OILS, ACIDS, ALKALOIDS, ETC. 57 



87. Tannin or Tannic Acid, which most abounds in older bark, is 

 probably a product of the oxidation or commencing decomposition 

 of the tissues. So, also, Humus, Humic Acid, Ulmine, Ulmic Acid, 

 and the numerous related substances distinguished by the chemists, 

 are products of further decomposition of vegetable tissue, rather 

 than true products of vegetation. 



88. Essential Oils, Turpentine, Caoutcliouc, &c. These are some of 



the Proper Juices of plants, pecuhar to certain plants, and occurring 

 under a great variety of forms in different species. It is not known 

 that they are of any account in vegetable growth or nutrition. They 

 undergo changes on exposure to the air, and become resins, gums, 

 &c. They are apt to be accumulated in intercellular cavities, or to 

 be excreted from the surface of the plant. Not kno'\\ang of what 

 use they are to the vegetable, .we are inclined to regard them as of 

 the nature of excretions. Caoutchouc exists in the form of minute 

 globules, diffused as an emulsion in the milky juice of plants, of 

 various families. The original India-Rubber of the East Indies is 

 the milky juice of a species of Fig. That of South America, now 

 so largely used for a great variety of purposes, comes from certain 

 trees of the Euphorbia family. It equally occurs in the juice of our 

 Milkweeds or Silkweeds. Gutta-Percha is a similar product of the 

 milky juice of a Sapotaceous tree of Borneo. 



89. The quaternary class of products (viz. those which consist of 

 the four elements, carbon, hydrogen, oxygen, and nitrogen, 79) are 

 of two kinds, the special and the general. The former are peculiar 

 to certain plants ; the latter are universal products of vegetation. 

 Examples of the special kind are found in Hydrocyanic or Prussic 

 Acid, already mentioned (8G), and the 



90. Alkaloids, such as Morphine, Strychnine, and Quinine. These 

 are principally formed in the bark and the leaves. We do not know 

 that they bear any part m vegetation, nor of what use they are to 

 the plant. In these substances reside the most energetic properties 

 of the vegetable, considered as to its action on the animal economy, 

 the most powerful medicines, and the most virulent poisons. That 

 they are of the nature of excretions may be inferred from the fact, 

 that a plant may be poisoned by its own products, introduced into its 

 ascending sap. 



91. The principal general quaternary product of plants is Pro- 

 teine, the nature and uses of which have already been explained 

 (27, 79). As it exists in living cells in a liquid or gelatinous 



