bably have got the name of balnea from water being mostly used, which is safest when no great heat is required; and this is called balneum maria. When sand is used, it is called balneum arena. Some have recommended steel filings, and Dr. Lewis quicksilver; but as quicksilver soon evaporates, I would prefer the softer metals, as bismuth, &c. It is often found useful to make use of air, which is called distilling in copella vacua. The vessel is to be placed in the middle of an iron vessel, suspended by iron hooks, and you must close the iron vessel so as to exclude the external air. The lamp furnace has been used both for the purposes of evaporation and digestion.-For various hints on furnaces, see Lewis's Commerce of Arts, and Mr. Nicholson's Dictionary of Chemistry.

Lutes are commonly used for joining two vessels together, and stopping the cavities. The ordinary lutes deserve the name less than others, having no clay in their composition: they are used to stop the junctures of vessels when the heat is gentle; but it is better to leave a hole made with a pin than to shut the juncture quite close. Flour and water with gum arabic is good for distilling; also linseed meal (from which the oil has been expressed) makes a remarkably tight luting. The most convenient linings are bladders cut into slips, and moistened; the inside of the bladder must be applied next the

vessel *. When the fumes are corrosive, fiery lutes must be used; the best of which is pipe clay mixed with seven times its quantity of white sand. The clay only serves to combine the particles of sand together, for sand resists the most violent heat. A good luting for lining the internal part of a furnace is made of charcoal dust and water with one fourth of clay, over which spread a covering of sand and clay, and beat them well with a hammer. The charcoal will last two or three years: the clay is mixed with the charcoal to prevent its cracking.

Mr. Nicholson† most judiciously remarks that the ingenious student in chemistry, when he has acquainted himself with the first principles, will easily perceive that there are few philosophical inquiries which require a large apparatus of furnaces or vessels. A tobacco pipe (the bore of which must be stopped with good clay) is a very useful crucible, in which many operations may be performed with a good pair of double bellows. An earthen pot, or an iron ladle, will contain a sand bath, and common phials, or Florence flasks, serve very well for

* When a bladder is used with the retort and receiver, a hole may be made in it with a pin, by which the operator may judge of the contents by the smell.

+ See his Dictionary of Chemistry.

matrasses. Chafing-dishes or small iron stoves will produce a great heat if well managed; and the blow-pipe and spirit lamp, with a set of small retorts and receivers, may be applied to the performance of almost every part of experi mental chemistry.

LECTURE XXIX.

CHEMISTRY.

SALTS.

THE objects of chemistry are commonly divided into salts, earths, combustibles, metals, waters, and vegetable and animal substances. By taking them in this order, we shall find that those of a similar nature, and which have most qualities in common, are united together.

Salts are commonly defined to be substances which are fusible, volatile, soluble in water, not inflammable, and sapid when applied to the tongue. These qualities are united to distinguish them from other bodies which will be afterwards mentioned. The first objection that has been made to this definition is, that their fusibility or volatility are not distinguishing characters, as we have reason to conclude there is no species of matter incapable of both but what is meant here is, that they are easily melted and converted into vapour with a moderate heat. Another objection is, that the absence of inflammability is mentioned as a characteristic; whereas many salts by a vigorous test show signs of inflammability.

Muriat of ammonia, as well as the pure salt of that name, will deflagrate.—But it must be remembered that hydrogen, which is a substance highly combustible, is one of the component parts of ammonia. This then is an imperfection in the definition not easy to be surmounted; but where objects are numerous and diversified, it is difficult to class them.

It is obvious from what was stated in the first lecture on chemistry, that all salts are compound bodies. The acids all consist of certain bases or radical principles which give the particular character to the salt, and the acidifying principle, or oxygen. Of the three alkalies the volatile or ammonia is a compound of hydrogen and nitrogen; and soda and potass have been ascertained to be metallic oxides.

The most simple state of salts is a mass, white, brittle, and in some degree transparent. Salts in certain degrees of heat, are fluid, like oil, and transparent; when cooled they return to their former state, and are semi-opake. They differ in their degrees of volatility and fusibility: some fly into vapour with the least heat, others in a violent heat remain nearly fixed; most of them require to be heated red-hot, yet all may be brought into fusion.

All saline substances dissolve more or less readily in water. The attraction of salts for water is however different in different salts.