234 THE ELECTRIC TELEGRAPH. 



its passage the particles of water are decomposed and their 

 constituents given off. Of these the bulk of hydrogen gas 

 given off at the negative electrode, in a certain time, is 

 double the volume of the oxygen given off by the positive. 

 The bubbles rise to the surface of the acidulated water, mix, 

 and form an explosive gas, and at the same time an equal 

 volume is forced out of the bottom of the S-shaped tube, 

 and rises up into the graduated measure (Fig. 121). 



In calculating the intensity of the current from the 

 volumes of water decomposed in given times, it is necessary 

 to make allowance for temperature and barometric pressure. 

 The common air is first expelled from the apparatus by 

 letting the gas bubble out a few minutes before putting 

 the graduated measure over the tube. The latter should 

 have as small a bore as possible, without having too much 

 resistance. 



According to Marriotte's law, the volume of a confined 

 gas is reciprocally proportional to the pressure upon it, 

 and therefore to its tension. Thus, if under the mean 

 pressure of the atmosphere 760 millimetres of mercury 

 the volume of the gas is v, under a pressure of two such 



atmospheres it would be only half as much, or . More 



generally, if h express the height of the barometer column 

 in millimetres, when the measurement of the volume 

 v h is made, and v t the volume of the same gas correspond- 

 ing to the mean pressure, 760 mm , the temperature being 

 constant, 



or, 



Physicists are generally agreed that the expansions of 

 all the gases are very nearly, if not absolutely, equal, with 

 equal increments of temperature ; in other words, that one 

 co-efficient of expansion is common to all, and that this 

 co-efficient for every degree of the centigrade scale, in the 



