that it may either sink or rise in the lower one. The upper part is moveable, and balanced by weights, (13, 13,) attached to chains passing over pulleys, that are fixed upon a hollow pillar, on which it is suspended. By this contrivance the upper part of the Gasometer can ascend or descend as the gas enters it from the Purifying Machine, or passes out to supply the lights. On that side of it which is opposite to (10, 10), is a similar upright pipe to convey the gas to the principal mains; and a slight pressure on the vessel produces the regular and necessary current of the gas, as it may be wanted for use.

PLATE II.

This plate exhibits the Apparatus for making Oil Gas. (A,) represents a section of the furnace, or oven, for heating to a red-heat the Retort.

(B,) shews the front of the Retort, which is a cylindrical cast-iron vessel, whose mouth is fitted with a cover, so that it may be completely closed; and it is partly filled with coke, or broken bricks, in order that the oil, which falls by drops upon the heated substance, may be exposed to a large portion of heated surface, to decompose and convert it into gas.

(C,) is a section of a copper vessel * to contain the oil, which, by means of the cock, (1,) is allowed to drop regularly through the pipe (2, 2) upon the heated matter in the Retort, where it is decomposed, and converted into gas and other volatile products; and these pass along the pipe (3) into the square cistern, called the Condenser.

* The usual height of the oil in the vessel is indicated where the fracture is shewn.

(D,) exhibits a section of the Condenser, which is composed of two iron vessels-one within the other; and the space between them is filled with cold water, which keeps the inner one cold, and therefore condenses the products of the distillation as they pass through it; the oil, which the heat may convert merely into vapour, condensing against the cold surface of its sides, falls down through the pipe (4) to pass again into the oil vessel; and the gas passes by the tube (5) into the purifying cistern.

(E,) is a section of the Purifier, which is filled with oil nearly to the top; and a board is placed slantingly across it, that the gas, by entering at the lower end, may have to travel over a large surface of the oil, and become thoroughly washed before it passes off through the pipe (7) to the Gasometer. The upright pipe (8) preserves the oil at an equal height in the Purifier; the funnel (9) is for supplying the oil vessel; and the small cock (10) is for drawing off any water that may accidentally mix itself with the oil.

(F, F,) represents the cast-iron stand which supports the Condenser, Purifier, and Oil-vessel.

This apparatus is so simple in its construction, and effective in its arrangement, that it may be adapted to small buildings as well as large: and the Engraving represents the most improved one, on the principle for which Messrs. Taylor and Martineau have the Patent.

INTRODUCTION.

In the course of a few years the science of Chemistry has disclosed so many of the wonderful processes by which nature performs her curious and astonishing operations, as to enforce admiration; and the application of these important discoveries to the improvement of the useful arts has been so extensive, as to confer inestimable benefits on society. Hence an acquaintance with its principles is desirable, not only on account of its connexion with many other branches of knowledge, but from the advantages it affords to those who may be engaged either in manufacturing, or in domestic occupations.

Every science having some words and expres sions particularly adapted to its use, one of the chief difficulties attending an attempt to render them familiar, consists in selecting such terms as will easily and correctly convey the information intended to be given. Among the modern improvements of Chemistry may be reckoned, the invention and adoption of a language peculiarly appropriated to its purposes. Formerly, the terms employed in this science were so arbitrary and

fauciful, that the mere retention of them required no ordinary effort of the mind; but those at present employed are simple, concise, and comprehensive; yet they are easy to be understood and remembered. The principle which guided the inventors in its formation was, to admit the use of such words only as were expressive of the general and distinctive qualities of the substances they denoted, at the same time connecting them with some determinate idea; so that while precise notions of things were conveyed to the understanding, the faculty of memory might be materially assisted. The advantages of this ingenious and philosophical contrivance are evident, in the simplicity and precision which it has been the means of introducing into the classification of the objects of chemical science; and it has also tended to remove some of the greatest obstacles to its easy and successful pursuit.

The processes of making and purifying Gas are chemical operations, and therefore the use of some chemical terms was sometimes unavoidably necessary, for the purpose of describing correctly the processes or substances. But to obviate this apparent inconvenience, as well as to facilitate the comprehension of the subject, a distinct and familiar explanation is given of such as most fently occur, that a ready and satisfactory

e may be made to them. Such an as

sistance will be found particularly useful to those persons who may be interested in obtaining a knowledge of the nature of Gas Lighting, but who may not possess any previous knowledge either of the language or principles of Chemistry.

M. Lavoisier observes, that "every branch of physical science must consist of three things-the series of facts, which are the objects of the science; the ideas which are represented by these facts; and the words by which these ideas are expressed. Like three impressions of the same seal, the word ought to produce the idea, and the idea to be a picture of the fact." These qualities are admirably displayed in the construction of the new terms which have been introduced into Chemistry, and they have been chiefly derived from the Greek language, because it possesses a peculiar facility and expressiveness in its combinations, adapted for the purposes of science.

N. B. The Explanation of Chemical Terms is placed at the end of the work.