INORGANIC CONSTITUENTS 17 1 



condition but in combination with acids, forming phosphates, car- 

 bonates, sulphates, chlorides, etc. When a plant is burned these 

 substances are found in its ash. Probably they do not all occur 

 in the plant in the same forms in which they are found in the ash, 

 the heat of combustion being doubtless responsible for new com- 

 binations. The essential salts of plants may form from 1.5 to 5 

 per cent, of their dry weight, where these only are to be had, 

 but when much unnecessary salt is available, the percentage of 

 mineral matter is usually much greater. 



240. Qualitative Determination of mineral Constituents. Be- 

 sides the mineral substances already mentioned as necessary, 

 many others also occur in plants, but for the present purpose it 

 will suffice simply to notice those which are most common. 



Ash for analysis may be obtained by carefully burning a quan- 

 tity of the tissue concerned. The ash should be separated into 

 three parts, first by removing as much as will dissolve in water, 

 then dissolving as much as possible of the remainder in hydro- 

 chloric acid, and retaining the residue insoluble in each. Boil 

 the ash with water, filter and wash the residue (Solution I). 



Treat portions of this solution as follows : 



1. Evaporate a portion to a small quantity and add hydro- 

 chloric acid. Effervescence indicates carbonic acid, probably from 

 carbonates of alkali earths. If lead acetate paper is darkened by 

 the escaping gas, sulphur in the form of sulphide is also present. 



2. Treat another portion as in 1. Apply a few drops to yel- 

 low turmeric paper, and dry at gentle heat. Boric acid is indi- 

 cated by a red color. Evaporate the solution to dryness and add 

 very dilute hydrochloric acid. Allow to stand for a few minutes 

 and filter. Divide the filtrate into two parts, (a) Add ammo- 

 nium hydrate and magnesia mixture ; phosphoric acid is in- 

 dicated by a white precipitate, (b) Evaporate to dryness on the 

 water-bath with excess of nitric acid. Dissolve the residue in ni- 

 tric acid and add molybdic solution. A yellow precipitate indi- 

 cates phosphates. 



3. Add silver nitrate till no more precipitate forms. A dark 



