making in the methods of analyzing bodies, or separating them into their component principles or elements, several other substances once supposed to be simple are now found to be compounds: and, as chemistry continues to advance, the list of simple substances may be reduced; our inability to decompose any body not proving it to be simple, but only, perhaps, that our methods of examination are still imperfect.

The substances which hitherto have resisted all the known methods of analysis, and which, in the present state of our knowledge, are considered as the elements of all bodies with which we are acquainted, are the following:

The utmost degree of mechanical division which we can effect in bodies, by pounding, grinding, and similar processes, can only reduce them to fragments so small that they can no longer be perceived by the sight; but we cannot thus arrive at those ultimate atoms, molecules, or particles, of which the various species of matter are supposed to consist, and which are, perhaps, incapable of subdivision.

Besides the attraction of gravitation possessed in common by all matter, these elementary substances possess peculiar attractions for each other, which are called chemical attractions. By these attractions, or affinities, as they are called, they combine together, and form compound bodies.

OPERATIONS AND INSTRUMENTS USED IN

CHEMISTRY.

The great principle of all chemical operations which enable us to decompose certain bodies, and to compound others, is, that every substance has a peculiar affinity or attraction for other substances, but that it has different degrees of attraction for different substances. This is called elective affinity

or attraction.

If some oil and some alkali be put together, they will unite and form soap. But if to this a little dilute sulphuric acid be added, the oil and alkali will be separated from each other again; the alkali having a stronger attraction for the acid than it has for the oil, will leave the latter and join the acid.

Dissolve some magnesia in nitric acid, and the solution will be transparent. Also dissolve some lime in water by letting it remain for some hours the solution of lime in water will also

be transparent. Pour them together, and immediately a turbid appearance will be presented, and a white powder will fall to the bottom. This powder will be found to be magnesia. The explanation is this: the nitric acid has a greater attraction for lime than it has for magnesia; therefore, it lets fall the latter and takes up the former. The substance thus thrown down is called a precipitate, and the process is called precipitation.:

If all the bodies presented to each other are compounds, sometimes two new substances are formed. Thus, if solutions of nitrate of barytes and of sulphate of soda be mixed together, the former being composed of nitric acid and barytes, and the latter of sulphuric acid and soda, two new products will be obtained, viz. nitrate of soda and sulphate of barytes. For the nitric acid will leave the barytes and join to the soda, and the sulphuric acid will give up the soda and seize the barytes. This is called double elective affinity, as the former example was of single elective affinity. When different substances unite chemically, and form a compound, they always unite in the same proportion. Thus, water, which is composed of oxygen and hydrogen, always contains the same proportion of each; that is, we do not find that in several specimens of water the proportions of oxygen and hydrogen vary. Also, if an acid and an alkali combine together, and thus form a certain salt, they always unite in the same proportion to form that salt; however, they will sometimes combine in another proportion to form another salt; but when substances unite in more than one proportion, the second, third, &c. proportions are multiples or divisors of the first. This is one of the latest discoveries in chemistry,

and has given rise to the doctrine of definite proportions.

Here it must be remarked that chemical combination and mechanical mixture are very different; since, although bodies only combine in definite proportions, yet, they can be mixed together in all proportions.

In general, before substances can be made to act chemically on each other, one of them, at least, must be in a fluid state; and, that solids may be acted on more easily, they are generally mechanically divided into small pieces, or reduced to a powder.

By trituration, pulverization, and levigation, is meant the reduction of solids into powders of different degrees of fineness. Brittle substances are reduced to powder by means of hammers, pestles and mortars, stones and mullers. Mortars and pestles are made either of metal, glass, porcelain, marble, agate, &c. according to the hardness and properties of the bodies to be pounded. WEDGEWOOD's ware affords a most excellent kind of mortar for most purposes, as it is very strong, and not liable to be acted upon by acids. Many bodies cannot be reduced to powder by the foregoing methods such are fibrous substances, as wood, horns of animals, elastic gum, and mettles which flatten under the hammer; for these, files, rasps, knives, and graters, are necessary.

The separation of the finer parts of bodies from the coarser which may want farther pulverization, is performed by means of sifting or washing.

A sieve for sifting generally consists of a cylin drical band of thin wood, or metal, having silk, leather, hair, wire, &c. stretched across it. They are of different degrees of fineness.

Washing is used for procuring powders of an uniform fineness, much more accurately than by means of the sieve; but it can only be used for such substances as are not acted upon by the fluid which is used. The powdered substance is mixed with water, or other convenient fluid: the liquor is allowed to settle for a few moments, and is then decanted off; the coarsest powder remains at the bottom of the vessel, and the finer passes over with the liquor. By repeated decantations in this manner, various sediments are obtained, of different degrees of fineness: the last, or that which remains longest suspended in the liquor, being the finest.

Filtration is a finer species of sifting. It is sifting through the pores of paper, or flannel, or fine linen or sand, or pounded glass, or porous stones, and the like; but it is used only for separating fluids from solids, or gross particles that may happen to be suspended in them, and not chemically combined with the fluids. Thus salt water cannot be deprived of its salt by filtration; but muddy water will deposit its mud. No solid, even in the form of powder, will pass through the abovementioned filtering substances: hence if water or other fluid, containing sand, insects, mud, &c. be placed in a bag or hollow vessel made of any of those substances, the sand, &c. will remain upon the filter, and the liquor will pass through, and may be received clear in a vessel under it. Unsized paper is a very convenient substance for making filters for chemical purposes. It is wrapped up in a conical form, and put into a glass funnel, which serves to strengthen the paper and support the weight of the fluid when poured into it.

Decantation is often substituted, instead of fil