118 LIGHT, VEGETATION AND CHLOROPHYLL 



Wurmser's theory (1930) is based on a photolysis of water, 

 dissociated into lU^ and O2 by hght — a dissociation, however, 

 which it is impossible to effect in the laboratory under visible 

 hght. The hydrogen thus produced, or the energy of its 

 recombination with oxygen, would be available for the 

 reduction of carbon dioxide. 



This theory is much simphfied and the combinations 

 concerned, in which the hving matter of the leaf takes part, 

 are, on the contrary, extremely complex. Nevertheless, it has 

 just received very clear confirmation through the use of an 

 isotope of oxygen. 



Chemical elements are generally mixtures of atoms, the 

 masses of which are very nearly whole numbers. Thus, 

 ordinary chlorine contains atoms of atomic mass 35 and others 

 of atomic mass 37. Both have exactly the same chemical 

 properties and are never separated, except by certain long and 

 dehcate physical methods only recently used in laboratories. 

 That is why they are always encountered in the same propor- 

 tion, which gives the mixture an average atomic mass of 

 35*5, a value found in chemical textbooks. These two kinds 

 of atoms are called isotopes of chlorine. 



Similarly, oxygen possesses two isotopes, one of mass 16, 

 the more abundant, and the other of mass 18, which is much 

 more rare. An important result has recently been obtained 

 through the use of this second variety of oxygen of atomic 

 mass 18. This isotope is always present, mixed with the 

 oxygen of mass 16, but in a smaller quantity. With oxygen 

 enriched in isotopes of mass 18, water and carbon dioxide are 

 prepared, and either the water or the carbon dioxide is offered 

 to illuminated chlorella. Oxygen is given off during photo- 

 synthesis and the measurement of the atomic masses (by the 

 mass spectograph) enables one to determine whether this 

 oxygen enriched in isotope 18 is present or not in the gas 

 liberated. The conclusion of these experiments is that it is the 

 oxygen of the water that is given off. The overall reaction 

 often written as 



CO2+H2O — ^CHaO+O^ 



