1870.] between the Sun's Altitude and Chemical Intensity. 303 



Chemical Intensity. 



No. of observations. Mean altitude. Sun. Sky. Total. 



15 9 51 0-000 0*038 0-038 



18 19 41 0-023 0-063 0-085 



22 31 14 0-052 0*100 0*152 



22 42 13 0*100 0*115 0-215 



19 53 09 0*13G 0-126 0*262 



24 61 08 0-195 0-132 0*327 



11 64 14 0*221 0-138 0-359 



Curves are given showing the relation between the direct sunlight (column 

 3) and diffuse daylight (column 4) in terms of the altitude. The curve of 

 direct sunlight cuts the base line at 1 0°, showing that the conclusion formerly 

 arrived at by one of the authors is correct, and that at altitudes below 10° 

 the direct sunlight is robbed of almost all its chemically active rays. The 

 relation between the total chemical intensity and the solar altitude is shown 

 to be represented graphically by a straight line for altitudes above 10°, the 

 position of the experimentally determined points lying closely on to the 

 straight line. 



A similar relation has already* been shown to exist (by a far less complete 

 series of experiments than the present) for Kew, Heidelberg, and Para ; 

 so that although the chemical intensity for the same altitude at different 

 places and at different times of the year varies according to the varying 

 transparency of the atmosphere, yet the relation at the same place between 

 altitude and intensity is always represented by a straight line. This varia- 

 tion in the direction of the straight line is due to the opalescence of the 

 atmosphere ; and the authors show that, for equal altitudes, the higher inten- 

 sity is always found where the mean temperature of the air is greater, as in 

 summer, when observations at the same place at different seasons are com- 

 pared, or as the equator is approached, when the actions at different places 

 are examined. The differences in the observed actions for equal altitudes, 

 which may amount to more than 100 per cent, at different places, and to 

 nearly as much at the same place at different times of the year, serve as 

 exact measurements of the transparency of the atmosphere. 



The authors conclude by calling attention to the close agreement between 

 the curve of daily intensity obtained by the above-mentioned method at 

 Lisbon, and that calculated for Naples by a totally different method. 



* Phil. Trans. 18G7, p. 555. 



