WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 163 



weigh the rest of the dried leaves, place them in a weighed 

 crucible, cover them with a weighed quantity of dried 

 sand, and after about ten minutes' heating turn out the 

 contents and find the weight of the charcoal (carbon 

 + ash). 



216. Ash Analysis. The chief elements to be tested 

 for in analysing the ash of plants are Calcium, Potassium, 

 Magnesium, Phosphorus, Sulphur (the two latter beiiig 

 present as acids) . The ash should not be heated so strongly 

 as to make it burst into flame. 



(a) Is the ash soluble in (1) water, (2) dilute hydrochloric acid, 

 (3) strong hydrochloric acid? Find out in each case by boiling 

 some of the ash in a test-tube with water or acid, allowing the un- 

 dissolved part to subside and evaporating some of the liquid, or 

 heating it to dryness, on a watch-glass or evaporating-dish. The 

 insoluble residue, after treatment with strong acid, contains chiefly 

 silica and carbon. 



(b) Place about 10 grams of ash in a 500 c.c. flask, moisten it 

 with a small quantity of strong nitric acid, then add about 20 c.c. 

 strong hydrochloric acid and heat on a tripod (or " digest "'it for 

 half an hour on a water or sand bath at boiling-point). Rinse the 

 contents of the flask into an evaporating basin and heat to dryness. 

 Moisten the residue with strong hydrochloric acid, add about 

 200 c.c. of water, and filter. Make the filtrate up to 600 c.c. with 

 water and divide it into four parts : 



(i) To one part add, in a large test-tube, some barium chloride 

 solution. The finely divided white precipitate (barium sulphate) 

 indicates the presence of sulphur (as sulphuric acid). Verify this 

 by mixing some dry ash with carbonate of soda, heat on charcoal 

 with the reducing blowpipe-flame, and (1) put a few drops of dilute 

 hydrochloric acid on the fused mass (the sulphuretted hydrogen 

 given off is easily recognised by its odour), (2) put a little of the 

 mass on a silver coin and add a drop of dilute acid (a black stain of 

 silver sulphide is formed). These "dry" tests may fail, however, 

 if but little sulphuric acid is present. 



(ii) To some ash solution in a test-tube add an equal bulk of 

 strong nitric acid, then three or four times its bulk of ammonium 

 molybdate. A yellow precipitate indicates presence of phosphorus 

 (as phosphoric acid). 



(iii) To test for iron, add potassium ferrocyanide : a dark blue 

 precipitate (Prussian blue) is produced. 



(iv) It is necessary to remove the phosphates from the ash solu- 

 tion, as follows. Neutralise with ammonia, then add acetic acid 



