position ensues, prussiat of iron falls down in a precipitate, and the alkali is attached to the sulphuric acid, forming sulphat of potass, which may be washed off from the colour. Phosphoric acid is the direct and sole product of the combustion or oxygenation of phosphorus, which consequently affords more than its own weight of it. This acid, deprived of all its water by heat, is solid, colourless, and of a glassy appearance. It has a very sour taste, without any smell; is not corrosive to the skin, and readily deliquesces. It dissolves in water, giving out much heat, like the sulphuric. It forms with lime an insoluble compound, phosphat of lime, which has the appearance and properties of earths, and is the solid part or base of animal bones, in which it is united to gelatine, or, in plain language, glue. Hence calcined bones, by proper treatment, yield phosphorus, the oxygen being abstracted by the charcoal employed in the distillation, which is conducted in a strong heat. The other animal acids are the uric (extracted from calculi), the rosacic, the amniotic, and the bactic, or acid of milk. The arrangement of the compound or neutral salts under the new chemistry is peculiarly easy. The barbarous jargon of the old school, which meant nothing, such as sal digestivus, sal mirabilis, sal polychrestis, is banished, and the name now discovers the materials of which the salt actually consists. Thus, when we hear of nitrat of potass, or muriat of suda, no person who knows any thing of chemistry can mistake; but must understand that these mean compounds of the nitric and muriatic acids with potass and soda. The different genera of compound salts are denominated from the acids. Thus, we have sulphats, nitrats, muriats, borats, fluats, tartrats, acetats, phosphats, &c. But it is necessary to remind the student, that where the salt is compounded of the acid, when not fully saturated with oxygen, the compound is distinguished by the termination ite. Thus, the sulphurous acid produces sulphites; the sulphuric acid sulphats. The species of compound salts are distinguished by subjoining the base; as sulphat of soda, which distinguishes the salt from sulphat of lime, or any other sulphat; and as some of the acids will unite with two bases, the triple salt is described by adding both bases, as tartrat of potass and soda. Where there is an excess of acid in the salt, it is expressed by prefixing the preposition super.-Thus sulphat of potass expresses the combination of the sulphuric acid with potass, and super-sulphat of potass denotes that there is a redundancy of acid. On the contrary, the preposition sub implies that there is an excess of base; thus sub-sulphat of potass is the salt before described, but with a deficiency of the acid, and a redundancy of the potass or base. } Salts have also been divided into the alkaline, the earthy, and the metallic, according to the nature of their respective bases. But it is unnecessary to enter into any particular description of them here, since the nomenclature itself defines the nature of the salts. Dr. Thomson calculates that there may be about 2000 different species of compound or neutral salts; and he remarks that "some idea may be formed of the progress which this branch of chemistry has made, by recollecting that forty years ago not more than thirty salts in all were known." LECTURE XXX. CHEMISTRY. EARTHS. We are led next, by the arrangement we have adopted, to consider earthy and stony substances. The common definition of these substances is, that they are bodies not soluble in water, not inflammable, and whose specific weight to that of water is not more than four to one. This definition, like that of salts, is not so precise as to be out of the reach of criticism, because there are some earths perfectly soluble in water, and many that are met with in the bowels of the earth that afford the greatest probability that they have been in a dissolved state. But the distinction, though not strictly just, may be proper in a looser sense; for there is a great difference of solubility between earths and salts, for a few grains of earth are sufficient to saturate a large quantity of water, and even these dissolve less perfectly than salts. There are two other circumstances, not included in the definition, that make part of the idea of an earthy substance. 1. A great degree of fixedness. 2. A disposition to assume the form of a glassy concretion when melted. By the first of these they are distinguished from all the other objects of chemistry, for we cannot in general convert a pure earth into vapour. The second characteristic is indispensable to an earthy substance, and in a great measure peculiar to this class. The bodies which come under the class of earths are nine in number, viz. 1. Lime. 2. Alumina (or clay). 4. Magnesia. 5. Barytes. 6. Strontian. 7. Yttria. 8. Glucina. 9. Zirconia. Some of these earths are much more plentiful than others, and the great mass of this globe may be said to be composed of lime, flint, and clay, variously mixed and disposed in the dif ferent species of stony bodies. The surface of the earth evidently consists of a confused mixture of decayed animal and vegetable substances, and earths rudely united together; but when we have got below the surface, we find the materials of the globe arranged in a regular manner. Sometimes, indeed, we find heaps of stone, which do not consist of layers, but are confused masses of unequal thickness; these are called rocks. The strata are in general extended through a whole country, and perhaps through the globe itself. Thus, in digging for coal, the workmen first come to a bed of clay, next to a bed of sand, for the thickness of which there is no particular rule; then |