frame accurate definitions for them. If possible it would perhaps be as well to banish the terms altogether; but as that would produce great inconvenience, it is better to take such imperfect definitions as we can get.

1. Acids. An acid is composed of hydrogen with one of those radicals, elementary or compound (page 86), which are called acid radicals. The hydrogen can be replaced by metals, in which case one of the compounds called salts is formed. Acids redden litmus, and are commonly sour.

The following are important examples :-Hydrochloric Acid, HC1; Nitric Acid, HNO,; Sulphuric Acid, H2SO1; Phosphoric Acid, H,PO,. In the first of these hydrogen is united with the elementary acid radical Cl, in the others, with the compound radicals NO, SO4, and PO4.

The imperfection of the definition will be apparent if we remember that we can only define an acid radical as a radical which, united with hydrogen, forms an acid.

Basity of acids.-Acids are said to be monobasic, dibasic, or tribasic, according as they contain one, two, or three atoms of hydrogen which can be replaced by acids. In the previous examples HC1 and HÑO, are monobasic, H2SO, is dibasic, and H,PO, is tribasic. Of course this is equivalent to saying that the radicals of those acids are monad, diad, or triad. PO, is a triad radical, and its acid may written in this way, H.(PO,)". Acids which contain more than one atom of replacable hydrogen are said to be polybasic.

be

Oxygen Acids, or Oxy-acids.-Most acids contain oxygen as a part of their radical, and these are related in a very simple manner to a particular series of oxides, which, for that reason, are called acid oxides or anhydrides. When one of these anhydrides combines with water, an acid is formed, and when, on the other hand, water is removed from an acid, the corresponding anhydride is obtained.

One molecule of nitric anhydride, uniting with one of water, forms two molecules of nitric acid. In a similar manner, if from one molecule of sulphuric acid one molecule of water be taken, one molecule of sulphuric anhydride will remain.

It must not, however, be supposed the acids are always, or even generally, prepared in practice from the anhydrides. One anhydride, indeed, silicic anhydride, SiO2, will not combine directly with water, although its acid can be obtained by indirect means. The anhydrides of many acids have not yet been obtained, and one anhydride (carbonic anydride, CO) is known to which no corresponding acid can be proved to exist. The formula for the acid should be

H.CO.

2. Salts.-A salt is a compound containing a metal and an elementary or compound acid radical.

A salt only differs from an acid by containing a metal in place of hydrogen. Taking, for example, the salts of sodium, those corresponding to the acids already mentioned are—

Na Cl Sodium chloride; corresponding to H Cl.

The proportion which the metal and radical bear to one another depends of course on the atomicity of each. Thus the chloride, nitrate, sulphate, and phosphate of the diad metal calcium, and of the triad metal bismuth, are formulated in this way:

It will be seen from the above remarks that acids are really hydrogen salts. Some chemists indeed name them accordingly, and call H.SO, "hydrogen sulphate," instead

H

of sulphuric acid. The term acid is applied by them to the anhydrides, so that the formula for sulphuric acid becomes SO.. Many salts are known which are more or less irregular in their composition, but modern chemical theory enables us to give a tolerably satisfactory account of all of hem.

3. Bases. A base is a metallic hydrate (that is, a compound of a metal with the radical HO) which is capable of reacting with acids to form salts.

As HO is a monad radical, the constitution of hydrates is closely analogous to that of chlorides, nitrates, &c. For example:

Sodium Hydrate.

Na HO is like Na C1 and Na NO ̧.

Calcium Hydrate.

Ca" (HO), is like Ca" Cl, and Ca" (N O3),.

Bismuth Hydrate.

Bi""' (HO), is like Bi"" Cl, and Bi (NO3)3.

Bases, like acids, are related to a particular series of oxides called basic oxides. These oxides are sometimes called bases. They differ from the true bases by the elements of water.

Some basic oxides, however (bismuth oxide, for instance), will not combine directly with water; and, on the other hand, some basic hydrates (sodium hydrate, for instance) cannot be dehydrated by heat. Calcium oxide and hydrate are examples of compounds which experience both changes with ease. The oxide (quick lime) combines eagerly with water, great heat is produced, and calcium hydrate (slacked lime) is formed. When slacked lime is heated, water is expelled, and quick lime once more obtained.

Ca" O+ H2O = Ca" (HO); and Ca" (HO), H2O Ca" O.

=3

Formation of Salts.-Salts can be formed by a variety of processes, only a few of which can be specified here, with one or two examples of each. It must not be supposed that every process is practicable in all cases.

1. By the action of metals on acids:

H2SO,+Zn" = Zn" SO, H2.

4

4

2. By the action of metallic oxides on acids:

Ca" O2 HCl = Ca" Cl2+ H2O.

3. By the action of bases on acids:

Na HO+ HCl = Na Cl + H2O;
Ca" (HO)2+2 HCl = Ca" Cl2+ 2 H2O.

This is the case before referred to.

4. By the action of anhydrides on metallic oxides:

O=

SO, Na, O Na, SO..

5. By the action of anhydrides on bases:

It will be seen that in most of these reactions water is

formed simultaneously with the salt.

PART II.

NON-METALLIC ELEMENTS.

INTRODUCTION.

THE old distinction between organic and inorganic chemistry is fast fading away. It was formerly believed that vegetables and animals had the power of producing in their organisms chemical compounds which could not be formed artificially in the laboratory. Vast numbers of compounds were known which owed their origin directly or indirectly to the animal or vegetable kingdom, and which could not be obtained from any other source. To the department of chemistry which dealt with such compounds, the name "Organic Chemistry" was very properly applied. But the progress of scientific research has taught us to manufacture a great number of these compounds from inorganic materials, and there is therefore no longer any reason why they should be separated from other compounds in a general system of classification.

The so-called organic compounds have, however, one feature in common. They all contain carbon, and it is therefore convenient for purposes of study to retain them in a separate department of chemistry. Moreover, although some few carbon compounds enter into the composition of important minerals, a very large number of them bear some direct or indirect relation to the processes of life, and the term " organic" may therefore, in this limited sense, still be applied to them. It is only necessary to bear in mind that what is still generally called organic chemistry is but the chemistry of carbon compounds. In the present division of the book we shall say as little about it as possible.