~Reduction.~ On account of its great affinity for oxygen, hydrogen has the power of abstracting it from many of its compounds. Thus, if a stream of hydrogen, dried by passing through the tube B (Fig. 15), filled with calcium chloride, is conducted through the tube C containing some copper oxide, heated to a moderate temperature, the hydrogen abstracts the oxygen from the copper oxide. The change may be represented as follows: hydrogen + {copper} {hydrogen} {oxygen}(copper oxide) = {oxygen }(water) + copper The water formed collects in the cold portions of the tube C near its end. In this experiment the copper oxide is said to undergo reduction. _Reduction may therefore be defined as the process of withdrawing oxygen from a compound._ ~Relation of reduction to oxidation.~ At the same time that the copper oxide is reduced it is clear that the hydrogen is oxidized, for it combines with the oxygen given up by the copper oxide. The two processes are therefore very closely related, and it usually happens that when one substance is oxidized some other substance is reduced. That substance which gives up its oxygen is called an _oxidizing agent_, while the substance which unites with the oxygen is called a _reducing agent_. ~The oxyhydrogen blowpipe.~ This is a form of apparatus used for burning hydrogen in pure oxygen. As has been previously stated, the flame produced by the combustion of hydrogen in the air is very hot. It is evident that if pure oxygen is substituted for air, the temperature reached will be much higher, since there are no inert gases to absorb the heat. The oxyhydrogen blowpipe, used to effect this combination, consists of a small tube placed within a larger one, as shown in Fig. 16. [Illustration: Fig. 16] The hydrogen, stored under pressure, generally in steel cylinders, is first passed through the outer tube and ignited at the open end of the tube. The oxygen from a similar cylinder is then conducted through the inner tube, and mixes with the hydrogen at the end of the tube. In order to produce the maximum heat, the hydrogen and oxygen must be admitted to the blowpipe in the exact proportion in which they combine, viz., 2 volumes of hydrogen to 1 of oxygen, or by weight, 1 part of hydrogen to 7.94 parts of oxygen. The intensity of the heat may be shown by bringing into the flame pieces of metal such as iron wire or zinc. These burn with great brilliancy. Even platinum, having a melting point of 1779 deg., may be melted by the heat of the flame. While the oxyhydrogen flame is intensely hot, it is almost non-luminous. If directed against some infusible substance like ordinary lime (calcium oxide), the heat is so intense that the lime becomes incandescent and glows with a brilliant light. This is sometimes used as a source of light, under the name of _Drummond_ or _lime light_. [Illustration: Fig. 17]