CHAPTER II 



Chemical Composition of Organisms 



Carbon. Heat a little yeast or any organic tissue in a dry test- 

 tube. It will char, owing to the production of carbon. 



Hydrogen. If the tissue had been previously dried in the above 

 experiment, the deposition of water on the upper part of the test- 

 tube will show the presence of hydrogen. 



Oxygen. That the oxygen contained in the water of the above 

 experiment comes from the tissue, and not from the air, can be 

 shown by performing the experiment in an atmosphere of coal 

 gas. The test-tube is closed by a cork through which pass two 

 glass tubes, one to the lower end, the other ending just below the 

 cork. The former is connected to the gas supply by means of a 

 rubber tube, the latter to a Bunsen burner. Gas is passed through 

 the tube until the air is displaced. The Bunsen burner is then 

 lit and the tube heated as before. 



Nitrogen. Mix the yeast, dried, with some dry soda-lime. 

 Heat in the test-tube. Ammonia is given off, detected by its 

 smell, and its turning moist red litmus paper blue. The fumes 

 given off when hydrochloric acid on a glass rod is held at the 

 mouth of the test-tube are also characteristic of ammonia. 



Sulphur and Phosphorus. Boil with a little strong nitric 

 acid to oxidise the sulphur to sulphate and the phosphorus to 

 phosphate. Dilute with water. Filter if necessary. Test a part 

 with barium chloride, precipitate shows sulphate. Add to another 

 part a few drops of a strong solution of ammonium molybdate and 

 heat. A yellow precipitate indicates phosphate. 



The Polarimeter 



A simple form of this instrument will serve to ascertain the 

 fact of optical activity. In fact, if a strong solution of sugar or 

 of egg-white be used in a vessel with flat glass sides, two Nicol 

 prisms may suffice. A beam of light is sent through one of these, 

 clamped in a retort stand, in order to polarise it. The beam then 

 passes through the solution and, lastly, through the second Nicol, 



181 



