When we buy a piece of iron from the blacksmith, we get more than iron for our money. Hidden in this iron are other substances (carbon, phosphorus, silicon, etc.), which are called "impurities" by the chemist. If all the impurities were taken out of the iron, however, we should have nothing but a powder left; this the chemist would call "chemically pure iron," but it would be of no value whatever to the blacksmith or mechanic. The impurities in iron and steel are just what are needed to hold the particles of iron together, and to make them valuable. By regulating the amount of carbon, phosphorus, etc., manufacturers can make different grades and qualities of iron or steel. When carbon is united with the _pure_ iron, we get what is commonly called iron. _=2. Kinds of Iron and Steel.=_ _Cast iron_ is the most impure form of iron. Stoves, large kettles, flatirons, etc., are made of cast iron. _Wrought iron_ is the purest form of commercial iron. It usually comes in bars or rods. Blacksmiths hammer these into shapes to use on wagons, machinery, etc. _Steel_ contains more carbon than wrought iron, and less than cast iron. _Soft steel_ is very much like wrought iron in appearance, and it is used like wrought iron. _Hard steel_ has more carbon in it than soft steel. Tools, needles, etc., are made of this. =EXPERIMENT 1. To study steel.= _Apparatus._ A steel sewing-needle (No. 1).[A] [Footnote A: _=NOTE. Each piece of apparatus used in the following experiments has a number. See "Apparatus list" at the back of this book for details. The numbers given under "Apparatus," in each experiment, refer to this list.=_] =3. Directions.= (A) Bend a sewing-needle until it breaks. Is the steel brittle? (B) If you have a file, test the hardness of the needle. _=4. Discussion.=_ "Needle steel" is usually of good quality. It will be very useful in many experiments. Do you know how to make the needle softer? =EXPERIMENT 2. To find whether a piece of hard steel can be made softer.= [Illustration: Fig. 1.] _Apparatus._ Fig. 1. A needle; a cork, Ck (No. 2); lighted candle (No. 3). The bottom of the candle should be warmed and stuck to a pasteboard base. =5. Directions.= (A) Stick the point of the needle into Ck, Fig. 1, then hold the needle in the flame until it is red-hot. (The upper part of the flame is the hottest.) (B) Allow the needle to cool in the air. (C) Test the brittleness of the steel by bending it. Test its hardness with a file (Exp. 1). _=6. Annealing.=_ This process of softening steel by first heating it and then allowing it to cool slowly, is called _annealing_. All pieces of iron and steel are, of course, hard; but you have learned that some pieces are much harder than others. =EXPERIMENT 3. To find whether a piece of annealed steel can be hardened.= _Apparatus._ The needle just annealed and bent; cork, etc., of Exp. 2; a glass of cold water.