44 SYNTHESIS BY SUNLIGHT IN KELATIONSHIP 



In carrying out the photo- synthetic experiments this solution 

 was either diluted fifteen to twenty fold with distilled water, and 

 carbon dioxide, evolved from marble and pure hydrochloric acid and 

 washed by water in a wash-bottle, passed through in a slow stream, 

 or it was diluted to the same degree with distilled water previously 

 saturated with carbon dioxide, and sealed up hermetically in glass 

 tubes, which were then exposed to such direct sunlight as was avail- 

 able on the flat roof of the laboratory. 



It will accordingly be observed that the concentration of the 

 colloidal uranic hydroxide in the solutions actually exposed to the 

 light was only 0-024 to 0-035 per cent. The stock solution of the 

 colloid had a pale lemon yellow colour, and the diluted solu- 

 tions were almost colourless, showing just the merest trace of 

 yellow colour, and the solutions throughout remained water clear. 

 The colloidal solution is very sensitive, and is precipitated by traces 

 of added crystalloid. 



The test solution for aldehyde used in our earlier experiments 

 was S chili's reagent, which consists of a dilute solution of fuchsin 

 (about 1 per cent, in water) through which sulphur dioxide is 

 passed until it is just decolorised. This reagent added in the pro- 

 portion of 3 drops to 10 c.c. of a dilute formaldehyde solution shows 

 the presence of the latter by a distinct pink coloration, developing in 

 about half an hour at a dilution of 1 in 500,000. 



At a later stage our observations were confirmed by the Schryver 

 test, which was found to be the most reliable. 



Schryver 's test was carried out by preparing a 5 per cent, 

 solution of potassium ferricyanide and a 1 per cent, solution of 

 phenylhydrazine hydrochloride, which was used as follows: To 

 10 c.c. of the solution suspected of containing formaldehyde 2 c.c. of 

 the phenylhydrazine solution are added, then 1 c.c. of the potassium 

 ferricyanide solution followed by 3 c.c. of concentrated hydrochloric 

 acid, when a pink coloration appears. If this is diluted with water 

 it becomes colourless ; this colourless solution is shaken with ether, 

 and the ethereal solution which is not coloured is separated off. 

 Finally the ethereal solution is shaken up with a few cubic 

 centimetres of concentrated hydrochloric acid, when, if formalde- 

 hyde be present, a strong pink or carmine colour appears in the 

 hydrochloric acid layer. 



All of the colour tests for formaldehyde have been obtained on 

 many occasions. 



