68 EXPKRIMENTS MADE IN THE GAEDEN 01' THE SOCIETY 



pcsed to light : but in tliis there was nothing unaccountable — 

 nothing but; what any observer of vegetation would have ex- 

 pected under the circumstances ; there was no decided disease. 

 Grown in fuller light, potatoes have of late, however, been 

 diseased ; and yet it is not to bo inferred that light has not 

 influenced tlie disease. Apparently, tlie solar light may not be 

 deficient, but it may be defective in some of its usual properties 

 as regards its action on vegetation. It exerts a chemical action 

 on tiie four organic elements, namely, oxygen, carbon, hydrogen, 

 and nitrogen. The grand result of its action is the decomposi- 

 tion of carbonic acid in the leaves, and consequent fixation of the 

 carbon, which being assimilated, constitutes the principal portion 

 of the solid parts of plants. 



It is generally supposed that the decomposition of carbonic 

 acid is directly effected by the action of light on tlie leaves of 

 plants, but Dr. Draper states in an article in the Philosophical 

 Magazine for September, 1843, that "there are many facts 

 which go to prove that the decomposition of carbonic acid is a 

 secondary result, brought about by the action of a nitrogenized 

 ferment in a state of eremacausis, the sunlight operating in the 

 first instance upon the ferment itself." Dr. Draper has satis- 

 factorily ascertained that both oxygen and nitrogen are evolved 

 in the process, and that the volume of mixed gases, namely, 

 oxygen and nitrogen, is precisely equal to the volume of carbonic 

 acid decomposed : but the relative proportions of the oxygen and 

 nitrogen are variable. 



The potato contains a greater quantity of easily separable 

 pure starch than is, perhaps, to be found in any other vegetable. 

 Starch is readily changed into other substances under certain 

 influences. It is decomposed by fermentation ; the latter is 

 induced by nitrogeiuzed substances. Liebig, if I am not mis- 

 taken, has stated that diseased potatoes contain an excess of 

 nitrogen. Tiiis may be owing to some change which has taken 

 place in the action of the sun's rays, and the potato plant may 

 liave been the most liable to be thereby affected, from its great 

 abundance of starch intermixed with comparatively litileof otiier 

 substances. 



Sir John Ilerschel found, in 1840, that the action of solar 

 light was exceedingly various, both as regards its total intensity 

 and the distribution of the active rays over the spectrum. 



It is well known that a ray of solar light, or what is termed a 

 ray, is actually compounded of several rays of different colours — 

 red, orange, yellow, green, blue, indigo, and violet. These rays 

 are found by persons engaged in photographic processes to exert 

 influences not only different in degree of intensity, but even 

 totally negative or antagonistic. Dr. Draper, however, cor- 



