614 PEOPESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEI^nCAL EESEAECHES. 
forces, in the second it suffers a transformation into mechanical energy, effecting the 
combination of the chlorine and hydrogen. It would seem as if the simplest mode of 
arriving at a conclusion regarding these two alternatives, were to determine the coeffi- 
cient of extinction for chlorine and hydrogen, by interposition of the transparent cylinders 
filled with the mixture : this plan, however, is accompanied by the unavoidable error 
arising from the impossibility of ascertaining when the mixture under examination has 
arrived at the point of maximum photo-chemical sensibility. We have therefore pre- 
ferred to deduce the coefficient of extinction for chlorine and hydrogen dii-ectly from 
the chemical actions effected by a constant amount of light in columns of sensitive gas 
of various length. For this purpose we have employed the apparatus (Plate XXVII.), 
which mainly differs from the instrument described in our fii’st communication 
in the arrangement of the insolation-vessel. The insolation-vessel ty consists of a 
glass tube with as even a bore as possible, 250 milhms. in length and 15 miUims. in 
breadth, of which the end f, exposed to the light, is a plane surface. The mixture of 
chlorine and hydrogen, evolved in the manner described in our first communication, 
enters the insolation-vessel through the tube p melted on to the side of the vessel, and 
ground air-tight into the glass stopcock h ; the gas issues through a tube similarly fused 
into the other end of the insolation-vessel, and thence passes into the observation-tube K. 
In the inside of the insolation- vessel is placed a semicircular diaphragm, d, of black 
glass, which is melted on to the glass rod ^l, moveable in the caoutchouc cap c. By 
moving this glass rod, on which a scale in millimetres can be etched, backwards or for- 
wards through the caoutchouc cap, the column of gas id^ exposed to the hght, can be 
shortened and lengthened at pleasure. The tube ig contains so much water, that when 
the diaphragm d is turned round to the right, at an angle of 90° from that represented 
in the figure, the surface of the water just touches the straight edge of the diaplmagm. 
In order to prevent entrance and collection of water in the tube g during the alteration 
of the diaphragm, the rod must never be moved, unless the ciu’ved side of the dia- 
phragm is undermost (as in the position represented in the figui’e), when the water is 
not carried back, and no inconvenience is experienced in altering the length of the 
column of gas. In spite of the number of ground-glass joints, and the consequent fra- 
gile nature of the apparatus, all these necessary operations may be executed 's’sithout 
danger, if the insolation-vessel is fii’mly fixed in one position by pressing it against the 
support at g with the finger and thumb. When the apparatus is to be filled \^'ith gas, 
all the precautions must be taken which are described in our first communication, and 
a thick piece of wood is placed under the board at A, so that the whole arrangement, 
including the evolution-tube, not represented in the figure, is placed in an oblique 
position, and all the water collects at the lower end, of the insolation-vessel, and is 
completely satm'ated with the gas which bubbles through from the tube ip. As soon as 
the gas has reached the maximum degree of photo-chemical sensibihty (determined by 
experiments made from time to time with the standard flame), the apparatus is again 
placed in a horizontal position by means of the screws B, B, B. 
