93 



so that light of the intensity 50, actin<2: durinj^ time t, i)roduces the 

 same bhK'kenin«r eft'ect as light of intensity / acting during the time 

 50. According to this method the ciieniical action of the total day- 

 light was determined for jNIanchester, England, many times a day 

 during 1864, and the total daily chemical intensity has been com- 

 puted for the year August, 1863, to September, 1864. Very large 

 changes in chemical intensity occur when the sky is cloudless and 

 unchanged as far a^ the eye can perceive. The total intensity for an 

 apparently cloudless day varies from 3.3 for December 21, 1863. to 

 110, June 22, 1864. This last number, compared with the figure 50.t) 

 for June 20, and 26.6 for June 28, shows the enormous variations that 

 take place in the chemical rays that reach the observer at Manchester 

 on cloudless days. This variation is undoubtedly due in part to 

 smoke and moisture, but possibly other unknown influences are also 

 at work. 



In 1867 H. E. Roscoe connnunicated to the Roj^al Society the 

 results of work done by his method at Kew, P^ngland, in 1865, 1866, 

 and 1867; at Heidelberg, 1862 and 1863, and at Para, Brazil, 1866. 

 The general results are that the chemical intensity attains its max- 

 imum at noon and not, like the temperature, at some time after noon. 

 P^verywhere the intensity increases from hour to hour with the alti- 

 tude of the sun, and is very closely proportional to it even when the 

 sky is partially clouded, but of course the rate of increase varies with 

 the season, the amount of cloud, and the degree of atmospheric opales- 

 cence. The total chemical intensity for each month, as determined 

 from numerous observations, is as follows for Kew : 



Total photochemical intensity of direct and diffuse light (Roscoe). 



Eoscoe compares these figures with the cloudiness, and finds that 

 the ratio between cloudiness, expressed on a scale of 10, and the chem- 

 ical intensity is as 1 to 5 in some months and as 1 to ^ in others. A 

 similar irregularity of ratio is found when he considers the absolute 

 moisture in the atmosphere; whence he concludes that the variations 

 in chemical intensity, as between the spring and autumn, are not 

 perfectly explained by either of these factors. He finds the high 

 autumnal and low vernal intensity fairly well explained as due to the 

 transparency or opalescence produced by finely divided solid particles 

 or dust. 



