80 ACTION OF LIGHT RAYS 



freezing-point method and the degree of ionisation as given by 

 conductivity determinations. It is shown in the case of ferric 

 chloride solutions by the darkening in colour of dilute solutions on 

 warming, approaching the colour shown by colloidal iron solutions, 

 and persisting after the heated solution has cooled again, for pro- 

 longed periods of time. Also, if a dilute solution of ferric chloride 

 be boiled a precipitation of a part of the iron as ferric oxide is ob- 

 tained, and the precipitate remains permanent on cooling although 

 the period of boiling is far too short to remove a corresponding 

 amount of hydrochloric acid. All these facts demonstrate that there 

 are ions and molecules in the ferric chloride solution of greater 

 molecular complexity than the mono-molecular condition. 



The explanation, then, of the greater effect with undialysed ferric 

 chloride, as contrasted with colloidal ferric oxide solution, is that the 

 mass of the complex molecule in the ordinary ferric chloride solution 

 is that which can best take up the light vibrations and absorb the 

 energy, whereas in the colloidal ferric oxide solution the solution 

 aggregate has become too heavy to take up the light energy and 

 convert it into chemical energy. 



On the other hand, with the uranium salts, the optimum solution 

 aggregate to act as a transformer for light energy lies nearer that 

 found in the dialysed solutions than in the ordinary uranium nitrate 

 solutions, and accordingly the former show a higher photo-synthetic 

 activity. 



Silicic acid and its salts show the same kind of effect as the 

 uranium compounds, for dialysed silicic acid solution gives a strong 

 photo-synthetic action, while both sodium silicate solution and 

 undialysed silicic acid solution are inactive. 



There appear, therefore, to be two factors in the production of 

 photo- synthetic activity by a given light source viz., (1) the specific 

 character of the inorganic catalyst or transformer, (2) the degree 

 of its molecular aggregation in solution. 



In view of criticisms that have been made as to the necessity of 

 inorganic catalysts in the solution, and as to the possibility of the 

 ultra-violet rays producing the synthetic effect when passed into 

 pure water charged with carbon dioxide, as also the view that the 

 formaldehyde obtained might be produced from the minute traces 

 of organic matter in the dialysed solutions and not synthetically 

 from carbon dioxide and water, the following critical series of ex- 

 periments was carried out. 



