OXIDATION AND REDUCTION 595 



But it is to be remembered that "protoplasm," as used here, means the cell 

 constituents as a whole. 



The organs may be divided into three groups : 



1. Those of high "oxygen saturation," in which indophenol blue is not reduced. 

 Such are the grey matter of the brain, the heart, and some other muscular organs. 



2. Those which reduce indophenol blue, but not alizarin blue. Such are the 

 greater number of the tissues, smooth muscle, most voluntary muscles, and secreting 

 glands. 



3. Those which reduce even alizarin blue. Lungs, liver, fatty tissue, Harderian 

 gland. 



As to the meaning of the facts, Ehrlich points out that, in activity, as also in 

 asphyxia, practically all cells become highly reducing, and that the state of a cell 

 at any given moment depends on the rate at which it consumes oxygen in relation 

 to that at which it is supplied. At the same time, it is necessary to assume that a 

 series of substances of different reducing power make their appearance.' Thus, a 

 substance which has less affinity for oxygen than alizarin blue has cannot reduce it, 

 and a further stage of reduction must occur before this takes place. 



The important question of the facility of access of oxygen belongs to those to 

 be discussed in the next chapter. 



The fact that fat tissue has so high a reducing power, shows that oxygen avidity 

 is not necessarily to be ascribed to great functional capacity. The chemical nature 

 of certain permanent constituents of the cell must be taken into account. This 

 should be kept in mind with regard to the paradoxical experimental fact of the 

 reducing power of the lung tissue. Ehrlich ascribes the property to the stroma 

 cells, not to the alveolar epithelium, and suggests that it may be due to an appropri- 

 ate relative impermeability of these cells to oxygen, so that they shall not unneces- 

 sarily retard the aeration of the blood. 



THE PRODUCTION OF LIGHT 



After what we have learnt in the present and preceding chapters in connection 

 with the phenomena of autoxidation and their relation to catalysts, together with 

 that of chemi-luminesceiice, further brief reference may profitably be made to the 

 problem of the emission of light by living organisms. 



One of the most important practical questions at the present time is that of the 

 improvement of the efficiency of our methods of artificial illumination. The 

 majority of these, as used now, depend on the emission of light by substances when 

 heated to a very high temperature. The higher they can be heated, the greater the 

 proportion of light to heat rays. Hence the advantage of the metallic filament 

 lamps over the old carbon filament, and especially that of the electric arc over other 

 forms of illuminant. 



We have seen, however (page 557), that, in the phenomena of chemi- 

 luminescence, we have light emitted at temperatures far below those to which it 

 would be necessary to heat a metallic wire in order to obtain light of the same 

 wave length. We may put it thus, chemical energy is transformed directly to 

 light energy, without passing through the state of heat. 



Now the problem appears to have been solved by numerous organisms, although 

 the quantity of light they emit is not great. Amongst these organisms we may 

 mention fungi (including bacteria), protozoa, medusae, insects, molluscs, and fish. 

 For individual details of these organisms, the reader is referred to the articles by 

 Mangold (1910), Dubois (1903, 1913), Coblentz (1912), and for plants, Molisch 

 (1904). 



The fact that living cells can emit light shows at once that the fact is not due 

 to their temperature, as in the case of the ordinary sources of light. Phosphorescence, 

 in the true sense, requires previous exposure to light, and is easily excluded. We 

 are left then with phenomena related to chemi-luminescence. 



Spectral examination shows, accordingly, that the light is limited to the 

 middle region of the spectrum, usually having its maximum in the green (Langley 



