large number of compounds called nitrates are known, which differ from nitric acid only in containing some metal in place of hydrogen. Thus we have sodium nitrate NaNO, and silver nitrate AgNO. In all these nitrates, however, the common quantity NO is found, and NO, is therefore called the radical of the nitrates. In the same way the formula for sulphuric acid is H2SO,, and the sulphates all contain the radical SO. Neither NO, nor SO, exist in a separate state. It is probably impossible for them to do so. We do not even know that they exist in the compounds, but we do know that their elements are present in the same proportions in a large number of compounds, and that all these compounds are linked together by a certain similarity of properties; and therefore, without troubling ourselves either to affirm or deny their existence as separate entities, it is convenient to assume their presence in certain compounds. We shall meet with many radicals as we go on, many of them very unlike one another in function; but for the present we have to regard them only in one point of view. In the compounds which contain them, radicals play the part of elements, and, like elements, every radical has its own proper atomicity. The acids which contain radicals afford good examples of this.

HNO, is like Hydrochloric Acid H Cl.
H2SO

Nitric Acid
Sulphuric Acid
Phosphoric Acid H, PO

Water
Ammonia

H2O.

H.N.

As Cl is united with one of H, so NO, is united with one of H. NO, is therefore called a monad radical. And as SO, is united with two of H, it may be compared to oxygen, and is called a diad radical. Their atomicity may be marked as that of elements is marked (NO,)', (SO,)", and (PO)"", and lastly it must be remembered that in all nitrates NO, is monad, and in all sulphates SO, is diad.

Measure of the Atomicity of Radicals.-The equivalent value of a radical is measured, just as that of an element is, by the number of atoms of hydrogen or other monad element with which it combines. NO3, for instance, is a monad radical, because in HNO, it is combined with one atom of hydrogen. A radical is in fact an incomplete compound, and it is monad, diad, or triad according as it lacks one, two, or three monads to complete it. The following table includes some

of the most important radicals of chemistry. Only a few of them have individual names.

NH, (Ammonium) in Ammonium Compounds, as in N H, Cl. NH, NO3.

CH, (Methyl) in Methyl Compounds.

C2 H, (Ethyl) in Ethyl Compounds

CH, Cl.

C2 H, Cl.

CH, (Methylene) in Methylene Compounds, as in C H2 Cl2.

2

K2 SO3.

H2SO4.

K2S 04.

Ca" SO3.
KHSO4.

K2CO3.

KHCO3.

Ca" CO3.

H2 Si O.

Mg" Si O..

CH in such compounds as Chloroform, C H Cl..

Si O, in Silicates

TETRADS.

as in H, Si O4. Mg", Si 04.

P2 O, in Pyrophosphates,, HP2O, Na, P2 O,. Mg"1⁄2 P2 07.

2

NOMENCLATURE.

2 2

Names are given to chemical compounds, according to rules which, though not perfect, are better than those employed in any other science. Only a few of the simplest need be indicated in this place.

1. Compounds of two elements only are distinguished by

the termination ide. Thus the compound of zinc and oxygen is called oxide of zinc, or zinc oxide; zinc and chlorine, zinc chloride; sodium and chlorine, sodium chloride; and so

on.

This rule is invariable, though for special reasons a different name is more often used for certain simple compounds.

2. When there are two compounds of the same two elements, the lower-that which contains the least oxygen, chlorine, &c.-is distinguished by the termination ous, and the higher by the termination ic. The lower chloride of tin SnCl2, for instance, is called stannous chloride, and the higher, SnCl4, stannic chloride. The same rule is applied to acids. HNO, is called nitrous acid, and HNO, nitric acid. 3. The substances called salts which are derived from an acid which ends in ous have the termination ite. All the salts, for instance, which contain the radical NO2, and which therefore correspond to nitrous acid, are called nitrites. Salts which correspond to acids ending in ic, have the termination ate. Nitric acid, for instance, forms nitrates, sulphuric acid, sulphates, and acetic acid, acetates. Almost the only exception to this rule is in the compounds called hydrates, which contain the radical HO, and which, instead of corresponding in composition to an acid, correspond to water. Other rules will be more conveniently given as instances

occur.

ATOMIC AND MOLECULAR HYPOTHESES.

Atoms.-Atomic Weights.-Dalton, to whom we are indebted for the first clear enunciation of the laws of combination by weight, contrived a beautiful hypothesis to account for them. In its simplest form it is known as Dalton's atomic hypothesis. According to this view, matter is composed of ultimate indivisible particles called atoms (aroμos, indivisible). Every element has its own atom, peculiar in properties and in weight. All compounds are formed by the union together of elementary atoms. The relative weights of the atoms are expressed by their combining weights, and afford a simple reason for those combining weights. Thus (taking the modern numbers), 35.5 parts of chlorine combine with 1 of hydrogen, because every atom of chlorine is attracted by and combines with one atom of hydrogen. In the same way, every one atom of oxygen, weighing 16, combines with two

atoms of hydrogen, weighing 2. In fact, the combining weights of the elements show the relative weights of the atoms, hydrogen being taken as unity, and this is the reason why the combining weights are generally called atomic weights. It follows from this that the ultimate particlethe smallest possible quantity-of any compound must contain at least two different elementary atoms. Compounds are groups of atoms of two or more kinds.

Molecules.-Molecular Weights.-The ultimate particles of compounds, consisting, as we have seen, of groups of atoms, are called molecules (moleculus, small mass). Their weight is, of course, the sum of the weights of their constituent atoms. Thus the molecular weight of hydrochloric acid, HCl, is 36.5, and of water, 18. Reasons have already been given for believing that the ultimate particles of most elements, at any rate in the gaseous state, consist of two or more similar atoms. The view adopted for compounds must therefore be extended to elements, and we must admit that in the gaseous state elements consist of molecules incapable of division, which molecules themselves consist of atoms of similar kind, sometimes of only one (mercury, zinc, and cadmium gases), sometimes of two (hydrogen, oxygen, &c.), and sometimes of more than two (ozone, phosphorus, arsenic, and sulphur gases).

By an extension of this view, the symbols and formulæ which are used to denote two volumes of any gas, elementary or compound, can also be applied with equal accuracy to denote respectively one atom and one molecule of it. The atom of hydrogen may be denoted by H, the molecule by H2; the atom of arsenic by As, the molecule (as gas) by As,; and single molecules of hydrochloric acid, steam, and ammonia may be represented by the formulæ HCl, H2O, and H.N. This leads us to another important hypothesis, which of late years has been very generally adopted. It refers only to gases.

Hypothesis of Avogadro and Ampere.-This hypothesis, suggested by Avogadro in 1811, and further developed by Ampere in 1814, may be stated in the following terms:

Equal volumes of different gases, at equal pressure and temperature, contain equal numbers of molecules.

This hypothesis explains two things.

1. The fact already mentioned (pages 34, 56), that all gases are equally affected in volume by variations of pressure and temperature. Physical agencies, such as heat and pressure, do not affect the nature of molecules, but only their distance from one another, and this being the same in all gases under similar conditions, the effect produced by those agencies must naturally be the same in all cases.

2. The law of gaseous volumes. If one molecule of chlorine enters into chemical action with one molecule of hydrogen, the same will be true of one thousand, or one million molecules of each. Now by the hypothesis one thousand, or one million, molecules of chlorine occupy a space equal to that occupied by one thousand, or one million molecules of hydrogen, and it therefore follows that the two elements should react together in the proportion of equal volumes, which experiment proves to be the fact. In like manner, two volumes of hydrogen react with one volume of oxygen, because one volume of oxygen contains n, and two volumes of hydrogen 2n molecules, and every molecule of oxygen reacts with two molecules of hydrogen.

Structure of Gases.-Summary of Hypotheses.-The modern doctrine may be summed up in these words.

1. Equal volumes of gases, at equal pressure and temperature, contain equal numbers of molecules.

2. Every molecule consists of one or more atoms, either similar in kind (elementary molecules), or dissimilar in kind (compound molecules).*

*The molecular hypothesis is sometimes referred to so loosely, that a few further remarks upon it may not be out of place.

1. The hypothesis asserts nothing of the relative bulk of molecules, or atoms. It does not assert that one molecule of hydrogen has the same bulk as one molecule of chlorine, but only that n molecules of hydrogen occupy, by their molecular movements, the same space as n molecules of chlorine.

2. Similarly, the hypothesis does not assert that the bulk of an atom of hydrogen is equal to the bulk of an atom of chlorine, but only that two atoms of hydrogen and two atoms of chlorine occupy, respectively, by their atomic movements, the space of one molecule. The atoms of matter may be almostly infinitely small, as compared with the molecules, and if the bulk of the molecules were known we should be no nearer to knowing that of the atoms.

A molecule of hydrogen may, for aught we know, be comparable to an inflated bladder, with two peas rotating in it.