172 Dr. B. Moore and Mr. T. A. Webster. 



exposed to direct sunlight. But much more experimentation is required 

 here. There is, however, no doubt that active rays penetrate both chlorophyll 

 and quinine solutions, as the reactions were most distinct, and it appeared as 

 if these solutions possibly had prevented the passage of rays with a slowing 

 effect on the reaction. 



Experiment VII. Dilute Colloidal Uranic Oxide Solutions Exposed to Light 

 from a " Uviol " Mercury Arc in a Transparent Quartz Test-tube. — 40 c.c. of 

 a colloidal uranic oxide solution made by diluting the stock solution 15-fold, 

 and so containing 0'03 per cent, of the colloid, were placed in a tube of 

 transparent quartz, and after saturation with washed carbon dioxide from 

 a Kipp apparatus were exposed about 2£ inches from a " Uviol " mercury 

 arc for the period during three days in which the lamp was lit, probably 

 about 12 hours in all. At the end of the period the fluid was distilled and 

 the distillate tested by Schryver's test. It gave a strongly marked reaction, 

 corresponding to at least 1 part in 100,000 of formaldehyde. 



Photo-synthesis by Colloidal Ferric Hydroxide. 



Preparation of Colloidal Solution. — The colloidal ferric hydroxide solution 

 was prepared after the method originally given by Graham.* A strong 

 solution of ferric chloride is taken, about 20 per cent., and to this a saturated 

 solution of ammonium carbonate is added gradually with shaking so long as 

 the precipitate first formed continues to dissolve. The solution is then 

 dialysed for some days, 10 days or more, until the reaction for chloride 

 becomes very faint. The solution so obtained is of a deep reddish brown 

 colour, even in a 1-per-cent. solution, and if converted back into the 

 crystalloidal form by boiling with a drop or two of acid the change is 

 remarkable, to a pale lemon yellow colour. At the dilutions used in the 

 experiments below the colour scarcely shows when reduced to the crystal- 

 loidal condition, but in the colloidal condition, even at this dilution, the 

 solutions possess a deep sherry colour. The dilute solution in an ordinary 

 small test-tube absorbs the blue of the spectrum completely, as shown by a 

 pocket spectroscope. When viewed by light transmitted from a " Uviol " 

 mercury lamp, the solution viewed directly transmits a yellowish green light, 

 and at the sides, reflected from the glass surfaces, there is a deep green 

 fluorescence, which reminds one strongly of the fluorescence of a strong 

 solution of chlorophyll. 



The colloidal iron solution so prepared is readily coagulated by boiling, 

 and is most sensitive to added crystalloids ; it is thrown out by 1 part in 



* ' Phil. Trans.,' 1861, vol. 151, p. 208. 



