FOODS AND THE THEORY OF DIGESTION 135 



O.iygert. Review experiments (page 114) showing the properties 

 of oxygen. 



Phosphorus. Examine a small piece of phosphorus, noting that it 

 has to be kept under water. Lay a small piece on the table and observe 

 the tiny stream of white smoke rising from it, formed by slow oxida- 

 tion. Dissolve a piece as large as a pea in a teaspoonful of carbon 

 disulphide in a test tube, pour this on a piece of porous paper, and lay 

 the paper on an iron support. When the carbon disulphide evaporates 

 the phosphorus takes fire spontaneously. (The heat from the slow 

 oxidation is sufficient to ignite the phosphorus in the finely divided 

 condition.) What is the most striking property of phosphorus? What 

 purpose does it serve in the match ? 



Sulphur. Examine some sulphur, noting its color and the absence 

 of odor or taste. (Impure sulphur may have an odor and a taste.) 

 Burn a little sulphur in an iron spoon, noting that the compound which 

 it forms with oxygen by burning has a decided odor. 



Other Elements. Magnesium. Examine and burn a piece of 

 magnesium ribbon, noting the white compound of magnesium oxide 

 which is formed. Iron. Examine pieces of the metal and also some 

 of its compounds, as ferrous sulphate, ferric chloride, and ferric oxide 

 or iron rust. Sodium. Drop a piece of the metal on water and observe 

 results. Sodium decomposes water. It has to be kept under some 

 liquid, such as kerosene, which contains no oxygen. (It should not be 

 touched except with the fingers wet with kerosene.) Chlorine. Pour 

 strong hydrochloric acid on a little manganese dioxide in a test tube, 

 and warm gently over a low flame. The escaping gas is chlorine. 

 Avoid breathing much of it. 



Composition of the Nutrients. The simplest way of determining 

 what elements make up the different nutrients is by heating them and 

 studying the products of decomposition, as follows : 



To show that Carbohydrates contain Carbon, Hydrogen, and Oxy- 

 gen. Place one half teaspoonful of powdered starch in a test tube and 

 heat strongly. Observe that water condenses on the sides of the tube 

 and that a black, charred mass remains behind. The black mass consists 

 mainly of carbon. The water is composed of hydrogen and oxygen. 

 These three elements are thus shown to be present in the starch. The 

 experiment may be repeated, using sugar instead of starch. 



To show that Proteids contain Carbon, Hydrogen, Oxygen, Nitrogen, 

 and Sulphur. Place in a test tube some finely divided proteid 



