explosive compound may be produced when acetylene is brought into contact with such alloys of copper as ordinary brass containing 64.66 per cent. of copper, or red brass containing 74.46 per cent. of copper, 20.67 per cent. of zinc, and 4.64 per cent. of tin; whereas none is obtained when the metal is either "alpaca" containing 64.44 per cent. of copper, 18.79 per cent. of nickel, and 16.33 per cent. of zinc, or britannia metal composed of 91.7 per cent. of copper and 8.3 per cent. of tin. Caro has found that when pure dry acetylene is led for nine months over sheets or filings of copper, brass containing 63.2 per cent. of copper, red brass containing 73.8 per cent., so-called "alpaca-metal" containing 65.3 per cent., and britannia metal containing 90.2 per cent. of copper, no action whatever takes place at ordinary temperatures; if the gas is moist very small quantities of copper acetylide are produced in six months, whatever metal is tested, but the yield does not increase appreciably afterwards. At high temperatures condensation occurs between acetylene and copper or its alloys, but explosive bodies are not formed. Grittner's statement that crude acetylene, with or without ammonia, acts upon alloys of copper as well as upon copper itself, has thus been corroborated by Caro; but experience renders it tolerably certain that brass (and presumably gun-metal) is not appreciably attacked in practical conditions. Gerdes' failure to obtain an explosive compound in any circumstances may very possibly be explained by the entire absence of any oxygen from his cylinders and gases, so that any copper carbide produced remained unoxidised. Grittner's gas was derived, at least partially, from a public acetylene supply, and is quite likely to have been contaminated with air in sufficient quantity to oxidise the original copper compound, and to convert it into the explosive modification. For the foregoing reasons the use of unalloyed copper in the construction of acetylene generators or in the subsidiary items of the plant, as well as in burner fittings, is forbidden by statute or some quasi-legal enactment in most countries, and in others the metal has been abandoned for one of its alloys, or for iron or steel, as the case may be. Grittner's experiments mentioned above, however, probably explain why even alloys of copper are forbidden in Hungary. (_Cf._ Chapter IV., page 127.)