42 



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 



tf=the volume of hydrochloric acid formed by the unit of light. 

 k=ihe thickness of sensitive gas through which the light passed. 

 5= the surface-area of the insulated gas. 

 a=the coefficient of extinction of the chlorine and hydrogen for 



the light employed. 

 /=the number of observed units of light in the time t. 



When these values are known, the volume of hydrochloric acid 

 which would be formed in the time t, 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 



V v l 



-'' 



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 

 of a place, and points the way to relations for the chemical actions 

 of the solar rays, which in the thermic actions are already repre- 

 sented by isothermals, isotherals, &c. 



In order to determine the chemical action exerted by the whole 

 diffuse daylight upon a given point on the earth's surface, the 

 authors were obliged to have recourse to an indirect method of ex- 

 perimenting, owing to the impossibility of measuring the whole 

 action directly, by means of the sensitive mixture of chlorine and 

 hydrogen. For the purpose of obtaining the wished-for result, the 

 chemical action proceeding from a portion of sky at the zenith, of 

 known magnitude, was determined in absolute measure, and then, 



