360 PROrESSOE BTJiS’SEN AXD DE. H. E. EOSCOE’S PHOTO-CHE^nCAE EESEAECHES. 
this cell from the circuit, the evolution of gas in the vessel a may be at any moment 
brought to a maximum or minimum amount. The chlorine and hydrogen mixture 
evolved by the current passes from the vessel a, through an accurately ground-glass 
joint rendered perfectly air-tight by a layer of water contained in the cup g, into the 
washing-bulbs w, partly tilled with water, whence it traverses the tube furnished with 
the glass stopcock li. Between this stopcock and the horizontal tube K, is placed the 
vessel in which the gas is exposed to the action of the light. This insolation-vessel 
contains from 2 to 3 cub. cent, of water, and is attached to the tubes at either end by 
air-tight ground-glass joints, so that when the glass stopcock is open, the gas bubbles 
through the water contained in the vessel «, then passes into the tube K and through the 
winter contained in the vessel and finally escapes into the condenser E, in which all the 
chlorine is absorbed by alternate layers of charcoal and slaked lime. "NATien the gas is 
allowed to pass through the whole apparatus, the liquids contained in the vessels a, w. i 
and I become by degrees saturated ; the composition of the free gas undergoes therefore 
a constant alteration, which diminishes in proportion as the liquids approach the point 
of saturation. As soon as this point of saturation is reached, and the equihbiium of 
absorption established, the composition of the free gas remains perfectly constant, and 
is represented by one volume of chlorine to one volume of hydrogen. 
The gas in different parts of the apparatus is subject to different pressures. At the 
surface of the carbon-poles the gas is under a pressure of the columns of liquid in a, w, 
i and I', ^tf, of the columns in w, i and 1] at A to the pressure of i and I ; and lastly, in 
the insolation-vessel and in the observation-tube ss, to the pressure of the column of 
liquid in 1. Any alteration in the pressure at any one of these parts of the apparatus 
causes an alteration in the composition of the free gas, and necessitates a ffesh satura- 
tion, in order to regain the required equilibrium. Hence the pressure must be kept 
constant, not only whilst the observations are being made, as the nature of the appa- 
ratus requires, but also during the time which elapses between each set of observations. 
For this purpose the following arrangement was made : — As soon as the volume of gas 
to be employed for the observations has been shut off by closing the stopcock /?, by which 
the pressure in no part of the apparatus is altered, the current of electricity is weakened 
by reversing the balance-wire of the gyrotrope, and thus the eA olution of gas in the decom- 
posing cell a is rendered very slight. This feeble cuiTent of gas, which merely serves to 
keep the hquids in a and w in a state of equilibrium, cannot escape by the stopcock, but 
finds its way through a small manometer, m, fused on to the tubey'and containing water, 
and thence by means of a caoutchouc connector into the tube Avhich dips under 
Avater in the bottle F. This bottle serves as a regulator of the pressure, Avhich may at 
pleasure be diminished or increased by raising or depressing the tube p in the liquid. 
From this bottle, F, the gas passes into the second charcoal and lime condenser, G. 
In order to protect the gas contained in the insolation-vessel from alterations of A'olume 
ensuing from external heat, either from the source of light or from the body of the 
observer, Ave have employed several arrangements of screens, which we shall more fully 
t 
