acids will be pointed out, also different modes of analysis, under the heads of Earths, Metals, and Mineral Waters. But in so very short a sketch of the doctrines of chemistry as these few lectures can comprise, it is not possible to go far into the detail; and the most effectual processes can only be found in the larger works on this science, particularly those of Bergman, Kirwan, and above all, that admirable compendium of modern science Dr. Thomson's System of Chemistry, and the Essay on Chemical Analysis by M. Thenard.

LECTURE XXVIII.

CHEMISTRY:

CHEMICAL APPARATUS.

THE instruments used in chemistry may be divided into three branches: 1st, The vessels that hold the subjects on which the effect is to be produced; 2d, The apparatus for producing heat; 3d, The means of regulating or applying heat in the best manner.

Was it in our power, we should wish that the vessels used in chemistry should possess the following properties: 1st, Transparency; 2d, The property of resisting the action of corroding substances; 3d, The power of enduring sudden changes of heat and cold; 4th, Strength, in order to confine elastic vapour, &c.; and 5th, The power of bearing great heat without melting. But no matter in nature is possessed of all these properties. The common materials are glass, metals, and earthen ware.

1. The greatest inconveniency of glass vessels is, that they do not well endure sudden changes of heat and cold. It is the rule to make glasses which are to bear heat as thin as possible, provided they are strong enough to be handled.

The effect of making them thin is, that their surfaces are more uniformly heated, that is, one surface is not heated before the other, and consequently they bear that disproportion of their form which heat occasions. They are more flexible too; and a spherical form contributes to this effect. White glass best resists sudden changes of temperature, and common green glass the higher degrees of heat in the sand furnace. One circumstance in particular to be regarded is the annealing of the glass. This is done by putting the vessels, while red-hot, into a furnace of nearly the same temperature, and letting them cool gradually; for if hastily cooled they are remarkably brittle. This phænomenon is hard to be accounted for; but we have a strong instance of it in what are called glass tears, or Prince Rupert's drops, which is melted glass dropped into cold water. The greatest number of these drops falling into atoms, those that remain are in the form of a pear, with a tail not thicker than a thread. If we break a little bit of the tail, the whole flies into powder with great explosion, and they sometimes fly without any thing that we can perceive disturbing them. A certain thickness of the glass is crusted over: and this crust being penetrated, the whole flies asunder; yet when the thick end is ground till a certain point is worn off, the explosión does not take place: this is a fact that cannot be explained till we understand better the nature of cohesion.

It is, however, somewhat analogous to the sudden crystallization of salts by agitation. The contraction necessary for developing the matter of heat is prevented in the first instance by the sudden formation of a solid crust. On the latter being split in several parts at once, the instantaneous mutual contact of the internal parts, and consequent liberation of the latent heat, perhaps, causes an explosion.

2. Metal vessels want transparency, yet their use is indispensable. Iron serves for sandheats, distilling pots, and other purposes requiring strength and durability; copper, for the more bulky utensils employed in boiling and distilling. Condensing worms and dyers' kettles are made of tin a little hardened by copper; lead, hammered out from a thick plate, makes good boilers for such acid and saline liquors as would corrode iron. It is also much used for lining wooden backs to perform crystallization in the great

way.

But the experimental chemist requires crucibles and evaporating vessels of pure silver and platinum; the former for the caustic alkalies, which have no sensible action on silver, though they attack almost every thing but that and gold. Platinum makes almost an universal implement, resisting the greatest heat of a charcoal fire, and the action of all the simple acids, but not of nitro-muriatic acid and the alkalies. Crucibles of these metals, being fitted with an alembic head

of glass, serve for most distillations by the heat of the lamp.

3. Earthen vessels of the best kind resist the strongest heat without melting, and are not easily corroded. Vessels made of clay alone are very close and compact, but are liable to break from their inflexibility, as well as to melt. To remedy these, sand is added, which endures violent heat without melting, and at the same time lessens the expansion of the clay, and consequently renders it less apt to break. If the vessel is required to be very compact, more clay and less sand must be used: they must be heated and cooled slowly. If vessels are necessary that will bear to be heated sooner, more sand must be employed. Mr. Pott, of Berlin, recommends powder of clay, burnt till it is quite hard, instead of sand, which has all the advantages without the inconveniences of that material. Chemists have long wished to discover a vessel in which lead may be vitrified without loss. This metal dissolves common vessels, as water dissolves salt. Mr. Pott's method answers this purpose extremely well. Dr. Lewis recommends a porcelain of bottle glass baked in sand*. Black lead vessels endure heat best.

The forms of the vessels vary according to their uses.

1. VESSELS for FUSION. The most common

See his Commerce of Arts.