Messrs. Bunsen and Roscoe’s Photochemical Researches. 68 
mination of a surface, that is, the amount of chemically active light 
which falls perpendicularly on the plane surface, can be obtained. 
It has thus been found that the distance to which two flames of 
coal-gas and carbonic oxide, each fed with gas at the rate of 4:105 
eubic cent. per second, must be removed from a plane surface, in order 
to effect upon it an amount of chemical action represented by one 
degree of light, was, in the case of the coal-gas flame, 0°929 metre, 
in that of carbonic oxide 0°561 metre. The chemical illuminating 
power, or chemical intensity, of various sources of light, measured 
by the chemical action effected by these sources at equal distances 
and in equal times, can also be expressed in terms of this unit of 
light ; and these chemical intensities may be compared with the 
visible light-giving intensities. In like manner, the authors define 
chemical brightness as the amount of light, measured photochemi- 
cally, which falls perpendicularly from a luminous surface upon a 
physical point, divided by the apparent magnitude of the surface ; 
and this chemical brightness of circles of zenith-sky of different sizes 
has been determined. Experiment shows that the chemical bright- 
ness of various sized portions of zenith-sky, not exceeding 0:00009 
of the total heavens, is the same; or, that the chemical action 
effected is directly proportional to the apparent magnitude of the 
illuminating surface of zenith-sky. 
It is, however, important to express the photochemical actions 
not only according to an arbitrary standard, but in absolute measure, 
in units of time and space. This has been done by determining 
the absolute volume of hydrochloric acid formed by the action of a 
given source of light during a given space of time. For this pur- 
pose, we require to know— 
v=the volume of hydrochloric acid formed by the unit of light. 
h=the thickness of sensitive gas through which the light passed. 
q=the surface-area of the insulated gas. 
a=the coefficient of extinction of the chlorine and hydrogen for 
the light employed. 
J=the number of observed units of light in the time ¢. 
When these values are known, the volume of hydrochloric acid 
which would be formed in the time ¢, by the rays falling perpendi- 
cularly on the unit of surface, if the light had been completely 
extinguished by passing through an infinitely extended atmosphere 
of dry chlorine and hydrogen, is found from the expression 
In this way the chemical illumination of any surface may be ex- 
pressed by the height of the column of hydrochloric acid which 
the light falling upon that surface would produce, if it passed 
through an unlimited atmosphere of chlorine and hydrogen. This 
height, measured in metres, the authors propose to call a Light- 
metre. ‘The chemical action of the solar rays can be expressed in 
light-metres ; and the mean daily, or annual height thus obtained, 
dependent on latitude and longitude, regulates the chemical climate 
