PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 61 1 
amount, according to the former considerations, is that which remains after the quan- 
tity lost by passing both the enclosing plates of glass has been subtracted from Ij. 
lo must therefore be multiplied with «, = 0‘811, found from several of the previous expe- 
riments, the difference between the coefficients of reflexion between glass and air and 
glass and chlorine being neglected as imperceptible. The value of which signifies, 
according to formula (2.), the depth of chlorine to which the chemical rays must 
penetrate in order to be reduced to one-tenth of their original amount, has been shown 
by Experiment VII. to vary proportionally with the density of the medium. For chlo- 
rine under O’ 76 pressure and at 0° C., the value of ^ is found from the equation 
l_ ^.P 
«~0-76(l +0-003660 (log flilo — log I)’ 
hence we get from Experiment VIII. 
-=168 millims. 
OL 
,, = 182 milhms. 
,, = 177 millims. 
,, = 160 millims. 
Mean .... 171-7 
This mean value, 171-7, gives a value of 0-00582 as the coefficient of extinction for 
chlorine at 0-76 pressure, and 0° C. for the flame of coal-gas. 
In order to prove experimentally that the coefficient of extinction varies, in the case of 
chlorine, in the inverse proportion of the dilution, another series of determinations was 
made in which the chlorine was mixed with varying quantities of atmospheric air. For 
this purpose the caoutchouc-tubes, connected with the cylinders previously used, filled with 
V volumes of chlorine at f C., and under the pressure P, were secured by screw-clamps, 
and into the end of one caoutchouc joining a fine capillai-y tube was fastened, whilst into 
the other was placed a chloride of calcium drying-tube. On opening the screw-clamps, air 
was forced by the mouth through the drying-tube into the chlorine cylinder, whence the 
gas escaped through the capillary tube into a reversed retort filled with a solution of 
iodide of potassium, and when sufficient chlorine had been expelled the screw-clamps were 
again closed. The volume of chlorine expelled, reduced to 0°C. and 0-76 pressure, was deter- 
mined by volumetric analysis, and this volume subtracted from that originally contained, 
also reduced to 0° C. and 0-76 pressm-e, gave the volume Vj remaining in the cylinder 
mixed with air. If H represent the depth of the cylinder, measured from the inner surface 
of one glass plate to the inner surface of the other, and /q represent the length of the 
H . P 
column of chlorine before displacement, and after displacement, 
V 
A.^= Y H. These various columns of chlorine, A, both of which were expanded to 
the height H in the cylinder, were placed between the source of light and the insolation- 
