80 
MR. P. V. BEVAN OA THE COMBINATION OF HYDROGEN 
capillary had to he carefully made, as slight irregularities occasion the stoppage of the 
tube by a drop of water at one end or a bubble of gas at the other. 
The insolation vessel from the tap (c) to the beginning of the capillary tube was 
enclosed in a wooden box packed witli cotton waste to keep the bulb from light and 
fiom temperature changes. In the side of the box was a tube of diameter about 
two-thirds that of the bulb, so that light could be thrown on the upper part of the 
bulb, the part with water in it being blackened and remaining in the dark. On this 
tube was fitted a Thornton-Pickard photographic shutter, so that an exposure of the 
bulb to the light of a given time could be made. With some practice, by counting 
the ticks of a watch, exposures of one-fifth of a second and upwards could be given 
with fair accuracy. lor the illumination of the bulb a Welsbach incandescent burner 
was usually used as a source of light. The lamji was enclosed in a box with a hole 
in it placed opposite to the shutter. The light was passed through a water trough 
to cut oft’ heat rays, and through a convex lens to be sent down the tube to the 
insolation vessel in as neaily as possible parallel rays. Other screens were arranged 
to cut oft extraneous light and accidental heating effects from the burner. With 
these precautions, working in a cellar, the temperature of the insolation vessel could 
lie kept constant for a considerable time. Fig- 3 shows a diagrammatic representation 
of the arrangements for illumination. 
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Fi^-. .3. 
It is necessary that the electrolytic gas should be continuously passed through the 
apparatus for nine or ten days, to thoroughly drive out all air, and to saturate the 
watei in the ^allous parts of the apparatus witli chloi'ine. It was found necessarv 
to run a certain quantity of gas through the apparatus before each experiment, 
especially if the gases had been standing for some time, as slight changes in 
