INTRODUCTION. 39 



a foliated expansion, as in liverworts ; in the centre of this a protuberance 

 makes its appearance, which is finally prolonged above into a sort of stem, 

 and below into a root ; from this stem the true leaves are afterwards deve* 

 loped, when the primary one decays. 



VEGETABLE CHEMISTRY. 



It has already been .stated that vegetables consist chiefly of carbon, oxygen, 

 and hydrogen, with some nitrogen, especially in their products; to these are 

 added minute portions of potash, soda, sulphur, silica, &c. ; these, however, 

 bear but a small proportion to the three first, which make up the greater part 

 of all vegetables and their products. These ultimate principles are variously 

 combined, and thus form certain compounds, some of which are common to 

 all vegetable bodies, whilst others are peculiar to certain orders or genera. 

 Those which are common, either contain nitrogen, or are devoid of that 

 element. Those into which nitrogen enters as a component part, are, fibrin, 

 albumen, legumin or casein, and the vegetable alkaloids, &c. 



Fibrin. Is found in vegetable juices, and separates from them, when at 

 rest, in the form of a slight coagulum. It is contained largely in gluten, or 

 that substance remaining after wheat flour has been well washed with water. 

 It is insoluble in that fluid, but when long boiled in it, is altered in composi- 

 tion. By the aid of heat it is partially soluble in acetic acid. It is dissolved 

 by the alkalies, and when heated with them is decomposed, and gives out 

 ammonia. 



Albumen. Also found in vegetable fluids, and in the seeds and other parts 

 of plants. It is present in two forms, either fluid or coagulated. All vege- 

 table juices when heated, become turbid, from the albumen they contain be- 

 coming coagulated. Its properties and composition are nearly the same as 

 those of fibrin. 



Legumin or Casein. This is found in abundance in the seeds of the le- 

 guminosse. It is not coagulated by heat, but readily so by the weak acids. 

 The scum formed on the surface of water in which peas or beans are boiled, 

 is principally composed of this substance. Its properties are similar to other 

 articles of its class. 



Vegetable Alkaloids. These substances are very remarkable, and may be 

 considered as the most active class of vegetable principles. They all contain 

 nitrogen, and, as a consequence, when subjected to a high temperature, give 

 off ammonia, from the nitrogen uniting with a portion of the hydrogen. They 

 restore a blue reddened by an acid, and usually have an extremely bitter 

 taste. Most of them are solid and fixed, and when pure, are crystal lizable ; 

 are usually soluble in alcohol, but sparingly so in water. They form salts 

 with the acids, which are in most cases more soluble in water, than the 

 bases from which they are made. In plants they usually exist in the state 

 of a salt, being united to one or other of the vegetable acids. 



Vegetable Neutral Principles. These may all be said to contain nitrogen ; 

 as it is probable that those in which this element does not exist, may be con- 

 sidered as belonging to extractive. They unite with acids, and form crystal- 

 lizable compounds, but their solutions do not exhibit alkaline reactions, and 

 the salts they form are acid. In some plants, a substance of this class 

 occurs, called amygdaline, which is very interesting, from the fact that 

 on its admixture with emulsinc (a kind of albumen), a decomposition takes 



