In previous chapters we have discussed the necessity of a thorough knowledge of the physical characteristics or properties of the various materials used in industry. It is equally important to understand the chemical "make up" of those materials; that is, their exact composition. Iron and steel, for example, are used more or less in every trade. The iron ore contains many other substances, such as carbon, silicon, phosphorus, sulphur, manganese, and so on. Experience has taught the steel-maker that it is desirable to have as little phosphorus and sulphur as possible in the raw pig iron from which he makes his steel. The foundry man requires pig iron without much manganese, because this property tends to make the iron hard and difficult to melt. Silicon in pig iron makes the carbon assume a form called graphite carbon. This tends to weaken the iron and steel bars, rails, sheets, etc., which are made from the pig iron, because it forms flakes between the particles of iron.

What has been said in regard to iron and steel, applies equally to other materials. A knowledge of the principles of chemistry is needed to understand the composition of these materials, and the chemical processes that take place when they are used in manufacture. In determining the chemical properties of a substance it is necessary to take a small amount of the mixture and analyze it (Fig. 66).