40 



that they have constructed a most delicate and trustworthy instrument 

 by which to measure the chemical action of light, and by help of which 

 they have been able to investigate the laws regulating this action. 

 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. 



2. 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 



