I860.] 



on the Chemical Action of the Solar Rays. 



213 



of action can be found which the sun would have produced if directly 

 shilling upon the instrument ; a condition, impossible of course to fulfil, 

 as the action would become too rapid and the whole apparatus would 

 be shattered by explosion. 



The day chosen for observation of the sun's action must obviously 

 be cloudless, if we wish to obtain an idea of the relation existing between 

 the chemical action and the height of the sun. Beginning the observa- 

 tions as near sunrise as possible, we find, for instance, on September 

 15th, 1858, one of the days on which such a series of experiments was 

 made, that at 7^* 9*"- a.m., when the sun's zenith distance was 76*^ 30', 

 the observed action amounted to 1 * 62. That is, in one minute the 

 column of water moved through 1 52 division ; or the quantity of 

 hydrochloric acid formed, when the sun stood at the height mentioned, 

 was represented by 1 '52 division on the scale. 



Gradually, as the day wore on, the observed action for each minute 

 became larger ; until at 9^- 14"- a.m., the latest observation possible on 

 the day in question, owing to the formation of clouds, the action 

 reached 18*5 divisions, or was thirteen times as large as at 7*" 9". In 

 the last column of the accompanying table is found the action, expressed 

 in degrees of light, which would have been observed at the foregoing 

 times, if the whole sunlight had been allowed to f^ll on the instrument. 



Table I. 



This great increase in the chemical action with the rise of the sun 

 in the heavens simply results from the fact that the solar rays, in pass- 

 ing through the air, are extinguished or absorbed, lost in fact as light ; 

 and that as the sun rises higher above the horizon, the column of air 

 through which the rays pass is constantly being lessened ; consequently 

 more of the direct rays reach the earth. 



Now, the law according to which the direct rays of the sun are 

 thus absorbed in the air can be obtained from the experiments, of which 

 the foregoing is only an example ; hence, if the action which the sun 

 produces, when at a given height, is known, it is possible to calculate 

 the action which it would produce at any other height. 



Vol. III. (No. 31.) Q 



