Book I. VEGETABLE CHEMISTRY. 225 



1499. The analysis of the ashes of plants, with a view to the discovery of the ingredients of which th^ 

 are composed, produces alkalies, earths, and metals, which must therefore be considered as ingredients in 

 the composition of the vegetable. But vegetable ashes contain also a variety of other principles, occur- 

 ring, however, in such small proportions as generally to escape observation. Perhaps they contain also 

 substances not capable of being volatilised by the action of fire. 



1500. 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 num- 

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

 monia 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 manufactured 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, and is extremely caustic and deliquescent. It dissolves all soft animal substances, and changes 

 vegetable blues into green. It dissolves alumina, and also 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 

 oxygen and a highly inflammable metal, which he denominates potassium, 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, Zostfera maritima, and various species of jPiici. 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 by 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 aHcalies, and the modes of obtaining 

 them. They are found generally in the state of carbonates, sulphates, or muriates, salts which form, beyond 

 all comparison, 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, turn- 

 sole, 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. 



1501. The utility of the alkalies, as obtained from vegetables, is of the utmost importance in the arts, 

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

 certain 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 may be moulded into almost any form, at the pleasure of the 

 artist : 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 silicious 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 made with the telescope and the 

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

 solution 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 a 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 found to be peculiarly efficacious in the reduction of 

 urinary calculi. 



1502. Earths. The only earths which have hitherto been found in plants are the 

 following : lime, silica, magnesia, and alumina. 



1503. Lwe is by far the most abundant earth. It is generally combined with a portion of phosphoric, 

 carbonic, or sulphuric acid, foifming 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 herbaceous 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 portion 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 1825. 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 145 parts of earthy phosphate ; in flower, only 135. 

 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 augmented. 

 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 carbonate 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 carbonate 

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

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



1504. 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 

 25 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. Perhaps 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 radical 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 inexpli- 

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

 of the genus jBquisfetum. Sir H. Davy 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 pro- 

 portions of silica were, in bonnet cane, 90 ; bamboo, 714 ; common reed, 48"1 ; stalks of corn, 66'5. 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 subtances where smoothness is required. The Dutch rush (jEquisfetum hyemale), a plant 

 of this kind, is used to polish even brass. 



1505. 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 i^uci ; but Salsbla Soda contain* 



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