through Oxygen and Hydrogen at Low Pressures. O24 
Preliminary experiments seemed to show that the fall of 
pressure is for a given pressure proportional to the quantity 
of electricity passed through hydrogen and oxygen, in other 
words that the chemical eftect depends on the product of the 
current by the time, when the pressure is constant ; but it is 
different for different pressures. This being so, the following 
methods were adopted for all the experiments the results of 
which are given. 
Hydrogen and oxygen were prepared in the equivalent 
proportions of two volumes to one. This was done by passing 
a current through a solution of caustic potash contained in 
a U-tube, one end of which was exposed to atmospheric 
pressure, for a certain small time, and then reversing the 
current and passing it through the solution for the same 
time as before. The mixed gases were passed through drying- 
tubes into the apparatus, and when the pressure had been 
reduced to a suitable small pressure within the range of the 
McLeod gauge, the stopcock 8 (fig. 2) was turned. The 
pressure was then carefully read. Let this be pp in mm. 
A current attended by a glow was then passed through the 
gas for several seconds, usually 30 or 60. This current was 
nearly always remarkably steady, and so the quantity passed 
through was accurately known. Let this be AQ coulombs. 
The pressure was again carefully measured. Let it be 
p, mm. Let Ap represent the fall of pressure in millimetres, 
and let p be the mean pressure, during the passage of the 
electricity. 
Then hp =p Th 
pa PEP 
ree 
It was found that p, could be read immediately after the 
current was stopped, for only a small additional fall of 
pressure was observed by reading the pressure again after a 
long interval. In this way, corresponding to the mean 
pressure p, the value of Ap/AQ was obtained. 
In the next experiment p, takes the place of py in the 
previous one, and the same course is followed. This process 
could be continued until nearly all the hydrogen and oxygen 
had disappeared. There was therefore no doubt that the 
effects observed were wholly or chiefly due to the formation 
of water-vapour, and not to the formation of ozone or to the 
disappearance of oxygen at the electrodes. 
When the pressure was too low to permit accurate ob- 
servations, the series of experiments was completed. Another 
series began with the introduction of fresh gases prepared as 
before, and it continued as before until the gas was more or 
