I56 OF GUN-METAL.

interstices of the copper, already in part congealed, descendstowards the bottom, and lodges in any cavity it finds.

113. This Circumstance did not escape the observationOf our ancestors, and various expedieirs were deviled inthe last century for remedying it; some thiew a quantity oftin into the trough that conducted the melted niatter fiomthe furnace to the mould, when they judged that the mould■was about ? full; others thought to prevent it bv the useof brass. Some are said in order suddenly to congeal themetal, to have drawn out the iron cores from the mouldswhile hot, and to have thrown water into the vacant space ;others again were anxious to form very long and weightyheads, that the copper being strongly compressed while set-ting, the tin might not be able to descend. Experiencehas proved that this last expedient is the best; since inpieces of large calibre, the difference between the hardnessof metal at the breech and muzzle is not very considerable,and hardly sensible in pieces os small calibre.

114. Gun-metal is fusible, not only by a great heat, buteven by a moderate one, with the assistance of solvents.It has been already observed that tin is a solvent of copper,of which it accelerates the fusion ; but as the quantity oftin cannot be increased without altering the quality of thebronze, this expedient can only be used so far as the pro-portion of the ingredients will permit. Tartar and nitreare the most powerful solvents; when mixed together inequal quantities, the chemists term the compound, a whitejinx ; when the quantity of tartar is double that of thenitre, a black jux. As these two fluxes, especially theformer, are very active, much care and circumspection mustbe observed in using them, to avoid the damage that thefurnace would be exposed to, from the violent agitation theyoccasion. When gun-metal is made extremely hot, it isoften very spungy and porous, particularly towards the muz-zle, so that when filled with Water, it exudes and oozesthrough. Tartar is used when from the great rarefactionof the air arising from extraordinary heat, the fire is ren-dered inactive, and the metal is infusible per fe. But iffire alone will answer, the founders stir and mix the substanceswell together before they open the furnace, the melted matterthen runs into the moulds; and this method has been foundsufficient to render it homogeneous ; for, if small quan-tities be taken out of the furnace with an iron ladle, while

the