J94 THEORETICAL CONDITIONS OF EQUILIBRIUM. 



well, however, to put a piece of platina foil in the bottle in which the supply of chlori- 

 nated muriatic acid is kept : it communicates to it slowly the proper golden tint. 



870. The liquid, being impregnated with chlorine in this manner until it exhales the 

 odour of that gas, is to be transferred to the siphon, a b c, of the tithonometer, and its 

 constitution finally adjusted as hereafter shown. 



871. Thirdly, of the Voltaic Battery. The battery which will be found most appli- 

 cable for these purposes consists of two Grove's cells, the zinc surrounding the platina. 



872. The following are the dimensions of the pairs which I use. The platina plate 

 is half an inch wide and two inches long; it dips into a cylinder of porous biscuit- 

 ware of the same dimensions, which contains nitric acid. Outside this porous 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 receives the dilute sulphuric acid. 



873. The force of this battery is abundantly sufficient both for preparing the fluid 

 originally and for carrying on the tithonometric operations ; it can decompose muriatic 

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



874. Before passing to the mode of using the tithonometer, it is absolutely necessary 

 to understand certain theoretical conditions of its equilibrium ; to these in the next 

 place I shall revert. 



875. Theoretical Conditions of Equilibrium. The tithonometer depends for its sen- 

 sitiveness 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 evolving the gases in exact, or, at all events, 

 known proportions. 



876. 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 subsist, 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 chlo- 

 rine would be dissolved, and an excess of hydrogen remain ; or, if the liquid was over- 

 charged with chlorine, an excess of that gas would accumulate in the sentient tube. 



877. It is absolutely necessary, therefore, that there should be an equilibrium be- 

 tween the gaseous mixture and the confining fluid. 



878. 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 exceedingly sensitive to the light. 



879. There are certain circumstances connected with the constitution and use of 

 the tithonometer which continually tend to change the nature of its liquid. The pla- 

 tina 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 chlorine from the narrow tube ef, for each time that the liquid 

 descends, a volume of air is introduced, which receives a certain amount of chlorine, 



