PROFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. G15 
The following precautions must be observed at the commencement of each experi- 
ment. In the first place, the ground-glass joint of the tube^ is rendered perfectly air- 
tight, by slightly pressing the ground edge into its place, and then moving the whole 
vessel at up and down as much as the elasticity of the glass joints allows. In the 
same way, care must be taken that the joint at ^ is also tight ; and in order to preserve 
these joints secure, pieces of caoutchouc tubing cut lengthways and filled with water are 
placed over the two tubes where the joint occurs, and the ground-glass surfaces are thus 
always kept wet. Secondly, the level of water in the tube must be so regulated by 
the screws B B B, at the commencement of each series of observations, that the same 
section throughout the whole length of the tube is filled with gas. Thirdly, the end of 
the insolation-vessel which is exposed to the light must be blackened as far as the upper 
level of water, in order to avoid any alteration in the constitution of the dissolved gases 
from the action of the fight. Fourthly, the insolation- vessel must be placed, before each 
experiment, exactly in a fine with the incident rays which are as nearly as possible 
parallel. Fifthly, the whole mass of gas contained in the tube, both before and behind 
the moveable diaphragm d, must be induced before each series of observations. We 
have always efiected this induction by allowing fight from a window opposite the door 
of our darkened room to fall on the tube, by opening the door previous to beginning an 
experiment, and allowing the action to proceed until it was evident, from the rate of 
motion on the index scale, that the maximum of the induction was attained. Sixthly 
and lastly, in order to eliminate all disturbing infiuence of change of temperature, the 
instrument was covered, during the experiment, with a tin case C, in which slits were 
cut, as seen in the figure, so that the case could fit closely over all parts of the insola- 
tion-vessel. 
From the following considerations, it is seen that in order to obtain the value of a„ 
the coetficient of extinction in the chlorine-hydrogen mixture, a determination of the 
chemical actions W and W, , in two columns of gas of difierent lengths h and A, , is 
sufficient. Let lo represent the intensity of the fight on entrance into the gaseous 
mixture, I the intensity remaining after it has passed through a length z of gas, we 
have then, according to the former equation, I=IoI0““‘®, in which a, signifies the 
coefficient of extinction of the um’educed chlorine mixture. If, also, wdz represents 
the chemical action effected in the unit of time in a film of gas of the thickness dz, 
we have w=NI, where N signifies a constant, and therefore w=NIo 10““*’'. If h is the 
whole depth of the insolated column of gas, and W the total action occurring therein, 
we have 
or more simply. 
W=- 
Nip 
log nat 10 
W=MIo(l-I0-“'"). 
For W, and A, we get, similarly, 
W,=MIo(l-10-“>*‘); 
4: L 2 
