47 



distance of the earth to the sun. Or the light which the sun 

 radiates into space during each minute of time represents a chemical 

 energy, by means of which more than 25 billions of cubic miles of 

 chlorine and hydrogen may be combined to form hydrochloric acid. 

 In like manner the amounts of chemical action have been calculated, 

 which the sun's rays, undiminished by atmospheric extinction, pro- 

 duce at the surface of the chief planets. The first column of num- 

 bers gives the mean distances of the planets from the sun, the second 

 contains the chemical action expressed in light-metres. 



Mercury 0-387 235-4 light-metres. 



Venus 0723 67'5 



Earth 1-000 35-3 



Mars 1-524 15-2 



Jupiter 5-203 1-3 



Saturn 9'539 0'4 



Uranus 19-183 0-1 



Neptune 30-040 0-04 



By aid of the formula already given, the authors have been enabled 

 to calculate the chemical action effected each minute by the direct 

 sunlight, not only at different points on the earth's surface, but at 

 various heights above the sea's level. Both these series of relations 

 are tabulated, and graphically represented. On comparing the 

 numbers and curves giving the action of the total diffuse light with 

 those of the direct solar light, the singular fact becomes apparent, 

 that from the North Pole to latitudes below that of Petersburg, 

 the chemical action proceeding from the diffuse light is, throughout 

 the day on the vernal equinox, greater than that effected by the 

 direct sunlight ; and that in lower latitudes, down to the Equator, 

 the same phenomenon is observed, if not for the whole, still for a 

 portion of the day. It is further seen, that for all places, and on 

 every day when the sun rises to a certain height above the horizon, 

 there is a moment at which the chemical action of the diffused light 

 is exactly equal to that of the direct sunlight. The times at which 

 these phases of equal chemical illumination occur can be calculated ; 

 they can also be actually determined, by allowing the direct sunlight 

 alone, and the whole diffuse daylight alone, to fall at the same time 

 upon two pieces of the same sensitized photographic paper; the 



