THE ACTION OF LIGHT 



559 



be confessed that, valuable as it is, it has not, as yet, thrown much light on the 

 problem before us. The brief account which follows is taken from the book by 

 Willstatter and Stoll (1913). 



The method of preparation will be found on p. 133 of the book. The most 

 important point is the first extraction of dried leaves (say of nettle or elder) with 

 80 per cent, acetone, which does not extract wax and fatty substances. 



Stokes (1864) had already pointed out that what is usually known as 

 chlorophyll consists of a mixture of two green substances, and that it is 

 accompanied in leaves by two yellow pigments. Willstatter confirms this 

 statement, and 

 calls the two 

 chlorophylls a 

 and b : these 

 are identical in 

 all plants, and 

 contain mag- 

 nesium and 

 nitrogen, but 

 neither iron 

 nor p h o s- 

 phorus. The 

 yellow pig- 

 ments, carotin 

 and xantho- 

 phyll, are free 

 from nitrogen. 



As to the 

 quantity of 



these four substances to be obtained from leaves, we find the following data : From 

 1 kg. of dried elder leaves ( = 4 kg. of the fresh leaves) were obtained : 



FIG. 177. ALLEGORICAL PICTURE REPRESENTING THE EFFECTS OF THE 

 LIGHT OF THE SUN ON GREEN PLANTS AND THE PURIFICATION OF THE 

 AIR THUS EFFECTED BY THEM. 



(Ingenhousz, 1780. Initial figure. ) 



Together with 



8 '48 g. of chlorophyll, consisting of 

 6 '22 of a-chlorophyll, and 

 2-26 of 6-chlorophyU. 



1'48 g. of carotinoids (yellow pigments), namely, 

 0'55 g. of carotin 

 0'93 g. of xanthophyll. 



The two chlorophylls are separated by partition between methyl alcohol and 

 petroleum ether. The latter takes up a-chlorophyll ; the former, 6-chlorophyll, 

 since it is insoluble in petroleum ether. Even a-chlorophyll is difficult to dissolve 

 in pure petroleum ether. It has a blue-green colour, with red fluorescence. By 

 rapid dilution with water a colloidal solution is obtained, devoid of fluorescence. 

 Chlorophyll-6 has a green or yellow-green colour in solution. There is a slight 

 difference between their absorption spectra, as will be seen later. The action of 

 acid on the a-substance gives an olive-green derivative, on the 6-substance, a red- 

 brown one. Chlorophyll-6 is an oxidation product of the a-substance, containing 

 an extra oxygen atom in place of two hydrogen atoms. Both are micro-crystalline. 

 They are adsorbed by charcoal, and cannot be extracted again by petroleum ether, 

 but by pyridine, no doubt a question of relative lowering of surface tension. 

 Their behaviour to reagents is essentially similar, and the word chlorophyll will be 

 used below to include both. 



Further information of their constitution is obtained by treatment with reagents. 



The Action of Acid on chlorophyll directly is to separate magnesium from it, 

 forming a derivative called " phaeophytin." The series of derivatives devoid of 

 magnesium are called in general "phytins." The magnesium in chlorophyll is in 

 organic combination, and by treatment of the magnesium-free phaeophytin with 

 Grignard's reagent (magnesium-methyModide), the magnesium is replaced and 

 chlorophyll obtained again. 



