362 PEOFESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHIDnCAL EESEAECHES. 
have been sharpened and hardened by hammering, are then firmly fixed into small holes 
bored in the carbon-poles by a needle, and over the platinum ■wire is placed a thread of 
glass drawn out before the blowpipe. This thread is melted in the blo-wpipe fiame at 
the point at which it touches the carbon, and pressed do'wn over the carbon and 'wire 
so as to form a button of glass, as is seen in fig. 3, Plate X^TII. The whole ■wire and 
glass thread is then slowly draum through the fiame, so that the glass is completely 
melted upon the platinum wire, forming an enamel throughout its whole length, and 
completely protecting it from the action of the chlorine. In order to preseiwe the ends 
of the "wires which are fixed into the carbon, from the action of the acid, it is sufficient 
to impregnate the upper end of the carbons mth a httle white wax. If the precaution 
be adopted of fusing the wires into the upper end of the tube, as is seen in fig. 2, so 
that the necessarily unprotected part of the ■wire does not come in contact "with the 
liquid, no destruction of the electrodes, or of the wires need be feared, even when the 
instrument is in continual use. Another very important precaution which must be 
attended to regarding the electrodes, is that the carbon-poles must never be allowed to 
come in contact "with the free gas, for carbon prepared in the manner described, acts 
upon the mixture of chlorine and hydrogen exactly as spongy platinum acts upon the 
mixture of oxygen and hydrogen. In the course of our experiments we have had 
many opportunities of obser'ring this catalytic action of the carbon, and more than once 
has this action proceeded so rapidly that infiammation of the gaseous mixtui-e has 
occurred, and the apparatus was shattered with a sharp explosion. 
The varying stand of the water in the horizontal tube ss can be conveniently obseiwed 
by the light of a distant candle or lamp, which exerts no perceptible action on the sen- 
sitive gas. f 
The following conditions, necessary in order to obtain exact photo-chemical measure- 
ments with a mixture of chlorine and hydrogen, are completely fulfilled in the apparatus 
just described. 
(1) The gas which is exposed to the action of the light consists of exactly equal 
volumes of chlorine and hydrogen. 
(2) It does not contain any foreign impurities. 
(3) Throughout the whole apparatus it does not come in contact with caoutchouc or 
other organic matter which could alter its composition. 
(4) The change of pressure during the whole series of observations is quite imper- 
ceptible, owing to the volume of water in the "s'essel I being much larger than that in the 
observation-tube ss. 
(5) The statical equilibrium betAveen the absorbed and free gases can be perfectly 
established. 
(6) The layer of water in the insolation-vessel i does not alter its position during the 
experiment, so that the hydrochloric acid formed by the action of the light is always 
absorbed under precisely the same conditions. 
(7) The vessel i is covered outside with black varnish, reaching as high as the level of 
