pression after dusting, etc., is then ready to receive the metal. The pieces of sand are removed with two needles stuck in a piece of wood. The relative position of the cores are readily recognised by their irregular forms.

Birds, insects, and parts of plants may be cast by fixing the object to be cast in the centre of a box by means of pieces of cotton. China clay mud, plaster of Paris, or any substance which is not combustible is thrown into the box, and the object covered. Plaster of Paris crumbles to powder when heated, but if it be mixed with a little potassium sulphate, alum, or borax, the plaster can be heated to redness without crumbling. When the object is covered with the non-combustible matter, sand is added until the box is full. It is then dried, and the box and its contents heated sufficiently to burn the object to ashes, which are carefully blown out. A place is left as usual for the metal to be poured in.

Dr. Branson's Method.-This method was devised for taking copies of ferns, seaweed, etc. A sheet of gutta-percha is softened in boiling water, put upon a warm metal plate, and dusted over with bronze powder; this dries the surface, makes it smoother, and prevents the specimens sticking to the plaster. A fern is then laid on the top of the guttapercha, then a smooth plate, and pressure applied. The plate is removed when cold, and from the beautiful impression thus prepared a cast is taken in plaster of Paris, and a casting may be obtained in type-metal, brass, etc.

Some articles, such as a pattern for a paraffin lamp-stand, are moulded as follows: The pattern is moulded in wax or plaster, laid upon a board, and an impression of the outside. taken. The mould is then inverted and the pattern removed. Parting sand is dusted in, a frame fixed on the top, and an impression taken of the sunken sand-mould. The thickness of the casting is then determined by placing a layer of clay between the two moulds. Each part of the casting is, in this case, of the same thickness throughout.

DIPPING AND PICKLING BRASS

§ 62. The layer of oxide on the surface of brass, which has been heated in contact with air, is removed by dipping in acid liquids and finally swilling in water. The corrosion

may be also due to sulphur and other bodies derived from the fuel, and with soldered articles from the fluxes used. It often happens that the incrustation is very persistent, and if it is attempted to remove it rapidly by using strong and hot aqua-fortis, much of the brass is also dissolved producing a very rough surface. For this reason it is preferable to employ at first a weak liquid containing salts of copper and zinc from previous operations, and termed pickle, which slowly removes the surface coating and leaves the metal smooth. Dilute sulphuric acid is also used as a pickle for sheet copper, being slower and more uniform in its action in proportion to the degree of dilution. Nitric acid, which is the chief constituent of aqua-fortis, exerts a more powerfully solvent action on zinc than on copper, so that the surface of dipped brass assumes a warmer tone, shading more or less into a reddish-yellow. To some extent the colour may be varied by using aqua-fortis of different strengths, probably depending on the component metals being dissolved in different ratios by acids of varying densities.

Nitric acid, containing a certain quantity of nitrous acid, is capable of producing different shades of colour. To obtain such a mixture small quantities of organic substances are used for the purpose of generating nitrous fumes, by the action of concentrated nitric acid upon them. Thus saw-dust added to strong nitric acid imparts an orange-yellow colour, due to the partial decomposition of the acid and the formation of nitrous acid.

Dead dipping is the name applied to the process of producing a dead yellow surface on brass-work by dipping in suitable liquids. The work is first pickled in dilute or spent acid until the scale can be removed by rubbing. It is then well swilled and placed in stronger acid, which acts much

more promptly, giving rise to a frothy appearance, this is removed by rinsing the work in water, after which it is dipped in strong nitric acid for a few seconds, washed in water, and then washed in water containing dissolved argol, finally being dried in hot saw-dust. The argol solution is said to prevent a brownish discoloration, or mottling of the surface, which would otherwise occur. In this last dipping it is important that each article should be dipped separately, and not a number strung together on wire, as is often the case in the former dippings.

CHAPTER III

BRONZE

§ 63. The term "bronze" will be applied in this work to all alloys consisting chiefly of copper and tin. These metals have been known from very remote times, and the importance of the mixture of copper and tin appears to have been among the first discoveries of the metallurgist. Instruments of various kinds were fabricated from these alloys, and weapons were made with a keen cutting edge, harder than iron, and almost rivalling that of steel. The bronzes of the ancients varied considerably in the proportions of the ingredients, for in the main copper and tin only were used, according to the purposes for which they were intended. Sometimes other ingredients were added, either purposely to produce a given effect, or it may be, in some cases, that bodies other than copper and tin were present as accidental impurities. This would arise from the use of impure metals, derived largely from ores of copper or tin associated with other ores, which is often the case.

Of late years very great attention has been devoted to the study of copper-tin alloys, and those proportions of the constituents which have been found by experience to give the greatest strength and the keenest cutting edge are the same as those used by the Greeks and Romans for their weapons of war and of the chase. The effect was produced by causing the bronze to undergo a process of hammering, as well as a method of hardening, by heating and slow cooling.

Many ancient coins were made of bronze, containing in some cases lead, zinc, or iron. The following table will show the composition of some ancient bronzes :

It will be observed from an examination of the foregoing table that the principal constituent of bronze is, in all cases, copper, the other components being added to harden or otherwise modify its properties, according to the purpose for which the alloy is intended. Tin has the property of