D.— ZOOLOGY, 81 



It will be admitted that these three reactions : ( 1 ) the formation of 

 formaldehyde, H.CO.H, from carbonic acid, OH. CO. OH, with libera- 

 tion of free oxygen, or, to put it more simply, the direct union of the 

 carbon atom of COo with a hydrogen atom of HoO ; (2) the formation 

 of sugars from formaldehyde, and (3) the formation from nitrites and 

 formaldehyde of nitrogenous organic substances, are the most funda- 

 mental and characteristic reactions of organic life. It is true that light 

 of such short wave-lengths (X= 200 (i-n) as were required in Baly's 

 experiments to synthesise formaldehyde do not occur in sunlight as it 

 reaches the earth to-day ; but, as we shall see later, the same author 

 has shown that, in the presence of certain substances known as photo- 

 catalysts, the reaction can be brought about by ordinary visible light; 

 and from Moore and "Webster's work it appears that colloidal hydroxides 

 of uranium and of iron are suitable photocatalysts for the purpose. 



If these results of the pure chemist are justified, they go far towards 

 bridging the gap which has separated the inorganic from the organic, 

 and make it not too presumptuous to hazard the old guess that even 

 to-day it is possible that organic matter may be produced in the sea and 

 other natural waters without the intervention of living organisms. "We 

 may note here, too, that if we take account of only the most accurate 

 and adequately careful work, the actual experimental evidence at the 

 present tim.e requires the presence of a certain amount of organic matter 

 in the culture medium or environment before the healthy growth of 

 even the simplest vegetable organism can take place. This was illus- 

 trated in some experiments made By myself some years ago when 

 attempting to grow a marine diatom, Thalassiosira gravida, in artificial 

 sea-water made up from the purest chemicals obtainable dissolved in 

 twice-distilled water. Even after nutritive salts, in the form of nitrates 

 and phosphates, had been added, little or no growtn of the diatom 

 occurred. But if as little as 1 per cent, of natural sea-water were 

 added excellent cultures resulted, in which the growth was as healthy 

 and vigorous as I was able to obtain when natural sea-water was used 

 entirely as the basis of the culture medium. There was clearly some 

 substance essential to healthy growth contained in the 1 per cent, of 

 natural sea-water, and from further experiments it became practically 

 certain that it was an organic substance. AVhen, for instance, the 

 natural sea-water was evaporated to dryness, the residue slightly heated 

 and redissolved in distilled water, 1 per cent, of this solution added to 

 the artificial culture medium was as potent in producing growth of the 

 diatom as the original natural sea-water had been. "When, on the 

 other hand, the residue after evaporation was well roasted at a dull 

 red heat and redissolved in distilled water, the addition of this solution 

 to the artificial culture medium produced no effect and growth did not 

 take place. Growth could also be stimulated by boiling a small frag- 

 ment of green seaweed (Ulva) in the artificial culture medium, the 

 weed being removed before inoculation with the diatom. All this points 

 to the necessity for the presence of some kind of organic matter in 

 the solution before growth can take place. One must not dogmatise, 

 however, for there are many pitfalls in the experimental work and the 

 necessai'y degree of accuracy is difficult to attain. My own experience 



H 2 



