varied considerably with the different homologues, and by taking advantage of this fact I was enabled with perfect safety to pronounce not only on the formula of the substances, but also on their boiling-point.

173. 66

The vapour density not only enables us, with non-metallic elements, to pronounce upon the atomic weight, but also, taken in conjunction with the analysis of a compound with silver, upon the class of bodies among which it is to be grouped.

174. "For the purpose of determining the atomic weight of a substance, it is very desirable to obtain at least one compound that consists of one atom of each of its constituents. Now, it is possible to obtain silver-salts of most of the non-metallic elements, in which we are sure of the constitution being of the kind required. If, therefore, we analyze a salt of this description, we at once possess the data for determining its atomic weight. Let us suppose that a silver-salt has yielded

175. "Now, on determining the density of its vapour, the non-metallic element gave 8.716; how are we to know to what class of bodies it belongs? A very little reflection will show, that as hydrogen is received as unity for atomic weights, if we received it also as unity for gaseous and vapour densities, the atomic weights of bodies of the same vapour volume would be expressed by their vapour density. It is obvious, then, that if a substance possesses the same vapour volume as hydrogen, its atomic weight will be obtained by dividing its vapour density by the density of hydrogen. If, therefore, the number obtained by dividing the vapour density of a non-metallic element by the density of hydrogen is identical with the number obtained by analysis of the silver-salt, it will be certain that the substance under examination possesses the same vapour volume as hydrogen, nitrogen, chlorine, &c. Let us try this in the present instance. The vapour density was, as

we have said, 8.716, and if we divide this by 0.0692, we obtain 125.95 as the result, which, although differing somewhat from 126.98, obtained as the atomic weight by analysing the silver-salt, is still quite a sufficient approximation to indicate that the body we have examined has a vapour volume the same as hydrogen. The body we have selected as an illustration is, as the student has probably perceived, iodine. If, on the other hand, it was necessary to divide the vapour-density by twice the density of hydrogen, to obtain the atomic weight, we may infer with safety that the vapour volume is that possessed by oxygen and elements of the same class."-Greville Williams.

CHAPTER III.

MOLECULAR ATOMS. *

Views hitherto held on the constitution of the elemental bodies, 176. Mr. Brodie's views, 177. An even number of atoms of an element required in many chemical combinations, 186. Synthesis of particles of the same element in opposite polar conditions, 187. The molecular and elementary atoms possess different properties, 191. Allotropic modifications of the elements, 192. Brodie's views upon allotropism, 192. Nascent state of the elements, 195. Ozone and its formation, 196. The existence of molecular atoms of the compound radicals, 197.

176. CHEMISTS have hitherto considered that chemical combination takes place only between unlike particles; that particles of the same nature are not influenced by the chemical, but only by the cohesive force. According to this view, no combination exists between the atoms of like bodies; they exist singly or detached. When, therefore, an element is liberated in the decomposition of compound bodies, its atoms are not supposed to enter into combination; and when an element enters into combination with other bodies, no decomposition of its molecules is supposed to take place.

The smallest quantity of an element which is supposed to be capable of existing in the free state. (See par. 120.)

H

177. Mr. Brodie, although not the first to question this view of the constitution of elementary bodies, was the first to establish, that at the moment of chemical change a chemical difference exists between the particles of which certain elemental bodies consist, perfectly the same in kind as that which exists between the particles of compound substances under similar circumstances, and on which the phenomena of combination and decomposition depend.

178. The proof, Mr. Brodie states, of the compound nature of a chemical substance is of a very simple kind. It lies in the fact, that it has been made by the combination of certain parts, or broken up into those parts, or at least in some phenomena which are supposed to be evidence of this. Indeed, the rational formula of a chemical substance is but a memorandum of its re-actions, and a particular mode of expressing the law of the synthesis and analysis of the body, apart from which it has but little meaning. The true nature, therefore, and chemical formula of the elemental bodies, as of all other substances, is to be discovered by the study of the series of chemical changes in which they are formed, and by the phenomena which they present when they pass into the combined condition. There are even well-known facts of great importance in this point of view, some unexplained, and some, I conceive, misinterpreted.

179. " The point which I shall seek to establish is this, -that, at the moment of chemical change, a chemical difference exists between the particles of which certain elemental bodies consist, perfectly the same in kind to that which exists between the particles of compound substances under similar circumstances, and on which the phenomena of combination and decomposition depend. That a peculiar chemical relation exists between two particles which combine, is generally admitted and expressed by the term affinity. The electro-chemical theory has defined more exactly in what this affinity consists, and states that the two particles are to one another in a positive and negative electric relation. But I do not know that it has ever been pointed out that this chemical relation-this

After the student has become thoroughly conversant with this chapter, he ought to study the section on Chemical Polarity in Graham's "Elements of Chemistry," vol. i., 2nd edit.

affinity between the particles of a substance-is an essential condition of the decomposition as well as of the composi tion of the body. As I am about to infer a chemical difference between the particles of the element from the fact of their chemical separation, I must say a few words upon this point, and I shall simplify the whole question by stating briefly the mode in which I consider chemical change to be effected. I may do this sufficiently for my present purpose in the following propositions:

"1. That when two particles chemically combine, a certain chemical relation exists between them, which is expressed by the terms positive and negative. The chemical difference of the particles I term the difference between their conditions in this respect.

"2. That when chemical combination takes place between the particles of which any two or more substances consist, a chemical difference exists between the particles of each substance, so that the particles of the same substance are to one another in a positive and negative relation.

"3. That the chemical relation between any two particles of these substances is determined by the chemical relation of all the other particles with which they are for the time being associated. Substances, the particles of which are to one another in this peculiar chemical relation, I term chemically polar.

180. "Silver cannot be oxidized by the direct action of oxygen on the metal, but oxide of silver is readily formed by boiling the chloride of this metal with potash. The particles of oxygen and silver have, therefore, acquired by this association with the chlorine and potassium a chemical relation or affinity, which at other times they have not. This is the fact. The rational conception of the fact is given in the expression,

+ +

Ag Cl K O= Ag 0 + K Cl,

in which I have indicated the polar relation of the substances. The chemical relation, therefore, between the oxygen and silver is essentially dependent on the chemical relation between the oxygen and potassium, in the same way as a negative and positive electricity are related to each other. The same is true of the relation between any other two particles of the system. Hence chemical

decomposition is an essential condition of chemical combination; so that when we see one of these events we may infer the other.

181. "On the other hand, where this polar division of the substance cannot take place, there is no chemical action, or, at any rate, it takes place with greater difficulty: thus, anhydrous sulphuric acid may be distilled off carbonate of potash without alteration; and generally, the socalled anhydrous acids have none of the combining properties of the hydrates to which they correspond; the reason of this being, that when these bodies combine they do not decompose, and that it is by the very fact alone of the decomposition of the substance, that the combining power is developed in the particles of which they consist, so that in the chemical change which is thus represented,―

HSO, KO=HO+ SO1K,

the two combinations which take place are not two combinations accidentally simultaneous, but correlative and mutually dependent phenomena, which we cannot separate.

182. "In the case of double decomposition, each of the four substances which enter into the change is combined; but it does not appear that this state of combination is necessary to the action. It can take place, also, and in the same manner, when the combining substances are only in contact with each other, and not in combination, provided always that there is the right chemical difference between them, which, however, is essential. Thus, for example, when iodine and phosphorus decompose water (in the usual mode of the formation of hydriodic acid), the chemical relation between the iodine and phosphorus is an essential condition of the action. The same remark applies to the decomposition of water between nitric oxide and chlorine, which can be effected by neither body separately; so that the changes which take place in these experiments are not simply due to the fact that the chlorine or iodine stand in one relation to water, or to the elements of water, and the nitric oxide or phosphorus in another, and that thus the water breaks up, being acted upon by two opposite forces; but that there is also, and must be, a certain chemical difference between the chlorine and nitric oxide, and between the iodine and phosphorus, which