OF GUN-METAL.

IO7

not suffice to make a thorough and exact mixture of the twometals in large furnaces; a motion infinitely greater thanthat which is sufficient to keep them in fusion must be forthis purpose excited. If the metals be well mixed togetherand drawn from the furnace so as to be suddenly condensed;the particles will retain their intimate connection when per-fectly solid: but if the quantity of melted matter be verygreat, and the receiver of such a form that it is some timebefore it sets, on examining it when cold, the tin willbe found to abound most towards the bottom, particu-larly in long receivers, like the moulds of cannon. Thecalcinations (18,21) which take place while the composi-tion is in fusion, are of different characters according to thequantity of melted metal, the exposure of its surface to theair, and the time that it remains in fusion : these changesare almost imperceptible in the more minute processes of themetallurgists ; but when in large furnaces a great quantity ofmetal is suffered to remain a long time in fusion, and thefire is increased, it will be found that the metal producedfrom the revivified calcinations, contains the tin in a greaterproportion, than when put into the furnace; as is seen inthe refined cakes, which the founders of artillery draw fromwhat they call the operation of the handle.

25. The mixture of copper with tin is called bronze, what-ever he the proportion between the component parts, providedthat the excess be on the side of the copper. In proportion tothe quantity of tin, the composition (2) is harder and less sub-ject to cavities; it even loses its tenacity and becomes crudeand brittle, if the quantity of tin be too great; the addition of aconsiderable proportion of brass renders it a little harder, andeven more brittle when heated (23) though the brass be of thebest quality. Since the use for which the bronze is designedmust determine the proportion between the component parts,in order that it may have the physical properties requisite forthe intended work (2, 3), it follows that guns may be castfrom bell-metal, and statues, &c from gun-metal, on addingcertain quantities of copper or tin.

26. It will be shewn in the following chapter that themetal for casting cannon, particularly battering Cannon, must(8, 9) indispensably be of a hard and tenacious nature. Tothis end it is necessary ;

1. That the metals be of a good quality and wellpurified.

2. That