226 SCIENCE OF AGRICULTURE. Tart II. 



more of magnesia than any other plant yet examined. According to Vauquelin, 100 parts of it contain 

 17-929 of Magnesia. 



1506. Alumina has been detected in several plants, but never except in very small quantities. 



1507. Metallic oxides. Among the substances found in the ashes of vegetables, we must class also metals. 

 They occur, however, only in small quantities, and arc not to be detected except by the most delicate 

 experiments. The metals hitherto discovered in plants are iron, manganese, and perhaps gold. Of these 

 iron is by far the most common. It occurs in the state of an oxide ; and the ashes of hard and woody plants, 

 such as the oak, are said to contain nearly one twelfth of their own weight of this oxide. The ashes of 

 Salsbla contain also a considerable quantity. The oxide of manganese was first detected in the ashes of 

 vegetables byScheele, and afterwards found by Proust in the ashes of the pine, calendula, vine, green oak, 

 and fig tree. Beccher, Kunckel, and Sage, together with some other chemists, contend also for the 

 existence of gold in the ashes of certain plants ; but the very minute portion which they found, seems more 

 likely to have proceeded from the lead employed in the process, than from the ashes of the plant. It has 

 been observed by Saussure, that the proportion of the oxides of iron and of manganese augments in the 

 ashes of plants as their vegetation advances. The leaves of trees furnish more of these principles in 

 autumn than in spring, as do those of annual plants. Seeds contain metals in less abundance than the stem ; 

 and if plants are washed in water, the proportions of their metallic oxides are augmented. 



1508. Such are the principal ingredients that enter into the vegetable composition. They are indeed 

 numerous, though some of them, such as the metallic oxides, occur in such small proportions as to render 

 it doubtful whether they are in reality vegetable productions or not. The same thing may be said of some 

 of the other ingredients that have been found in the ashes of plants, which it is probable have been 

 absorbed ready formed by the root, and deposited unaltered, so that they can scarcely be at all regarded as 

 being the genuine products of vegetation. 



1509. Other substances. Besides the substances above enumerated, there are also several others which have 

 been supposed to constitute distinct and peculiar genera of vegetable productions, and which might have 

 been introduced under such a character; such as the mucus, jelly, sarcocol, asparagin, inulin, and ulmin, 

 of Dr. Thomson, as described in his well known Si/ste?n of Chemistry i but as there seems to be some 

 difference of opinion among chemists with regard to them, and a belief entertained that they are but 

 varieties of one or other of the foregoing ingredients, it is sufficient for the purposes of this work to have 

 merely mentioned their names. Several other substances, of a distinct and peculiar character, have been 

 suspected to exist in vegetable productions : such as the febrifuge principle of Seguin, as discovering itself 

 in Peruvian bark ; the principle of causticity or acridity of Senebier, as discovering itself in the roots of 

 i?anvinculus bulbbsus, 6'cilla maritima, .Bryonia alba, and .4Vum maculatura, in the leaves of Digit^ilis pur- 

 purea, in the bark of Daphne Meztreon, and in the juice of the spurges : to which may be added the fluid 

 exuded from the sting of the common nettle, the poisons inherent in some plants, and the medical virtues 

 inherent in others ; together with such peculiar principles as may be presumed to exist in such regions 

 of the vegetable kingdom as remain yet unexplored. The important discoveries which have already 

 resulted from the chemical analysis of vegetable substances encourage the hope that further discoveries will 

 be the result of further experiment ; and, from the zeal and ability of such chemists as are now directing 

 their attention to the subject, every thing is to be expected. 



Sect. II. Simple Products. 



1510. A very few constituent and uncompounded elements include all the compound 

 ingredients of vegetables. The most essential of such compounds consist of carbon, 

 oxygen, and hydrogen ; a small proportion of nitrogen is said to be found only in cruci- 

 form plants. The remaining elementary principles which plants have been found to 

 contain, although they may be necessary in the vegetable economy, yet they are by no 

 means principles of the first importance, as occurring only in small proportions, and be- 

 ing dependent in a great measure on soil and situation ; whereas the elements of carbon, 

 oxygen, and hydrogen form as it were the very essence of the vegetable subject, and 

 constitute by their modifications the peculiar character of the properties of the plant. 

 This is conspicuously exemplified in the result of the investigations of Gay Lussac, and 

 Thenard, who have deduced from a series of the most minute and delicate experiments 

 the three following propositions, which they have dignified by the name of Laws of Ve- 

 getable Nature (Traite de Chem. Element., tom. iii. chap, iii.) : 1st, Vegetable sub- 

 stances are always acid, when the oxygea they contain is to the hydrogen in a greater 

 proportion than in water ; 2dly, Vegetable substances are always resinous, or oily, or 

 spirituous, when the oxygen they contain is to the hydrogen in a smaller proportion than 

 in water ; 3dly, Vegetable substances are neither acid nor resinous, but saccharine, or 

 mucilaginous, or analogous to woody fibre or starch, when the oxygen and hydrogen they 

 contain are in the same proportion as in water. (See Dr. Thomson* s System of Chemistry.) 



Chap. IV. 



Functions of Vegetables. 



1511. The life, growth, and propagation of plants necessarily involve the several 

 following topics : germination, nutriment, digestion, growth and developement of parts, 

 anomalies of vegetable developement, sexuality of vegetables, impregnation of the 

 vegetable germen, changes consequent upon impregnation, propagation and dispersion 

 of the species, causes limiting the dispersion of the species, evidence and character of 

 vegetable vitality. 



