190 PRACTICAL PHYSIOLOGY 



Nitrogen may also be detected by the soda lime test, which is 

 perhaps more easily carried out, but is not quite so reliable as the 

 metallic sodium reaction (Lassaigne's test). 



Mix a small quantity of the substance with many times (at least 20) 

 its weight of soda lime in a dry test tube and heat strongly. Test 

 the vapours formed for ammonia by (a) red litmus paper, (6) a 

 glass, rod moistened with hydrochloric acid, (c) smell. 



Sulphur. To another portion of the nitrate add a few drops of a 

 freshly prepared solution of sodium nitro-prusside. If sulphur be 

 present solution assumes a reddish violet colour. 



The presence of sulphur can also be shown by adding a few drops 

 of concentrated sulphuric acid to a portion of the filtrate, place a 

 piece of filter paper moistened with lead acetate solution across the 

 mouth of the test tube and heat. If sulphur be present black lead 

 sulphide is formed. 



Phosphorus. To another portion of the filtrate add nitric acid, 

 until markedly acid, then a few drops of ammonium molybdate 

 solution. If phosphorus be present a yellow precipitate of am- 

 monium phospho -molybdate will form on standing. 



The presence of halogens may also be detected in the filtrate by 

 the ordinary methods. 



Tests for carbon and hydrogen can also be carried out by heating 

 the substance mixed with at least ten times its own weight of 

 dry cupric oxide in a hard glass test tube fitted with a rubber 

 cork carrying a length of glass tubing. Tubing is led to a bottle 

 containing baryta water. Heat the test tube containing the 

 mixture, small drops of water will condense in the upper colder part 

 of the test tube and glass tubing and a precipitate of barium 

 carbonate will form in the baryta water. 



A general test for carbohydrates should also be carried out. 

 Molisch's furfurol test with a-naphthol is too delicate for general 

 use. Ordinary hydrolysis with a strong mineral acid and a reduction 

 test is preferable. Hydrolyse a small portion of the substance 

 suspended in water by boiling, after the addition of 1-2 c.c. of 

 sulphuric or hydrochloric acid, for five minutes. Cool under the 

 tap. Add a drop or two of copper sulphate solution, then strong 

 sodium hydrate solution cautiously and with frequent shaking until 

 a clear blue solution results. Boil. If carbohydrate be present 

 reduction will take place. 



The only reliable general test for the presence of fat is extraction 

 with ether by grinding up a portion of the material in a porcelain 

 basin with small amounts of ether, filtering the ethereal extracts 

 and then evaporating. This must be done without the use of a 

 free flame ; an electric hot plate serves admirably. 



Choice of Methods 



As regards quantitative methods it may be stated, speaking 

 generally, that a gravimetric method is to be preferred to a volu- 



