The quality of the steel depends upon the presence and the relative proportions of these ingredients, and a variation of a tenth of 1 per cent. in certain of them will make a different metal out of it. For instance, the steel becomes stronger and tougher as the proportion of nicked is increased up to about 15 per cent. Raising the percentage to 25 we get an alloy that does not rust or corrode and is non-magnetic, although both its component metals, iron and nickel, are by themselves attracted by the magnet. With 36 per cent. nickel and 5 per cent. manganese we get the alloy known as "invar," because it expands and contracts very little with changes of temperature. A bar of the best form of invar will expand less than one-millionth part of its length for a rise of one degree Centigrade at ordinary atmospheric temperature. For this reason it is used in watches and measuring instruments. The alloy of iron with 46 per cent. nickel is called "platinite" because its rate of expansion and contraction is the same as platinum and glass, and so it can be used to replace the platinum wire passing through the glass of an electric light bulb. A manganese steel of 11 to 14 per cent. is too hard to be machined. It has to be cast or ground into shape and is used for burglar-proof safes and armor plate. Chrome steel is also hard and tough and finds use in files, ball bearings and projectiles. Titanium, which the iron-maker used to regard as his implacable enemy, has been drafted into service as a deoxidizer, increasing the strength and elasticity of the steel. It is reported from France that the addition of three-tenths of 1 per cent. of zirconium to nickel steel has made it more resistant to the German perforating bullets than any steel hitherto known. The new "stainless" cutlery contains 12 to 14 per cent. of chromium. With the introduction of harder steels came the need of tougher tools to work them. Now the virtue of a good tool steel is the same as of a good man. It must be able to get hot without losing its temper. Steel of the old-fashioned sort, as everybody knows, gets its temper by being heated to redness and suddenly cooled by quenching or plunging it into water or oil. But when the point gets heated up again, as it does by friction in a lathe, it softens and loses its cutting edge. So the necessity of keeping the tool cool limited the speed of the machine.