CHAPTER XI.

OF OXIDES.

WHAT is an oxide?

Any one or more of the simple substances, when united to a less quantity of oxygen than is necessary to form an acid, is called an oxide.

What substances are capable of forming oxides?

The mineral, the animal, and the vegetable kingdoms, all furnish matters which are convertible into oxides by an union with oxygen.

*

In what way do metals become united to oxygen There are several ways in which metallic ox

* The oxides appear to range themselves into two classes. There are oxides which are permanently such, so long as they retain the oxygen which enters into their formation; and there are others which seem to possess only a kind of intermediate state between combustibles and acids, being convertible into acids by a further portion of oxygen. This will appear as we proceed.

According to the old theory, metals were supposed to be oxidized by the loss of phlogiston; and when these oxides were reduced to a metallic state, it was imagined that they recovered their phlogiston from the carbonaceous matter employed in their reduction. Those persons who have not been in the habit of reading the works of the older chemists may understand their phraseology by attending to the following particulars: In most cases, all that is necessary is to substitute the word oxygen for phlogiston, with a slight inversion of the language. For, the effects which they attributed to the combination of phlogiston, appear to be due to the extrication of oxygen; and what they supposed to be owing to the loss of phlogiston, was really occasioned by the absorption of oxygen.

ides are formed, the chief of which are by the access of atmospheric air, by the decomposition of water, and by the decomposition of acids.

Will all metals become oxidized by exposure to the air?

No: gold, silver, and platina, cannot be oxidized, unless in a very high temperature; though iron,* copper, and lead, merely by long exposure to the air, will become oxidized in the coldest atmosphere. Manganese, by such exposure, will in a few hours be converted into a perfect oxide.

Are metals ever exposed to the air with the design of converting them into oxides?

The common red-lead of the shops, which is a true oxide of lead, is made by melting that metal in ovens so constructed as to have a free access of atmospheric air.†

How is it known that the change of common lead to red lead is caused by the absorption of oxygen?

This is known by the increase of weight which the metal acquires during the operation; and to

* Metals not only become oxidized by atmospheric air, but sometimes, by exposure to its action, pass from a lower to a higher degree of oxidizement. Thus, if a solution of the common sulphate of iron be exposed to the atmosphere, it acquires a further dose of oxygen, and by degrees a portion of the metal, in a higher state of oxidizement, is precipitated.

Some of the metallic solutions cannot be formed but in contact with atmospheric air or oxygen. Thus, copper or lead, placed in acetic acid, and excluded from the air, does not form any solution; but if the mixture be exposed, oxygen is absorbed, and the solution takes place.

The following account of the manufacture of red lead, from Watson's Chemical Essays, will be a satisfactory proof of this

confirm the fact, the oxide may be again reduced, and the original quantity of metal left unaltered. Do all metals increase equally when converted into oxides?

No: each metal has its extremes, between which it absorbs oxygen in various proportions:* and one metal may not only have a greater or less capacity, but also a greater or less attraction for oxygen than another; so that one will often rob the other, thus reducing the oxide to its primitive metallic form.t

doctrine. In the manufactories of red lead in Derbyshire, the melted lead is exposed to atmospheric air; the surface soon becomes covered with a dusky pellicle; this pellicle being removed, another is formed; and thus, by removing the pellicle as fast as it forms, the greater part of the lead is changed into a yellowish green powder. This powder is then ground very fine in a mill, and, when washed and properly dried, is thrown back into the furnace, and by constant stirring for 48 hours, so as to expose every part to the action of the air, it becomes red lead, and is taken out for use. Twenty cwt. of lead generally give 22 cwt. of red lead; so that 2 cwt. of oxygen is absorbed from the atmosphere during the process.

This may be shown by keeping a given weight of ironwire red-hot for some time in the bowl of a common tobaccopipe, and weighing the iron before and after it has been submitted to the experiment.

Zinc, by its powerful attraction for oxygen, decomposes a great number of salts and metallic solutions, and precipitates the metal from them in a metallic form. This may be exemplified by the experiment of the metallic tree, which has been attributed to Dr. Black. For the method of conducting it, see the Chapter of Experiments, No. 123.

Sir Humphry Davy has found that whenever one metal precipitates another from its arid solution, the body that falls down is usually free both from acid and oxygen; and that the whole of the oxygen and the acid is transferred from one metal to the other. Davy's Chemical Philosophy, vol. i. page 121. The pin-manufacturers whiten their pins on the same prin

What metals are oxidized by the decomposition of water?

Iron, zinc, tin, and antimony have the property of decomposing water, and become oxidized by the process. Thus the rust which forms upon polished iron is occasioned by the iron imbibing the oxygen of the water which it decomposes, as it meets it in the atmosphere. This metal, when heated, decomposes water with great rapidity.†

How do these metals operate in thus decomposing water?

This effect, like most of the operations of Nature, depends on chemical affinity. These metals having a greater affinity for oxygen than oxygen has for hydrogen, the oxygen of the water unites

ciple. They fill a pan with alternate layers of pins and grain tin, into which they pour a solution of super-tartrate of potass, and then boil the whole for four or five hours. In this process, the tartaric acid first dissolves the tin, and then gradually deposits it on the surface of the pins, in consequence of its greater affinity for the zinc, of which the brass wire is composed.

*An increase of near 30 per cent. may be given to iron, by heating it red hot, and passing a continued stream of the vapour of water over it when in that state. This increase of weight arises from its decomposing the water, and imbibing its oxygen.

The metals, which are capable of decomposing water, effect it much sooner when assisted by heat than without. In this case, the caloric unites with the hydrogen of the water, and carries it off in hydrogen gas; while the oxygen, the other component part of water, unites with the metal. Hydrogen requires a very large portion of caloric to give it the gascous form. This accounts for the necessity of heat, when water is decomposed by

means of some metals.

with the metal to form a metallic oxide,* while the hydrogen, the other ingredient of the water, escapes in the form of gas.

Is it known what proportion of oxygen each metal requires, in order to its being converted to an oxide?

Most of the metals are capable of combining with different proportions of oxygen, according to the mode by which they are oxidized.†

In what instances are acids used to oxidize metals?

Many instances of this mode of forming metallic oxides might be adduced: thus, common white lead is made by exposing sheet lead to the fumes of acetous acid; and the oxide of tin,

* See the account of an experiment, page 105, which explains the operation of metals in the decomposition of water.

With respect to some metals, one dose of oxygen occasions the loss of their metallic splendour; a larger dose destroys that splendour entirely; and the proportion of oxygen may be increased so far as to give to the oxides an appearance altogether earthy. We have reason to believe that most of the metals are capable of indefinite degrees of oxidizement, producing oxides of various colours, and possessing very different properties.

In the oxidizement of metals by acids, though there be no oxygen gas sensibly present by which it is effected, oxygen exists in the acids, also in the water with which the acids are diluted; and the effect is owing to the passage of a portion of oxygen from one of these substances to the metal; and the increase in weight which the metal acquires, is always equal to Whenever a metallic the weight of the oxygen absorbed.

oxide dissolves in an acid, it causes the acid properties to disappear exactly as if an alkali had been employed; and saturates corresponding quantities of the different acids. Berthollet.

The manufacture of white lead is conducted in the following manner: A number of earthen crucibles, holding from thrce to six quarts each, and nearly filled with vinegar, are placed in