164 WATER CULTURE, PHOTOSYNTHESIS, RESPIRATION. 



till the solution is distinctly acid again, and then ammonium acetate 

 in excess. Now add ferric chloride till no further buff-coloured 

 precipitate (ferric phosphate) is produced and the solution becomes 

 red (owing to ferric acetate). Boil the solution till it is colourless, 

 filter, and reject the precipitate. 



To the solution thus obtained add ammonium chloride, ammo- 

 nia, and ammonium carbonate ; a white precipitate indicates the 

 presence of lime. Filter, and to the filtrate add sodium phos- 

 phate : a white precipitate (often formed only after shaking the 

 liquid and letting it stand for some minutes) shows that magnesia 

 is present. 



Filter, evaporate the filtrate to dryness, and test the residue for 

 soda and potash. Add a few drops of platinic chloride to the 

 residue, evaporate again, then add some alcohol : a yellow crystal- 

 line precipitate shows that potash is present. Or dip a clean 

 platinum wire into hydrochloric acid and hold it in a Bunsen or 

 spirit-lamp flame until it no longer colours the flame yellow (owing to 

 presence of soda). Then dip the wire, moistened with hydrochloric 

 acid (strong), into the residue and put it in the flame. Potash. 

 turns the flame violet, but if the yellow (soda) colour is too strong 

 look at the flame through a thick piece of blue glass : the soda colour 

 is cut off and the reddish-violet potash flame is seen. 



217. Water Culture. Analysis shows that in all 

 plants at least 13 elements are present Potassium (K), 

 Sodium (Na), Calcium (Ca), Magnesium (Mg), Iron (Fe), 

 Carbon (C), Hydrogen (H), Oxygen (O), Nitrogen (N), 

 Sulphur (S), Phosphorus (P), Silicon (Si), and Chlorine 



(01). 



Since various other elements occur in different plants, 

 analysis alone leaves it doubtful whether all these elements 

 are essential for life and healthy growth although this 

 is obviously the case with the elements C, H, and which 

 are present in all the organic compounds found in plants, 

 with N and S which are present (along with C, H, and 0) 

 in proteins, and also with P which is an essential element 

 (along with C, H, O, N, and S) in iiucleo-proteins. 



However, in order to ascertain definitely which elements 

 are indispensable for the nutrition of a green plant, we 

 must offer its roots a solution of salts containing (1) all 

 the essential elements, (2) solutions in which one or other 

 of these elements is omitted. This method of research is 

 called " water culture," and the solutions used a*re called 

 " culture solutions." 



