BETWEEN" THE STJN’S ALTITUDE AND CHEMICAL INTENSITY. 
315 
Fig. 2 gives the graphical representation of the relation of the total chemical intensity 
as ordinate to the sun’s altitude as abscissa. The relation between the altitude and che- 
mical intensity for altitude above 10° is here seen to be accurately represented by a 
straight line. The position of the experimentally determined points are noted, and serve 
to show how closely they lie to the straight line. 
In former communications*' it has been shown that a similar relation between altitude 
and chemical intensity of total daylight has been found to hold good at Heidelberg, 
Kew, and Para ; and that although the chemical intensity for the same altitude at dif- 
ferent places and at different times of the year may greatly vary according to the vary- 
ing transparency of the atmosphere, yet that the relation at the same place between 
altitude and intensity is always represented by a straight line. Thus the mean inten- 
sities at Lisbon and Para for 30° are 0T5 and 0-44 respectively, whilst for 60° they are 
0'32 and 0-80. That this variation in the direction of the straight line expressed by 
the constant in the formula, given in the paper last referred to, is due to the opalescence 
of the atmosphere, we have evidence of in the fact that, for equal altitudes, the higher 
intensity is always found where the mean temperature of the air is greater, as in summer, 
when we compare the same place at different seasons* or as we approach the equator, 
when we compare different places. The first of these conditions of variation is clearly 
seen if we compare the Kew observations for the same altitudes, but for different times 
of the year. The following Table clearly shows that the altitude in the warmer half of 
the year is invariably accompanied by a higher chemical action than that in the colder, 
and this is attributable to the varying opalescence, which is certainly a function of the 
atmospheric temperature, and is less marked as we approach the summer solstice or pass 
towards the equator. 
Comparison of Chemical Intensities at Kew, 18G6. 
Month. 
Time of 
observation. 
Corresponding 
altitude. 
Chemical intensity 
of total daylight. 
I. 
f October 
li m 
2 30 p.m. 
23 10 
0-059 
August 
4 42 „ 
23 58 
0-115 
November ... 
2 27 
14 52 
0-035 
September ... 
4 43 
14 14 
0-058 
March 
2 30 
28 36 
0-075 
June 
4 43 
29 52 
0-106 
,v.| 
f April 
2 30 
38 06 
0116 
July 
4 39 
30 05 
0-141 
It is interesting to observe the close agreement which exists between the measure- 
ments made at Lisbon with sensitive paper, and the luminous intensities calculated from 
observations made at Heidelberg by a totally different method. In 1859 Professor 
Bunsen, and one of us in Part IV. of ‘ Photochemical Researches, ’ gave curves of the 
* Roscoe, Philosophical Transactions, 1867, p. 555. 
