210 Professor Roscoe, [March 2, 



WEEKLY EVENING MEETING, 



Friday, March 2, 1860. 



Sir Henry Holland, Bart. M.D. F.R.S. Vice-President, 

 in the Chair. 



Professor H. E. Roscoe, 

 . On the Measurement of the Chemical Action of the Solar Rays. 



> 

 Those portions of the solar rays which vibrate most slowly, and are 

 situated near the red end of the spectrum, are those which mainly 

 regulate the alterations of temperature on the surface of our planet. 

 They are, par excellence, the heating rays. They principally produce 

 all those motions in our atmosphere which we term winds ; they effect 

 those grand phenomena of distillation and deposits which we call rains ; 

 and the amount and distribution of those heating rays at any point on 

 the earth's surface determines the thermal climate of that point. 



On a scale, perhaps less grand, but certainly not less important as 

 regards their effects, are the actions produced by the most rapidly 

 vibrating portion of the sun's rays ; those, namely, which are situated 

 near the violet end of the spectrum. These rays have been called the 

 chemical rays, because it is by these especially that the chemical 

 action of the sunlight is effected. It is in presence of these rays alone 

 that the plant is enabled to decompose the carbonic acid of the air, to 

 assimilate the carbon, restoring the oxygen for the subsequent use of 

 animals. Hence the amount and distribution of these rays at any given 

 place regulates to a great extent the character of the fauna and flora ; 

 gives, in short, the " chemical climate " of the place. 



The measurement of the quantity of this solar energy, falling at 

 any time on a given spot upon the earth's surface, must be a subject of 

 primary importance in the determination of the physical history of our 

 globe. We fortunately possess a method, although it is only a com- 

 parative one, for measuring the amount of effect which the heating rays 

 produce, that is for measuring temperature. No such mode of measure- 

 ment for those of the solar rays which especially effect chemical action 

 has, up to the present time, been adopted ; not that meteorologists 

 have ignored the importance of the subject, but because the difficulties 

 which beset the establishment of a measuring instrument for chemical 

 action were considered to be insurmountable. 



