SUGAR. 83 



nitric, sulphuric, or muriatic acid during thirty-six hours, it first 

 changes its opalescent appearance, and becomes colorless and trans- 

 parent ; losing at the same time its power of striking a blue color 

 with iodine. After a time, it begins to acquire a sweet taste, and 

 is finally altogether converted into a peculiar species of sugar. 



Secondly, by contact with certain animal and vegetable sub- 

 stances. Thus, boiled starch mixed with human saliva and kept 

 at the temperature of 100 F., is converted in a few minutes into 

 sugar. 



Thirdly, by the processes of nutrition and digestion in animals 

 and vegetables. A large part of the starch stored up in seeds and 

 other vegetable tissues is, at some period or other of the growth of 

 the plant, converted into sugar by the molecular changes going on 

 in the vegetable fabric. It is in this way, so far as we know, that 

 all the sugar derived from vegetable sources has its origin. / 



Starch, as a proximate principle, is more especially important as 

 entering largely into the composition of many kinds of vegetable 

 food. With these it is introduced into the alimentary canal, and 

 there, during the process of digestion, is converted into sugar. 

 Consequently, it does not appear in the blood, nor in any of the 

 secreted fluids. 



2. SUGAR. This group of proximate principles includes a con- 

 siderable number of substances, which differ in certain minor 

 details, while they resemble each other in the following particulars : 

 They are readily soluble in water, and crystallize more or less 

 perfectly on evaporation ; they have a distinct sweet taste ; and 

 finally, by the process of fermentation, they are converted into 

 alcohol and carbonic acid. 



These substances are derived from both animal and vegetable 

 sources. Those varieties of sugar which are most familiar to us 

 are the following six, three of which are of vegetable and three of 

 animal origin. 



c Cane sugar, f Milk sugar, 



Vegetal J Grape sugar> Animal 1 Liver sugar? 



su S ars - 1 Sugar of starch. su S ars ' I Sugar of honey. 



The cane and grape sugars are held in solution in the juices of 

 the plants from which they derive their name. Sugar of starch, or 

 glucose, is produced by boiling starch for a long time with a dilute 

 acid. Liver sugar and sugar of milk are produced in the 

 tissues of the liver and the mammary gland, and the sugar of 



