. Elementary physics and chemistry: second stage . Fig. 6g.—When red lead is heated, a gas is given off which will re-kindle a glowing splinter. REASONS AND RESULTS. Where to look for the active part of air.— It has been seen several times that many substances take the active part out of the air, and combine with it when exposed to it under suitable condi- tions. A candle does so when it burns, iron when it rusts, and copper and lead when they are heated in contact with the air. Since these substances take the active part out of the air, and unite with it to form fresh substances, it should no
. Elementary physics and chemistry: second stage . Fig. 6g.—When red lead is heated, a gas is given off which will re-kindle a glowing splinter. REASONS AND RESULTS. Where to look for the active part of air.— It has been seen several times that many substances take the active part out of the air, and combine with it when exposed to it under suitable condi- tions. A candle does so when it burns, iron when it rusts, and copper and lead when they are heated in contact with the air. Since these substances take the active part out of the air, and unite with it to form fresh substances, it should not be difficult to make these or similar substances give up the part of the air which they take up, and to thus procure the active constituent of air by itself in a pure form. But a little more thought suggests that probably some of these substances would do better than others. It is quite certain that some are formed more easily than others. Will those which are most easily formed be the best from which to get the active part ? No. The reason is this. When a chemical change takes place very easily it generally ^This is the chemist's name for the rust of mercury, or quicksilver.
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