216 Prof, Roscoe, on Chemical Action of Solar Rays, [March 2, 



action effected by direct sunlight in one minute at the places and hours 

 named on the 21st of March. Curves were exhibited, showing the 

 rise of the action, with the progress of the sun through the heavens. 

 By comparing the numbers in the table, it is seen how greatly this 

 chemical action differs at various points on the earth's surface ; and we 

 can understand how it is, that at the latitude of Cairo, where the 

 chemical action of the direct sunlight is twice as great as it is in that 

 of Manchester, the whole flora and fauna assume a more tropical and 

 luxuriant character. 



The speaker stated, that he was only able briefly to notice the 

 principles upon which the new mode of measuring the chemical action 

 of light depends ; adding one or two illustrations of the measurements 

 actually made. He was unable even to refer to one of the most inter- 

 esting and important applications, viz , the measurement of the chemical 

 action effected by the diffuse daylight. This has, however, been 

 accomplished, and we are now able to calculate the amount of chemi- 

 cal action produced by both diffused and direct solar light, on a cloud- 

 less day, at any place situated above the latitude of Cairo. The 

 following table shows the results of such a calculation. 



Table V. — Total Chemical Action effected by the Solar Rays from 

 Sunrise to Sunset on the Vernal Equifiox, at 



Melville Island 



Rejkiavik 



St. Petersburg 



Manchester . 



Heidelberg . 



Naples 



Cairo 



Knowing the intimate connection of the chemically active solar 

 rays with the plant and animal-producing power of a country, no one 

 can doubt the immense importance of the establishment of a regular 

 series of measurements of the variations of the amount of these chemi- 

 cal rays at different situations on the earth's surface. Such a series 

 would open an entirely new field to the science of meteorology, and 

 would reveal to us relations and points of difference as regards the 

 chemical climate, at present wholly unknown. 



The chemical photometer, in the hands of an experienced experi- 

 menter, is a perfectly accurate and extremely sensitive instrument ; and 

 the method described is a most valuable one for scientifically investi- 



