440 Prof. C. Timiria/pfr. [Apr. :JO, 



here a chlorophyll print of a fern leaf. The leaf was applied to a 

 plate coated with a film of collodion tinged with chlorophyll. After 

 a short exposure to direct sunlight we observe that the whole ground 

 is bleached, the parts protected by the leaf retaining their original 

 hue. The image is fixed by a short immersion in a bath of copper 

 sulphate. On submitting such chlorophyll collodion plates to different 

 rays of the spectrum, I convinced myself that this bleaching effect 

 is due to the same rays which effect the reduction of carbon dioxide. 



I may perhaps be allowed to dwell a little longer on this important 

 question of chlorophyll playing the part of a sensitiser. The sensi- 

 tisers are in general divided into two groups chemical and optical. 

 The former are considered simply absorbents of one or more of the- 

 products of the reaction, the latter are at the same time absorbents of 

 radiant energy. An optical sensitiser is supposed to be at the same 

 time a chemical sensitiser, but the reverse of course does not hold 

 good. The existence of chemical sensitisers was admitted long before- 

 Vogel's discovery ; many instances of their action may be found in 

 Becquerel's well-known book, and in consequence at a very early date, 

 in 1871, in my Russian work "Spectrum Analysis applied to Chloro- 

 phyll," I admitted that chlorophyll may be considered a sensitiser in 

 the purely chemical acceptance of the word. I insisted that the- 

 reduction of carbon dioxide being essentially a process of dissociation, 

 and the " rapidity of dissociation depending on the removal of the- 

 products of dissociation," " the plant acts as an absorbent, continually 

 disturbing the equilibrium between the carbon dioxide and the pro- 

 ducts of its dissociation," and finally that we must probably admit 

 " the existence of two modifications of chlorophyll somewhat corre- 

 sponding 'to haemo- and oxyhsemoglobin of the blood, the latter beinsr 

 perhaps capable of originating a product analogous to carbonyl 

 haemoglobin." 



At the time it seemed to me that these two states of oxidation corre- 

 sponded to the normal green chlorophyll and to Sir George Stokes's- 

 modified chlorophyll (fig. 2), my experiments having put it beyond 

 doubt that the latter was the product of oxidation of the former ; but 

 later I discovered another reaction of chlorophyll, which I look upon 

 as highly important from the physiological point of view we are now 

 considering. When a moderately concentrated solution of chlorophyll 

 is acted upon by hydrogen in the nascent state, it is transformed into 

 a substance nearly colourless, or of a pale yellow hue, but possessing 

 a beautiful purple colour when highly concentrated. This substance 

 can exist only in a total absence of oxygen. On being brought into 

 contact with air it almost immediately recovers its natural green 

 colour. The spectrum of this reduction product of chlorophyll (I pro- 

 posed to call it protochlorophyllin, or simply protophyllin) in a diluted 

 state, is characterised by a total absence of bands in the less. 



