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 be distinguished by their parts of fructification, and illustrating his method by the description of 1476 different genera and species, in alphabetical order. An enlarged edition of this book was published by Daniel Nebel, in 1700, with the addition of the characters of Tournefort and Hermann. For the complete list of Paul Amman’s writings, see Haller, ''Bibl. Med.'' and Eloy, ''Dict. Hist.''

AMMONIAC,, a saline substance, formerly much used in dyeing, and some other arts. At present, not much of it is employed in this country, most of the sal-ammoniac manufactured in Great Britain being sent to Russia.

Sal-ammoniac is usually in the form of a hard white cake, opaque or only slightly translucent. Its taste is cooling, saline, and rather disagreeable; though it has been occasionally employed as a seasoner of food. Its specific gravity is 1.441, according to the mean of the experiments of Wallerius, Watson, and Kirwan. It requires rather more than three times its weight of cold water to dissolve it. By proper evaporation, the sal-ammoniac may be obtained in crystals. The primitive form of these crystals, according to Hauy, is the octahedron; but most frequently they appear in the state of long four-sided pyramids, or of thin plates, like sword blades. It often assumes the form of plumose crystals, consisting of long hexahedral pyramids attached to a stem. These crystals contain a little water, and therefore dissolve in somewhat less than three times their weight of cold water. A hundred parts of alcohol, of the specific gravity 0.834, dissolve 1½ parts of this salt.

When exposed to a moist atmosphere, it gradually absorbs water and deliquesces, though very slowly. When heated, it sublimes unaltered in a white smoke, having a peculiar smell, very characteristic of sal-ammoniac. If a cold body be presented to this smoke, the sal-ammoniac condenses on it, and forms a white crust. When thus sublimed, it has the property of carrying along with it various bodies, which, when heated by themselves, are perfectly fixed. Thus, if it be mixed with gold leaf, filings of iron, or oxide of copper, these substances rise along with it, and tinge the sal-ammoniac purple, yellow, or red.

If quicklime or potash be triturated with sal-ammoniac, a strong smell of ammonia exhales. If sulphuric acid be poured upon it, vapours of muriatic acid are separated in abundance. If equal bulks of muriatic acid gas, and ammoniacal gas be brought into contact, they immediately combine and condense into sal-ammoniac. These facts, which are well known, show us, that sal-ammoniac is a salt composed of muriatic acid and ammonia. The composition of this salt seams to have been first discovered by Tournefort in 1700. The experiments of Geoffroy junior in 1716, and 1723, were still more decisive, and those of Duhamel, in 1735, left no doubt upon the subject. Various experiments have been made by modern chemists, to determine the proportion of the constituents of this salt. But its analysis is attended with peculiar difficulties, on account of the volatility of the sal-ammoniac, and the want of a simple method of determining the proportion of ammonia. Dr Thomson first pointed out a process by synthesis, which has the advantage of being very simple, and at the same time rigidly accurate. He observed, that when muriatic gas, and ammoniacal gas, both as dry as possible, are brought in contact with each other, they always combine in equal volumes. Therefore, sal-ammoniac is composed of 100 measures of muriatic acid gas, united with 100 measures of ammoniacal gas. Now, 100 cubic inches of muriatic acid weigh 38.979 grains, and 100 cubic inches of ammoniacal gas weigh 18 grains. Hence, sal-ammoniac is composed of

Or of

This gives us the exact proportions of the acid and base. But, at present, we are not in possession of any exact method of determining the quantity of water which the salt contains. From the property which the salt has of deliquescing in a moist air, it is obvious, that the affinity of water must be considerable. Hence, it is probable, that the quantity of water in this salt is pretty great.

An integrant particle of muriatic acid weighs 4.762, and an integrant particle of ammonia 2.149. Hence, it is obvious, that sal-ammoniac is composed of one integrant particle of muriatic acid, united to one integrant particle of ammonia.

But there is another way in which the composition of this salt may be viewed. Muriatic acid is a compound of chlorine and hydrogen. It has been shown, that most of the substances hitherto called muriates are in fact chlorides, or combinations of chlorine, with the metallic bases of the alkalies, earths, or metallic oxides respectively. Thus, common salt is a chloride of sodium, or a compound of chlorine and sodium; horn silver is a chloride of silver, or a compound of chlorine and silver. Now, of all the bases capable of combining with acids, amounting to fifty at least, every one is known to contain oxygen except ammonia. It would be singular if such a solitary exception were to exist. Hence, analogy is in favour of the supposition that ammonia contains oxygen. If a hole be dug in a piece of sal-ammoniac; if this hole be slightly moistened by breathing on it; if a globule of mercury be put into it, and this globule be subjected to the action of a tolerably powerful galvanic battery, the mercury speedily acquires the consistence of butter, and swells up so, as to amount to nearly four times its original bulk. In short, it is converted into an amalgam. Ammonia itself may be substituted for sal-ammoniac; but the experiment, in that case, is attended with greater difficulty. In this ease, the ammonia is evidently decomposed by the galvanic energy, and one of its constituents has combined with the mercury, and converted it into an amalgam. If the amalgam be put under water, it is speedily reduced to the state of pure mercury, while, in the meantime, ammonia and hydrogen gas