62 Royal Society :— 
In the present memoir, the authors proceed, in the first place, to 
establish a general and absolute standard of comparison for the 
chemical action of light; and in the second place, to consider 
the quantitative relations of the chemical action effected by direct 
and diffuse sunlight. They would endeavour, in this part of their 
research, to lay the foundation of a new and important branch of 
meteorological science, by investigating the laws which regulate the 
distribution, on the earth’s surface, of the chemical activity ema- 
nating from the sun. 
The subject-matter of the present communication is divided under 
five heads :— 
1. The comparative and absolute measurement of the chemical 
rays. 
%. Chemical action of diffuse daylight. 
3. Chemical action of direct sunlight. 
4, Photochemical action of the sun, compared with that of a 
terrestrial source of light. 
5. Chemical action of the constituent parts of solar light. 
The first essential for the measurement of photochemical actions, 
is the possession of a source of constant light. This the authors 
secured with a greater amount of accuracy than by the method de- 
scribed in their former communications, by employing a flame of pure 
carbonic oxide gas, burning from a platinum jet of 7 millims. in dia- 
meter, and issuing at a given rate, and under a pressure very slightly 
different from that of the atmosphere. The action which such a 
standard flame produces in a given time on the sensitive mixture of 
chlorine and hydrogen, placed at a given distance, is taken as the 
arbitrary unit of photochemical illumination. This action is, how- 
ever, not that which is directly observed on the scale of the instru- 
ment. The true action is only obtained by taking account of the 
absorption and extinction which the light undergoes in passing 
through the various glass-, water-, and mica-screens placed between 
the flame and the sensitive gas. These reductions can be made by help 
of the determinations given in Part III. of these Researches, as well 
as by experiments detailed in the present Part. When these sources 
of error are eliminated, it is possible, by means of this standard 
flame, to reduce the indications of different instruments to the same 
unit of luminous intensity, and thus to render them comparable. 
For this purpose, the authors define the photometric unit for the 
chemically active rays, as the amount of action produced in one 
minute, by a standard flame placed at a distance of one metre 
from the normal mixture of chlorine and hydrogen; and they 
determine experimentally for each instrument the number of such 
units which correspond to one division on the scale of the instru- 
ment. By multiplying the observed number of divisions by the 
number of photometric units equal to one division, the observations 
are reduced to a comparable standard. It is proposed to call this 
unit a chemical unit of light, and ten thousand of them one chemical 
degree of light. ’ 
According to this standard of measurement, the chemical illu- 
OO Ee 
