from the atoms of any other element, and the relative weight compared with hydrogen is termed the "atomic weight." The following elements are referred to in the present work.

Compounds are formed, as already stated, by the combination of different elements, thus; FeO represents oxide of iron, and MnO oxide of manganese. In many cases two elements unite in more than one proportion, such as FeO, Fe2O3, Fe3O4, each of which requires a distinguishing name. There are several systems of nomenclature, but the simplest for compounds containing two elements-is that of writing the name of the metal first, and the non-metal or least metallic element afterwards, giving it the termination "ide." When two non-metals combine, the one which is most unlike a metal is written second. Sometimes Greek prefixes are used for the element of the second position, such as mono, di, tri, tetr, etc., to indicate the number of atoms present.

B

Another system is to make the metal terminate in

"ic" or "ous." That compound which contains the

greater proportion of the non-metallic constituent is distinguished by the suffix "ic" and that containing the lesser in "ous." The following list will illustrate these points.

When three elements--one being a metal and another oxygen-are combined together, the name of the second is made to end in "ate.' In the following list a few. compounds are given to illustrate this, but it should be observed that the order of placing the symbols is immaterial. In works on metallurgy the arrangement of formulæ in the last column is most

common.

Equations. When two or more elements unite to form a compound, or two compounds unite to form a more complex compound, the change may be represented by a chemical equation, thus;

Any chemical change may be so represented, the bodies taking part in the change being placed on the left hand side of the sign of equality

66 ", and the bodies formed

after the change, on the right hand side.

The quantities involved in any such change can be seen at a glance, since the symbol of an element represents a definite weight, given in the table, p. 17, as the atomic or combining weight. Thus, in the first equation 3 x 56 × parts of iron unite with 4 × 16 parts of oxygen to form 168 +64 232 parts of the black oxide of iron.

=

When a chemical change occurs, the production or absorption of heat is the result, the former by the union of elements or compounds, the latter by the forcible separa

tion of the constituents of a compound. When the temperature produced is considerable, it is usual to speak of it as combustion; thus, carbon combines with oxygen to produce carbonic acid and generates intense heat.

In common language it is said to burn, and the burning of fuel is simply the result of chemical combination.

When substances combine with oxygen they are said to be "oxidised," and the substance which imparts the oxygen is termed an oxidising agent. Conversely, substances which remove oxygen from a body are termed reducing agents. The following lists give some examples of both kinds.

Iron tetroxide. Carbonic acid.

Fe3O4 + со

Ferrous oxide. Carbonic acid. Iron tetroxide. Carbonic oxide.

Ferric oxide. Carbonic oxide.

Fe2O3 + CO

=

Ferric oxide. Carbonic oxide. Ferrous oxide. Carbonic acid.

It will be obvious that in the cases of oxidation and reduction, the change may be partial or complete. Thus, iron is completely oxidised when converted into Fe2O, and oxide of iron is completely reduced when all the iron is brought to the metallic state.

The following is a short description of the chief substances mentioned in the present chapter and which are subsequently alluded to.

Oxygen (O). This is the most abundant element, forming probably one half the solid mass of the earth, ths of all water, and about 21 per cent. by volume of the air. It is necessary for life and all ordinary processes of combustion. In the air it is a gas, but in its compounds it is chiefly a solid or a liquid. Its oxidising action has been already mentioned. It is the chief supporter of combustion, that is, it forms the active medium in which bodies burn.

Oxides, as the compounds of oxygen with other elements are termed, are divided into two groups :-10. Those which have an acid character, chiefly oxides of the non-metals, and often termed acids, such as carbonic acid CO, and silica SiO,. 2°. Those of a basic character, chiefly oxides of the metals, which are termed bases. These two classes are opposite in character, and when united in equivalent proportions, neutralise each other, forming what are termed "neutral" bodies, which do not possess the characteristic properties of either kind. Thus