rious salts, resulting from the combination of the acids already mentioned, and benzoic, acetic, and phosphoric acids, with soda, ammonia, lime, or magnesia. TEARS, and MUCUS, and the HUMOURS OF THE EYE, SYNOVIA, and the rest of the animal fluids, differ so little in their constituent parts, as not to demand particular notice in this Compen dium.

It is necessary to observe, that animal as well as vegetable substances yield, besides the constituents already mentioned, a great number of peculiar acid products, which contribute to the vast range of physical and chemical properties which these bodies possess.

We shall now proceed to a description of some processes and instruments employed in chemical experiments. Solid substances are reduced into powders by trituration, pulverization, and levigation. Brittle substances are pulverized by means of ham. mers, pestles and mortars, stones and mullers. Pestles and mortars are made of metal, glass, porcelain; marble, agate, &c., according to the hardness and properties of the bodies to be reduced to powder.

The separation of the finer parts of bodies from the coarser is performed by means of sifting or washing. Sieves are well known instruments, made of different degrees of fineness, according to the purposes for which they are wanted. Washing is used for procuring powders of an uniform fineness, much more accurately than can be done by a sieve;

but it can only be employed for such substances as are not acted upon by the fluid which is used. The powdered substance is well mixed with water, or other 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 parts pass over with the liquor. By various decantations in this way, several sediments are obtained of different degrees of fineness, the last being the finest.

Filtration is a finer species of sifting, performed through the pores of paper, flannel, fine linen, sand, &c. It is used only for separating fluids from solids. No solid, even in the form of powder, will pass through the above-named filtering substances; of course, 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 or other solids will remain upon the filter, and the liquor will pass through. Unsized paper is a very convenient substance for making filters for chemical purposes. It is wrapped into 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.

Evaporation separates a fluid from a solid, or a more volatile fluid from another which is less volatile. Various degrees of heat are employed in evaporation, according to the nature of the substances. It is performed in vessels of wood, glass, metal, porcelain, &c. Basons made of Wedgewood's ware

are not apt to Small flasks of

are extremely convenient, as they break by sudden changes of heat. thin glass are likewise well adapted to the purposes of evaporation: these are placed either over the naked fire, or in a vessel filled with sand, which is then called a sand-bath. This affords a more regular degree of heat, and prevents the vessels from being broken. When the fluid which goes off in the act of evaporation is to be preserved, the operation is called distillation. Distillation is the evaporating in close vessels when we wish to separate two fluids of different degrees of volatility, and to preserve the most volatile, or both of them. The substance to be distilled is put into a vessel that will resist the action of heat, called a retort, an alembic, or still, having a neck projecting from it, to which is attached another vessel to receive the fluid that rises first, which is called the receiver. The vessel that contains the liquor to be distilled is placed upon the fire, or in a sand-bath, or over a lamp: the heat causes the most volatile fluid to rise in the form of vapour, which passes into the receiver, where it is again condensed by cold. In many cases the condensation is assisted by immersing the tube, through which the vapour passes, in a vessel containing cold water.

In plate Chemistry, fig. 1, AB represents a retort used for distillation: it is made of glass, baked earth, &c. When it has a small neck a with a stopple fitted to it for introducing materials, it is called a tubulated retort. D is the receiver for condensing the vapour which is raised, and into which the neck of

the retort is inserted. The joining at C is made airtight by means of some substance applied to it called a lute. There are several methods made use of for supporting both retort and receiver, according to the degree of heat employed in the process. When great heat is required, earthen retorts are used, which are placed in or on the fire; but if a moderate degree of heat only be required, glass retorts are employed, which may be placed in a sand-bath, or suspended over a lamp, as will be shown hereafter in fig. 3. In this case, the receiver may be placed upon some stand convenient for the purpose, with a ring of hay under it to keep it steady.

Fig. 2 is a phial with a bent glass tube fitted into it for disengaging gas.

Fig. 3 represents an improved pneumatico-chemical apparatus, completely fitted up with a lamp furnace. A is a vessel filled with water, in which a shelf is placed so as to be two, three, or four inches under the surface, having holes bored through it, to which small funnels may be attached underneath. The glass receiver B, which is to contain the gas, is filled with water, and, being inverted with its mouth under water, it is raised gently and placed upon the shelf over one of the holes. The receiver will remain full of water by means of the pressure of the atmosphere. The materials from which the gas is to be disengaged are now to be put into the retort C, which is afterwards suspended in one of the rings of the lamp furnace D. A common Argand's lamp is placed on the shelf F, and adjusted to the proper height below

the retort, the neck of which rests upon the edge of the cistern, and the end of its neck opens in the funnel under the jar standing upon the shelf. The lamp thus adjusted may be lighted, and as soon as the substances in the retort act upon each other sufficiently, the gas will begin to be disengaged, and will ascend through the hole in the shelf into the receiver B, and displace the water with which it had been filled. When all the water is displaced, the receiver is full of gas which was disengaged from the retort, and may be preserved in it by keeping its mouth always under the water in the cistern. The receiver may, in this state, be removed to any other place by first sliding it carefully into a deep plate or dish previously filled with water, so that it be not once raised above the surface of the fluid; or the gas may be transferred from the vessel B into any other vessel in the following manner: Fill the vessel into which the gas is to be transferred with the water in the trough, and place it on the shelf as before directed, over one of the holes. Then take the receiver, and, keeping its mouth still under the fluid, bring it under the hole on which the vessel is placed, depressing its bottom, and elevating its mouth, so as to bring it more to a horizontal position: the gas, being the lighter body, will escape and rise up through the hole on which the other vessel has been placed, and will fill it by displacing the fluid. In this manner any gas may be formed, or transferred from one vessel to another.

Experiment. Put some pure oxygenated muriateof potash into the retort C, and then apply the heat

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