156 SCIENCE OF GARDENING. Tart II. 



705. Alkalies. The alkalies are a peculiar class of substances, distinguished by a caustic taste and the 

 property of changing vegetable blues to green. They are generally regarded as being three in number, 

 potass, soda, and ammonia, of which the two former only are found in the ashes of vegetables. Ammonia 

 is, indeed, often obtained from vegetable substances by means of distillation, but then it is always formed 

 during the process. If the ashes of land vegetables, burnt in the open air, are repeatedly washed in water, 

 and the water filtered and evaporated to dryness, potass is left behind. The potass of commerce is manufac- 

 tured in this manner, though it is not quite pure. But it may be purified by dissolving it in spirits of wine, 

 and evaporating the solution to dryness in a silver vessel. When pure it is white and semi-transparent, 

 aud is extremely caustic and deliquescent. It dissolves all soft animal substances, and changes vegetable 

 blues into green. It dissolves alumina, and alsc a small quantity of silex, with which it fuses into glass by 

 the aid of fire. It had been long suspected by chemists to be a compound substance ; and according to 

 the notable discovery by Sir H. Davy, its component parts are at last ascertained to be a highly inflam- 

 mable metal, which he denominates potassium, and oxygen — one proportion of each. Soda is found 

 chiefly in marine plants, from the ashes of which it is obtained by means of lixiviation. It exists in great 

 abundance in salsola soda, zostera maritima, and in various species of fuci. It is generally obtained in the 

 state of a carbonate, but is purified in the same manner as potass, to which it is similar in its properties; 

 but from which it is easily distinguished bv its forming a hard soap with oil, while potass forms a soft soap. 

 It consists, according to Sir H. Davy, of one proportion of a metal which he denominates sodium, and 

 two proportions of oxygen. Such are the only vegetable alkalies, and modes of obtaining them. They 

 are found generally in the state of carbonates, sulphates, or muriates, salts that form beyond all compa- 

 rison the most abundant ingredient in the ashes of green herbaceous plants whose parts are in a state of 

 vegetation. The ashes of the golden rod, growing in an uncultivated soil, and of the bean, turnsol, and 

 wheat, were found by Saussure to contain at least three fourths of their weight of alkaline salts. This 

 was nearly the case also with the leaves of trees just bursting from the bud. But the proportion of alkaline 

 salts is found to diminish rather than to augment as the parts of the plant are developed. The ashes of 

 the leaves of the oak, gathered in May, yielded 47 parts in the 100, of alkaline salts; and in September, 

 only 17. . , 



70S. The utility of the alkalies, as obtained from vegetables, is of the utmost importance in the arts, par- 

 ticularly in the formation of glass and of soaps. If a mixture of soda or potass, and silex or sand, in cer- 

 tain proportions, is exposed to a violent heat, the ingredients are melted down into a fluid mass, which is 

 glass in a state of fusion. In this state it mav be moulded into almost any form at the pleasure of the ar- 

 tist And accordingly we find that it is manufactured into a great variety of utensils and instruments, 

 under the heads of flint-glass, crown-glass, bottle-glass. Bottle-glass is the coarsest ; it is formed of soda 

 and common sand, and is used in the manufacture of the coarser sort of bottles. Crown-glass is composed 

 of soda and fine sand : it is moulded into large plates for the purpose of forming window-glasses and 

 looking-glasses. Flint-glass is the finest and most transparent of all : that which is of the best quality is 

 composed of 120 parts of white siliceous sand, 40 parts of pearl-ash, 35 of red oxide of lead, 13 of nitrate of 

 potass, and 25 of black oxide of manganese. It is known also by the name of crystal, and may be cut and 

 polished so as to serve for a variety of ornamental purposes, as well as for the more important and more 

 useful purpose of forming optical instruments, of which the discoveries of the telescope and the micro- 

 scope are the curious or sublime results. If a quantity of oil is mixed with half its weight of a strong so- 

 lution of soda or potass, a combination takes place which is rendered more complete by means of boiling. 

 The new compound is soap. The union of oil with potass forms soft soap, and with soda hard soap; sub- 

 stances of the greatest efficacy as detergents, and of the greatest utility in the washing and bleaching of 

 linen. The alkalies are used also in medicine, and are found to be peculiarly efficacious in the reduction 

 of urinary calculi. 



707. Earths. The only earths which have hitherto been found in plants are the following : lime, silica, 

 magnesia, alumina. . . 



708. Of these earths, lime is by far the most abundant. It is generally combined with a portion of phos- 

 phoric, carbonic, or sulphuric acid, forming phosphates, or carbonates, or sulphates of lime. The 

 phosphate of lime is, next to the alkaline salt, the most abundant ingredient in the ashes of green herbace- 

 ous plants, whose parts are all in a state of vegetation. The leaf of a tree, bursting from the bud, contains 

 in its ashes a greater proportion of earthy phosphate than at any other period: 100 parts of the ashes of the 

 leaves of the oak, gathered in May, furnished 24 parts of earthy phosphate ; in September, only 18"25. In 

 annual plants the proportion of earthy phosphate diminishes from the period of their germination to that 

 of their flowering. Plants of the bean, before flowering, gave 14-5 parts of earthy phosphate; in flower, 

 only 13-5 Carbonate of lime is, next to phosphate of lime, the most abundant of the earthy salts that are 

 found in vegetables. But if the leaves of plants are washed in water the proportion of carbonate is aug- 

 mented. This is owing to the subtraction of their alkaline salts and phosphates in a greater proportion 

 than their lime. In green herbaceous plants, whose parts are in a state of increase, there is but little car- 

 bonate of lime ; but the ashes of the bark of trees contain an enormous quantity of carbonate of lime, and 

 much more than the alburnum, as do also the ashes of the wood. The ashes of most seeds contain no car- 

 bonate of lime; but they abound in phosphate of potass. Hence the ashes of plants, at the period of the 

 maturity of the fruit, yield less carbonate of lime than at any previous period. , 



709 Silica is not found to exist in a great proportion in the ashes of vegetables, unless they have been 

 previously deprived of their salts and phosphates by washing ; but when the plants are washed in water, 

 the proportion of their silica augments. The ashes of the leaves of the hazel, gathered in May, yielded 

 2-5 parts of silica in 100. The same leaves, washed, yielded four parts in 100. Young plants, and leaves 

 bursting from the bud, contain but little of silica in their ashes ; but the proportion of silica augments as 

 the parts are developed. But nerhaps this is owing to the diminution of the alkaline salts. The ashes of 

 some stalks of wheat gathered\a month before the time of flowering, and having some of the radicle leaves 

 withered, contained 12 parts of silica and 65 of alkaline salts in 100. At the period of their flowering, and 

 when more of their leaves were withered, the ashes contained 32 parts of silica and 54 of alkaline salts. Seeds 

 divested of their external covering, contain less silica than the stem furnished with its leaves ; and it is 

 somewhat remarkable that there are trees of which the bark, alburnum, and wood, contain scarcely any 

 silica, and the leaves a great deal, particularly in autumn. This is a phenomenon that seems inexplicable. 

 The greater part of the grasses contain a very considerable proportion of silica, as do also the plants ot the 

 •*enus equisetum. Sir H. Daw has discovered that it forms a part of "the epidermis of these plants, and in 

 some of them the principal part. From 100 parts of the epidermis of the following plants the proportions 

 of silica were, in bonnet cane, 90; bamboo, 71 '4; common reed, 481; stalks of corn, 66 p. Owing to the 

 silica contained in the epidermis, the plants in which it is found, are sometimes used to give a polish to the 

 surface of substances where smoothness is required. The Dutch rush, equisetum hyemale, a plant of this 

 kind, is used to polish even brass. ,. ... 



710 Magnesia does not exist so abundantly in the vegetable kingdom as the two preceding earths. It 

 has been found, however, in several of the marine plants, particularly the fuci ; but salsola soda contains 

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

 17 '929 of magnesia. . 



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



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

 metals They occur, however, only in small quantities, and are not to be detected except by the most de- 

 licate 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, arc said to contain nearly one twelfth of their own weight of this oxide, lne ashes 



