OF THE LUMINOUS RAYS. 



AVc liave already noticed some chemical phenomena 

 due to heat, particularly those experiments of Count 

 llumford's, which appeared to him to prove that the 

 chemical agency of the sun's rays was due to its calorific 

 power. Certain chemical phenomena, we know, may be 

 produced by thermic action but the only variety of 

 thermo-chemical action which connects itself imme- 

 diately with the solar radiations, belongs to a class of 

 rays to which the name of Parathermic has been given, 

 and to which the scorching, as it is called, of plants, the 

 browning of the autumnal leaves, and the ripening of 

 fruits, appear to be due.* When we come to the con- 

 sideration of those physical phenomena which belong to 

 the. growth of plants, all these peculiarities of solar 

 action must be attended to in detail. 



The manner in which we find the actinic power 

 influencing electrical action, also shows us that the 

 equilibrium of forces is continued through all the great 

 principles of nature. If a galvanic arrangement is made, 

 by which small quantities of metals may be slowly pre- 

 cipitated at one of the poles in the dark, and a similar 

 arrangement be exposed to sunshine, it will be found 

 that no metal is deposited: the sun's rays have- 

 interfered with the decomposing power of the electrical 

 current. At the same time we learn, that by throwing 

 a beam of light upon a plate of copper which forms one 

 of a galvanic pair, whilst it is under the influence of an 

 acidulated solution, an additional excitation takes place, 

 and the galvanometer will indicate the passage of an 

 increased current of electricity. These two dissimilar 

 actions appear enigmatical ; but they may, there is no 

 doubt, receive some solution from the 'influence of 

 different rays on the contrary poles of the battery. One 



* Sir John Herschel's Memoirs already referred to ; and Reports 

 on the influence of the Solar Rai/s on the growth of Plants, by Robert 

 Hunt : lieport of the British Association for the Advancement of 

 Science, for 1847. 



