SYNTHESIS BY SUNLIGHT 39 



and taking energy from the sunlight, built up at first simple organic 

 bodies, and now these in turn reacting with one another formed 

 more and more complex organic compounds. In any such trans- 

 formation external energy is necessary, because the reacting bodies, 

 carbon dioxide and water, are fully oxidised, and must be reduced 

 with evolution of oxygen and uptake of energy in what is called an 

 endothermic reaction. To this reaction, the inorganic colloid plays 

 the part of an activator or catalyst, the solar energy being converted 

 into chemical energy of the organic compound, so serving as a reser- 

 voir of the energy necessary for the coming living organic world. 



It was first suggested by von Baeyer 1 that the initial stage in the 

 synthesis of organic matter from inorganic by the green plant 

 consisted in a reaction of carbon dioxide and water to produce 

 formaldehyde and oxygen, the energy for the endothermic reaction 

 being supplied from the energy of the light vibrations. This has 

 been confirmed by delicate reactions, for although the change is a 

 transitory one, the formaldehyde being condensed into other 

 organic substances as it is formed, yet colour reactions for aldehydes 

 are known so delicate that they will clearly indicate 1 part in 

 1,000,000 of aldehyde. 



Any accumulation of formaldehyde would rapidly kill the living 

 cell, and it is soon transformed into other products, but the colour 

 tests are so delicate that its presence in traces has now been confirmed 

 by several observers. 2 



Acting on the hypothesis outlined above, experiments with a 

 view to testing the synthetic action of sunlight in presence of 

 inorganic colloids were commenced in 1911, but for over a year only 

 negative results were obtained, on account of difficulties in adjusting 

 proper concentrations of solution, securing adequate illumination 

 in vessels made of suitable material, and obtaining delicate enough 

 methods for separating and identifying formaldehyde. After over- 

 coming these experimental difficulties, and with the aid of the 

 brighter sunshine, a large number of positive results have been 

 obtained, and the synthesis of formaldehyde under the conditions 

 to be described below has been shown quite unmistakably by all the 

 usual colour reactions for formaldehyde. 



It has also been shown that it is the ultra-violet rays which are 



1 Berichte d. deut. chem. Gesettsch., 1870, vol. iii., p. 68. 



2 See especially Usher and Priestley, Roy. Soc. Proc., 1S06, B, vol. Ixxvii., 

 p. 369, where references to other papers are given. 



