224 SCIENCE OF AGRICULTURE. Part II. 



to analysed Is yet but very limUed. It is the grand and principal source of vegetable aliment, and may be 

 regarded as being somewhat analogous to the blood of animals. It is not made use of by man, at least in 

 its natural state. But there are trees, such as the birch, whose sap may he manufactured into a very 

 pleasant wine ; and it is well known that the sap of the American maple-tree yields a considerable 

 quantity of sugar. 



1470. T/ie proper juice. "When the sap has received its last degree of elaboration from the different or- 

 gans through which it has to pass, it is converted into a peculiar fluid, called the proper juice. This fluid 

 may be distinguished from the sap by means of its color, which is generally' green, as in periwinkle ; or 

 red, as in logwood ; or white, as in spurge ; or yellow, as in celandine ; from the two last of which it may 

 readily be obtained by breaking the stem asunder, as it will then exude from the fracture. Its principal 

 seat is in the bark, where it occupies the simple tubes ; but sometimes it is situated between the bark and 

 wood, as in the juniper-tree ; or in the leaf, as in the greater part of herbs ; or it is diffused throughout 

 the whole plant, as in the fir and hemlock ; in which case, either the proper juice mixes with the sap, or 

 the vessels containing it have ramifications so fine as to be altogether imperceptible. It is not, however, 

 the same in all plants, nor even in the different parts of the same plant. In the cherry-tree it is mucila- 

 ginous ; in the pine it is resinous ; in spurge and celandine it is caustic, though resembling in appearance 

 an emulsion. In many plants the proper juice of the bark is different from that of the flower ; and the 

 proper juice of the fruit different from both. Its appearance under the microscope, according to Senebier, 

 is that of an assemblage of small globules connected by small and prism-shaped substances placed between 

 them. If this juice could be obtained in a state of purity, its analysis would throw a considerable degree 

 of light upon the subject of vegetation. But it seems impracticable to extract it without a mixture of 

 sap. Senebier analysed the milky juice of euphorbia cyparissias, of which he had procured a small 

 quantity considerably pure, though its pungency was so great as to occasion an inflammation of the eyes 

 to the person employed to procure it. It mixed readily with water, to which it communicated its color. 

 When left exposed to the air a slight precipitation ensued ; and when allowed to evaporate a thin and 

 opaque crust remained behind. Alcohol coagulated it into small globules. Ether dissolved it entirely, as 

 did also oil of turpentine. Sulphuric acid changed its color to black ; nitric acid to green. The most ac- 

 curate experiments on the subject are those of Chaptal. When oxymuriatic acid was poured into the 

 peculiar juice of euphorbia, a very copious white precipitate fell down, which, when washed and dried, 

 had the appearance of starch, and was not altered by keeping. Alcohol, aided by heat, dissolved two 

 thirds of it, which the addition of water again precipitated. They had all the properties of resin. The 

 remaining third part possessed the properties of woody fibre. The same experiment was tried on the juice 

 of a variety of other plants, and the result uniformly was that oxymuriatic acid precipitated from them 

 woody fibre. 



1471. The virtues of plants have generally been thought to reside in their proper juices, and the opinion 

 seems indeed to be well founded. It is at least proved by experiment in the poppy, spurge, and fig. The 

 juice of the first is narcotic, of the two last corrosive. The diuretic and balsamic virtues of the fir reside 

 in its turjientine, and the purgative property of jalap in its resin. If sugar is obtained from the sap of the 

 sugar-cane and maple, it is only because it has been mixed with a quantity of proper juice. The bark 

 certainly contains it in greatest abundance, as may be exemplified in cinnamon and quinquina. But the 

 peach-tree furnishes an exception to this rule : its flowers are purgative, and the whole plant aromatic ; 

 but its gum is without any distinguished virtues. Malpighi regarded the proper juice as the principle 

 of nourishment, and compared it to the blood of animals ; but this analogy does not hold very closely. 

 The sap is, perhaps, more analogous to the blood, from which the proper juice is rather a secretion. In 

 one 'respect, however, the analogy holds good, that is, with regard to extravasated blood and peculiar 

 juices. If the blood escapes from the vessels it forms neither flesh nor bones, but tumors ; and if the pro- 

 per juices escapes from the vessels containing them, they form neither wood nor bark, but a lump or 

 deposit of inspissated fluid. To the sap or to the proper juice, or rather to a mixture of both, we must re- 

 fer such substances as are obtained from plants under the name of expressed juices, because it is evident 

 that they can come from no other source. In this state they are generally obtained in the first instance 

 whether with a view to their use in medicine or their application to the arts. It is the business of the 

 chemist or artist to separate and purify them afterwards according to the peculiar object he may happen 

 to have in view, and the use to which he purposes to apply them. They contain, like the sap, acetate of 

 potass or of lime, and assume a deeper shade of color when exposed to the fire or air. The oxymuriatic 

 acid precipitates from them a colored and flaky substance as from the sap, and they yield by evaporation 

 a quantity of extract. But they differ from the sap in exhibiting no traces of tannin or gallic acid, and 

 but rarely of the saccharine principle. 



1472. Ashes. When vegetables are burnt in the open air the greatest part of their substance is evapo- 

 rated during the process of combustion ; but ultimately there remains behind, a portion which is alto- 

 gether incombustible, and incapable of being volatilised by the action of fire. This residuum is known by 

 the name of ashes. Herbaceous plants, after being dried, yield more ashes than woody plants ; the leaves 

 more than the branches ; and the branches more than the trunk. The alburnum yields also more ashes 

 than the wood ; and putrified vegetables yield more ashes than the same vegetables in a fresh state, if the 

 putrefaction has not taken place in a current of water. The result of Saussure's experiments on 1000 parts 

 of different plants was as follows : 



Gathered in May, dried leaves of the oak - .... s."? farta qf ashes. 



preen leaves of the oak - - - 13 



dried leaves of the rhododendron - - - 50 



dried leaves of the aesculus hippocastanum 72 



trunk and branches of aesculushippocastanum 35 



Gathered in September, dried leaves of the tcsculus hippocastanum 86 



dried leaves of the oak .... 55 



green leaves of the oak - - - 24 



GoMeredwAenin^ower, leaves of pisum sativum - - - 95 



Gathered tv hen in /ruzV, leaves of pisum sativum - - 81 



leaves of vicia juba - - - 20 



Gathered before comine intojlower, the leaves of the vicia faba . 16 



Oak, the dried bark 60, the alburnum 4, wood ... - 2 



1473. The analysis of the ashes of plants, with a view to the discovery of the ingredients of which they 

 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 all sub- 

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



1474. 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, arid 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 tbe notable discovery by Sir H. Davy, its component parts are at last ascertained to be a 



