert. 



Theoretical Conditions of Equilibrium.— The tithonometer 

 depends for its sensitiveness on the exact proportion of the 

 mixed gases. If either one or the other is in excess a great 

 diminution of delicacy is the result. The comparison of its 

 indications at different times depends on the certainty of evol- 

 ving the gases in exact, or at all events, known proportions. 



Whatever, therefore, affects the constitution of the sentient 

 gases alters at the same time their indications. Between those 

 gases and the fluid which confines them certain relations sub- 

 sist, the nature of which can be easily traced. Thus, if we 

 had equal measures of chlorine and hydrogen, and the liquid 

 not saturated with the former, it would be impossible to keep 

 them without change, for by degrees a portion of chlorine 

 would be dissolved, and an excess of hydrogen remain ; or, 

 if the liquid was overcharged with chlorine, an excess of that 

 gas would accumulate in the sentient tube. 



It is absolutely necessary, therefore, that there should be 

 an equilibrium between the gaseous mixture and the confining 

 fluid. 



As has been said, when muriatic acid is decomposed by a 

 voltaic current, all the chlorine is absorbed by the liquid and 

 accumulates therein, the hydrogen bubbles however as they 

 rise withdraw a certain proportion, and hence pure hydrogen 

 passed up through the tithonometric fluid becomes exceed- 

 ingly sensitive to the light. 



There are certain circumstances connected with the consti- 

 tution and use of the tithonometer which continually tend to 

 change the nature of its liquid. The platina wires immersed 

 in it by slow degrees give rise to a chloride of platina. It is 

 true that this takes place very gradually, and by far the most 

 formidable difficulty arises from a direct exhalation of chlo- 

 rine from the narrow tube ef, for each time that the liquid 

 descends, a volume of air is introduced, which receives a cer- 



