And for purposes such as welding and cutting this flame has an interesting and peculiar advantage over all other kinds of heat. When a metal is heated in the air there is usually trouble from oxidation. The domestic poker, for example, after it has been left to get red-hot in the fire is seen to be coated, in the part which has been heated, with scales which will flake off if the thing be struck. Those scales are oxide of iron, caused by the union of iron and oxygen when the poker was hot. But if the heat be applied by the oxyacetylene flame that will not happen. The oxygen and the carbon from the acetylene will burn, and if the supply of the former be properly regulated it will be entirely used up in the process. The hydrogen from the acetylene is, strange to say, unable to unite with oxygen at such a high temperature as that of the oxygen and carbon, so that it passes on beyond the oxygen-carbon flame and ultimately burns on its own account with the oxygen from the atmosphere in a second flame surrounding the first. Thus there is a double flame: inside, a little pointed cone of white flame, that is the oxygen and carbon; and outside that a bluish flame, the hydrogen and the atmospheric oxygen. The latter flame forms a kind of jacket entirely enveloping the former. And so when one melts metal by means of the white cone the hydrogen jacket shields the molten metal from oxygen and prevents the oxidation. Only one who knows the bother caused by oxidation whenever metals are heated can realise the wonderful advantage of this. And now we can turn to even another source, also quite modern, of high temperature.