PEOrESSOE BUNSEN AND DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 889 
As an example of such a measurement, we cite the experiment made with the coal-gas 
flame, detailed in the Philosophical Transactions for 1857, page 379. This flame gave 
an action of 14'2 divisions of scale No. 2 in one minute at a distance of 0“'216 at a 
temperature of 22°’7 C., and under 0“-753 barometric pressure. From the capacity 
of the observation-tube, the volume of hydrochloric acid produced by one unit of light 
is found to be 
v=0’001155 cubic centimetre. 
The exposed area of the insolation-vessel was 
5 '= 3-3 square centimetres. 
The thickness of gas (A) through which the light passed is obtained from the thickness 
of the insolation- vessel centimetre, by help of the following equation:-— 
. d{?-p) 
(H-O-OO3660O-76’ 
in which P represents the barometric pressure, and j) the tension of aqueous vapour at 
the temperature t of the experiment. By calculation h is found to be O' 837. Accord- 
ing to our former experiments*, the reciprocal of the coeiflcient of extinction for gas- 
light in pure chlorine and hydi'ogen, at 0° C. and 0'76, is centimetre. As between 
flame and insolation-vessel two plates of mica and a water-screen were placed, the number 
of units of light {1) corresponding to the observed alteration on the scale is obtained by 
multiplying this alteration by the factors ^=2'005 and ?^=0•6612. When this is carried 
out a value ^=18-56 is obtained, which, when substituted in formula (4.), gives 
V=0 '08204 cubic centimetre. 
We thus see that the chemical rays which fall on a surface of one square centimetre 
from the flame of coal-gas of above dimensions, placed at a distance of 21'6 centimetres, 
produce in one minute the combination of a column of hydrochloric acid of 0 '082 04 
centimetre in height over the whole area of this square centimetre, under the supposi- 
tion that the chemical action of the rays had been completely expended in passing 
through a layer of sensitive gas of indeflnite thickness. If Ave imagine the flame to be 
placed in the centre, and the insolation-vessel at the surface of a sphere whose radius is 
equal to the distance of the flame from the insolation-vessel, r=21'6 c. m. ; the amount 
of hydrochloric acid formed in one minute is V cubic centimetre on every square cen- 
timetre of surface, or on the whole surface of the sphere 
47rr^V=481'l cubic centimetres. 
As the rate of issue of the coal-gas was 4- 105 cub. cent, per second, the quantity of coal- 
gas which was burnt during the time in Avhich 481 '1 cub. cent, of hydrochloric acid was 
formed is 246'3 cub. cent. 
The result of this calculation may be thus expressed : when one cubic centimetre of 
coal-gas burns in the above-mentioned manner, such an amount of chemical rays is 
* Philosophical Transactions, 1857, p. 616. 
