SCIENCE AND PRACTICE. 235 



neighbourhood of the zero point, is equivalent to the 

 0,003665 part of the volume. 



Therefore, v t being the volume under mean atmospheric 

 pressure, at the temperature t, its volume v at the tem- 

 perature will be 



1 -|- 0,003665 t 



As a numerical example, let the quantity of explosive 

 gas developed in a minute be 25*64 cubic centimeters, 

 measured in the graduated tube, the temperature of the 

 gas 21 C., and the height of the barometer, at the moment, 

 775 -5 mm , we have then 



775-5 x 25-64 centimetres 



760(14-0,003665 x 21) 



for the volume of the explosive gas which would have been 

 observed had the temperature been 0and the barometer 760 mm> 



Faraday has proved that the chemical and magnetical 

 effects of galvanic currents are proportional to their strengths 

 and to the volumes of water decomposed by them in stated 

 times. It is therefore only necessary, in order to compare 

 the intensities of two currents, to compare the respective 

 volumes of gas developed in the same time and under 

 the same conditions. Unfortunately, however, we can only 

 measure in this way the currents in circuits of which the 

 measuring apparatus itself forms part, and as the con- 

 ducting power of water, even when highly acidulated, is 

 very small, the resistance of the voltameter quite overpowers 

 it, when the electro-motive force is feeble, so that no decom- 

 position, or very little, occurs, and no satisfactory result can 

 be obtained. 



15. For this reason we are obliged to have recourse to 

 the deflection of a magnetic needle suspended within the 

 coils of a multiplier ; and when we know what function of 

 its deflections is proportional to the currents producing them, 

 the method affords us the most delicate measure we can desire. 



The object of all measurement is the comparison of some 

 unknown with some known magnitude ; and this known 



