PEOrESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 361 
describe when treating of the testing of the instrument. For the sake of simphcity, in 
figure 2 only a part of these screens has been represented ; namely, the screen L, the 
convex lens M, and the cylinder N, closed by plates of glass and filled with water. In 
all the observations, in addition to these arrangements, the double metallic screen, fig. 4, 
afterwards described, was employed. 
The following is the method adopted for filling the apparatus. The washing-bulbs 
w, the insolation-vessel ^, and the small vessel I, having been supplied with the proper 
amount of water, the evolution-tube a is filled with hydrochloric acid of about sp. gr. 
IT 48, and the neck carefully placed upon the ground end of the tubey. The lower 
end of the tube a dips in mercmy, which acts as a spring, pressing the tube against the 
end of the washing-bulbs the upper part of the tube is surrounded by water, to pre- 
vent the platinum "wires fused into the glass from being heated by the passage of the 
electricity. A current from three or four elements is now led through the acid by 
means of the gyrotrope ; the mixed gases are rapidly evolved, and can pass in two 
directions, according to the depth to which the tube f dips under the level of the water 
in the bottle F. If the lower end of this tube is placed at such a depth in the liquid 
that the sum of the pressures rr, m is larger than the sum of the pressures of the 
columns of liquid at w, ^ and /, the gas will pass through y w, /q ^, I into the con- 
denser E, supposing, of course, that the stopcock h is open. If, on the contrary, the 
tubep be drawn up through the caoutchouc cap t, in which it moves air-tight, until the 
sum of the pressures of the columns rr and vv is less than that of w, i and the gas 
takes the direction m, vv, p, rr, and issues into the condenser G. The tube p is usually 
fixed in such a position that the slightest displacement is sufficient to cause the gas to 
pass in either direction. Diuing the evolution of the gas and saturation of the liquids, 
the equilibrium between the free and absorbed gases and the complete expulsion of the 
air, is effected by allowing the gas to pass through the apparatus in both directions. The 
apparatus being thus filled with the sensitive gas, and the stopcock h shut, the observa- 
tions are made by measuring the diminution of the volume of enclosed gas on the scale 
of the horizontal tube ss of accurately determined capacity. 
It being necessary, as we shall hereafter show, in order completely to saturate the 10 
or 12 grammes of water contained in the Avhole apparatus, to evolve not less than from 
6 to 10 litres of the electrolytic gas, we found that the platinum wires in the evolution- 
tube were so quickly acted upon by the chlorine, that they had to be renewed after two 
or three days’ use. This continual reparation of the apparatus was the more inconve- 
nient, because, in the original saturation of the water, the gas has to be led through the 
apparatus for a period of from three to six days. After many fruitless attempts, we 
have at last overcome this difficulty by the following arrangement of the carbon-poles. 
Small carbon points or plates are cut off from the cylinder of a carbon-zinc element 
which has been long in use, and after they have been boiled in aqua regia, they are 
heated to whiteness in a stream of dry chlorine until all sublimation of chloride of iron 
or other volatile chlorine compounds has completely ceased. Platinum wires, whose ends 
MDCCCLVII. 3 B 
