FOOD OF VEGETABLES. 143 



hydrogen and oxygen in tlie same proportions as water. These three bodies, formed 

 from the same elements in similar proportions, are called isomerous bodies ; the 

 difference between them consists entirely in the manner in which their molecules 

 are grouped ; it is therefore simply necessary to derange these molecules to convert 

 dextrine, cellulose, and starch into each other. 



The sugar yielded by the Sugar-cane, Beetroot, and many other vegetables is also 

 a ternary compound very similar to the preceding ones, containing one molecule 

 more of water than starch, dextrine, and cellulose contain. 



Glucose or grape-sugar only differs from cane-sugar in containing three molecules 

 more of water. Thus starch or dextrine, with an additional molecule of water, 

 becomes cane-sugar ; and grape-sugar from which three molecules of water are 

 abstracted becomes cane-sugar. 



Organic acids, such as acetic acid, which is found in the sap of plants, and forms 

 in sour wine, pectic acid in the gooseberry, tartaric acid in grapes, malic acid in 

 apples, citric acid in the lemon and other fruits, gallic acid in oak-galls and bark, &c., 

 are ternary compounds which contain carbon and the elements of water (oxygen and 

 hydrogen), plus a certain quantity of oxygen. Oils, essences, resins, chromule or 

 chlorophyll, are ternary compounds, formed by the combination of carbon with the 

 elements of water, plus a certain quantity of hydrogen. 



Besides these, vegetables contain, especially in their bark, quaternary compounds 

 of carbon, hydrogen, oxygen, and nitrogen ; these crystallize, and are always found 

 in union with an organic acid which forms a salt with them, whence their name of 

 vegetable alkalies. 



The Poppy contains morphine, narcotine, &c. ; the Nux vomica, strychnine ; the 

 genus Cinchona, quinine, cinchonine, and cusconine. Experience has proved that the 

 poisonous and medicinal properties of vegetables reside in the organic alkalies. 



Other organic substances frequently found in vegetables are still more com- 

 plicated ; for, besides oxygen, hydrogen, carbon, and nitrogen, plants contain sulphur 

 and phosphorus : these are albumen, fibrine, and casein ; the proportions of their 

 elements are similar, although their physical properties are different ; whence the 

 name ot protein by which chemists designate the essential principle- of all those 

 substances that are collectively designated albuminous. Protein has been alluded 

 to under the nucleus ; it constitutes the nutritious element of vegetables, for without 

 it no blood can be formed, and it is always found in this liquid. Fibrine is a 

 compound substance, insoluble in water, like cellulose ; it may be looked upon as the 

 origin of all the parts of a plant ; it always exists in them, and especially in the 

 seeds of cereals. Albumen coagulates with heat like starch; it constitutes nearly 

 all the serum of blood and the white of eggs, and abounds in the juices of plants. 

 Casein, which forms with starch the nutritive part of beans, lentils, and peas, 

 constitutes essentially, in the milk of animals, the nutriment that the young receives 

 from its mother. Gluten, which forms the base of leaven or yeast, exists in 

 most seeds, and is composed of the same elements (less the sulphur and phosphorus) 

 as albumen, fibrine, and casein. 



The elements of carbonic acid {oxygen and carbon), of ammonia {hydrogen and 



