352 Prof. J. Sebelien on the Distribution of Actinic Sunlight 



diminishes towards the equator as well as towards 70° N. 

 lat. ; from here it increases again, and reaches at 90° a 

 secondary maximum that, however, has a smaller value than 

 that quantity of light which on the same day reaches the 

 regions of 60° K lat. 



But even the results of Spitaler are incomplete, since his 

 calculations only refer to the direct insolation, without 

 regard to the diffused daylight. Moreover, those calculations 

 are of a purely theoretical nature, without the support of 

 any experiment. In the fifties Bunsen and Boscoe made 

 their famous photochemical researches * of the light in- 

 fluencing the chlorine and hydrogen explosive mixture. 

 These experimenters found that the said activity is especially 

 localized between the lines G and M of Fraunhofer in the 

 solar spectrum ; from here it decreases more quickly and 

 regularly towards the red end of the spectrum than towards 

 the ultra-violet one. Their researches showed, further, that 

 the sun-rays, before entering the atmosphere, have a photo- 

 chemical effect of 35*5 light-metres, that is, by complete 

 absorption in an infinite atmosphere of chlorine-hydrogen 

 electrolytic gas at 760 mm. pressure and 0° C, they will be 

 able to transform during a minute a column of the said 

 mixture of 355 metres height into hydrogen chloride. 

 Having passed through the atmosphere, the sun-rays on 

 reaching the surface of the ocean will only possess a photo- 

 chemical effect of 14*4 light-metres ; thus, in passing the 

 atmosphere they will have lost about two-thirds of their 

 photochemical effect. 



Bunsen and Roscoe generally express the effects of light 

 that they measured and calculated in chemical photo-units* 

 each one of which is determined by the chemical action upon 

 a normal explosive mixture of hydrogen and chlorine con- 

 tained in an insolation vessel of such small dimensions that 

 the variability of the extinction appearing in large vessels 

 may be neglected when the said explosive gas is illuminated 

 at a distance of 1 metre from a so-called normal flame, that is, 

 a flame of burning carbonic oxide at a certain pressure issuing 

 from a platinum burner of accurately determined dimensions. 

 10,000 of these photo-units are called 1 chemical light-unit. 



The sun-ra)S reaching a horizontal area of the surface of 

 the earth at an angle $ with the vertical will produce in 

 one minute on each square unit of this area a photochemical 

 effect that may be expressed in chemical light-units by the 

 formula _ o- 47 58 p 



W = 318-3. cos £.10 cos * , . . . (a) 



* Poggendorff's Annalen, Bd. xcvi. &c. ; Ostwald's Klassiker Ausc/ahen, 

 Bd. xxxiv. & xxxviii. 



