DETERMINING THE CONSTANTS OF A VOLTAIC CIRCUIT. 
321 
of which is to be measured. I then adjust the rheostat to bring the needle of the 
galvanometer to a determined point ; this having been noted, I draw the piston back 
through the entire remaining space of one inch, and fill the vacancy with the same 
liquid ; the needle will recede towards zero. I then diminish the resistance of the 
circuit by means of the rheostat and the resistance-coils, until the needle stands at 
the same point that it did when only a quarter of an inch of the liquid column was 
interposed. The reduced length of the wire thus taken out of the circuit will be the 
measure of the resistance of one inch of the liquid. The contrary electro-motive force 
arising from the decomposition of the liquid exists in the circuit during the whole 
process, and therefore does not affect, the result. 
The measure of the resistance of a liquid must be made immediately after it is 
placed in the circuit, because if a current be allowed to act upon it for any length of 
time the nature of the solution changes. In the case of sulphuric acid, for instance, 
the solution is rendered stronger by the decomposition and consequent diminution 
of the water, while, in the case of a metallic salt, not only is the water decomposed, 
but the metal is reduced, and free acid is liberated. Under the conditions, however, 
of my experiments, the chemical action is so slow, and the time of operation is so 
short, that no sensible changes of this kind ta.ke place. 
The resistance of liquids to the transmission of electricity is, no doubt, one of their 
most important physical properties An investigation of all the circumstances which 
occasion changes in this property, especially if accompanied with accurate quanti- 
tative determinations, must necessarily lead to important and hitherto unobserved 
relations. To investigate the changes due to different degrees of dilution and tem- 
perature alone will be a task requiring considerable patience. I have made many 
measures of the specific resistances of different conducting liquids, by the aid of the 
preceding* process, but as they have not been sufficiently numerous to enable any 
general conclusions to be drawn, and as I am at present engaged in a more extensive 
series of experiments in which strict attention will be paid to all the known influen- 
cing circumstances, I shall defer an account of them to a future occasion. 
As bodies differ so much from each other in their specific resistances, and as the 
means of determining this property are so easy, it cannot be doubted that hereafter 
this process will be extensively employed to detect the purity of substances and to 
distinguish them from each other. 
Another method of measuring the resistance of a conducting liquid is the following : 
E 
— Prepare a circuit the electro-motive force and resistance of which is known, ^ = F. 
Interpose the liquid which is to be the subject, of experiment in a small cell with two 
E — 6 
parallel platinum electrodes ; the expression for the circuit will then be jj— — = F' ; 
e being the contrary electro-motive force, and x the resistance of the liquid which is to 
be determined. Having ascertained the value of e by the process described in § 10. sub- 
