114 PHYSIOLOGICAL CHEMISTRY. 



formly distributed. Then stir in enough flour to make a thick cream and 

 allow to stand over night at room temperature. In this time fermenta- 

 tion of the small amount of sugar in the flour begins and the " sponge " 

 swells up by the escape of bubbles of gas. At this stage mix in uniformly 

 and thoroughly enough flour to make a stiff dough, using for the purpose 

 perhaps 25 gm. Put the dough in an evaporating dish, keep it for an 

 hour or more at a temperature of 30 to 35 C. and observe that it in- 

 creases very greatly in size, from the continued action of the yeast in 

 liberating bubbles of carbon dioxide. If a good hot air oven is at hand 

 the experiment is completed by baking the leavened mass. The nature 

 of the yeast fermentation will be explained later. 



Milk. In milk we have a substance in which all the essen- 

 tial food elements are present. The average composition of 

 cow's milk is given in this table. 



Water 874 



Fat 3-5 



Sugar 4-5 



Proteins 3-9 



Salts .. 0.7 



IOO.O 



Human milk contains more sugar and less protein than the 

 milk of the cow. Details of this will be given later. 



