558 PRINCIPLES OF GENERAL PHYSIOLOGY 



chemical energy into light energy, without passing through heat. In the same 

 way, certain photo-chemical reactions are direct conversion of light energy 

 into chemical energy. 



The bearing of these phenomena on the emission of light by organisms will be 

 seen later. 



The light emitted by the Welsbach mantle appears to be of a shorter wave 

 length than that corresponding to the temperature of the Bunsen flame. It is 

 difficult, however, to determine accurately what is the temperature of the flame. 

 That of a body in it depends on the ratio of its powers of emission and absorption 

 of radiation. For example, a bead of sodium phosphate on a loop of platinum wire 

 is barely luminous in the Bunsen flame, while the platinum glows brightly. 



PHOTO-ELECTRIC EFFECTS 



When a ray of light falls upon a metallic electrode, the potential of this latter 

 is changed. 



The discharge of a charged electroscope by ultra-violet light, the Hallwachs 

 effect, will be referred to later. 



The change in the resistance of selenium on exposure to light has been made 

 practical use of in the transmission of pictures by electric current. 



Two kinds of explanation have been given of the origin of the potential 

 difference in photo-electrical cells. One may say that the light which falls upon 

 an electrode of silver chloride raises the tension of chlorine, and secondarily the 

 potential of the electrode, or that electrons are torn off from their combination by 

 the increased kinetic energy. 



For further information the reader may consult the book by Allen (1913). 



THE CHLOROPHYLL SYSTEM 



We are now in a position to discuss with more profit the action of chlorophyll 

 in the decomposition of carbon dioxide and evolution of oxygen, perhaps the most 

 interesting of all natural phenomena. 



History. There are three important dates to be noted. 



Priestley (1774, pp. 89-92) observed that air " spoilt " by mice, that is, incapable 

 of supporting animal life, was made good again by allowing green plants to remain 

 in it for some time. 



Ingenhousz (1780) showed that this action of green plants only takes place in 

 the light. He says : " The light of the sun is alone capable of producing in the 

 leaves that movement which can develop dephlogisticated air " (that is, oxygen) : "as 

 soon as the light ceases to act on the leaves, their operation ceases at the same 

 time, and another of a different nature commences." (Translated from p. 17 of the 

 French edition.) He also shows clearly that it is not the heat that is responsible 

 for the result (p. 38). In Fig. 177 I have reproduced his little allegorical initial 

 picture of how light destroys noxious things. 



Senebier (1783, see pp. 410-442 of his book, 1788) showed that the chemical 

 change involved is the conversion of fixed air into dephlogisticated air, that is, 

 carbon dioxide into oxygen. De Saussure (1804, Chap. 2) investigated the 

 phenomena quantitatively. 



General Nature of the Reaction. As already mentioned, the process, taken as 

 a whole, results in a large storage of light energy, and is one of those complex 

 reactions which are the most difficult to investigate. We have to deal with several 

 reactions chemically coupled, some sensitive to light, others apparently not; 

 together with both optical and chemical sensitisation. 



As is well known, it is the presence of the pigment, chlorophyll, which enables 

 the reactions to take place. Those parts of variegated leaves which are devoid of 

 chloroplasts, although otherwise similar to the green parts, are incapable of photo- 

 synthesis, as it may be called for convenience. 



The Chemistry of Chlorophyll. Although much very interesting work has been 

 done on the chemical constitution of chlorophyll, especially by Willstatter, it must 



