Dr. Odling delivered a lecture on the above before the Chemical Society, London, February 2, 1882.

The lecturer said that it had been found useful to occasionally bring forward various points of chemical doctrine, on which there were differences of opinion, to be discussed by the society. On this occasion he wished not so much to demonstrate certain conclusions, or to make a declaration of his opinions, as to invite discussion and a thoughtful consideration of questions of importance to chemists. Originally three questions were proposed: First, Is there any satisfactory evidence deducible of the existence of two distinct forms of chemical combination (atomic and molecular)? Second, Is the determination of the vapor density of a body alone sufficient to determine the weight of the chemical molecule? Third, In the case of an element forming two or more distinct series of compounds, e.g., ferrous and ferric salts, is the transition from one series to another necessarily connected with the addition or subtraction of an even number of hydrogenoid atoms? He would, however, limit himself to the first of these questions; at the same time the three questions were so closely associated with one another that in discussing the first it was difficult to know where to begin. The answer to this question (Is there any satisfactory evidence deducible of the existence of two distinct forms of chemical combination?) depends materially on the view we take of the property called in text-books valency or atomicity; and before discussing the question it is important to have a clear idea of what these words valency and atomicity really mean. It is necessary, too, to start with some propositions which must be taken for granted. These propositions are: First, that in all chemical changes, those kinds of matter which we commonly call elementary, do not suffer decomposition. Second, That the atomic weights of the elements as received are correct, i.e., that they do really express with great exactitude the relative weights of the atoms of the individual elements. If we accept these two propositions, it follows that hydrogen can be replaced atom for atom by other elements not only by the hydrogens but by alkali metals, etc. Hydrogen is, it may here be remarked, an element of unique character; not only can it be replaced by the elements of the widely different classes represented by chlorine and sodium, but it is the terminal of the series of paraffins, CH; CH, CH, H. The third proposition which must be taken for granted is, that the groups of elements, CH, CH, behave as elements, and that these radicals, ethyl, methyl, etc., do not suffer decomposition in many chemical reactions.

Now as to valency or atomicity, accepting the received atomic weights of the elements, it is certain that there are at least four distinct types of hydrogen compounds represented by ClH, OH, NH, CH. The recognition of these types, and their relations to each other as types, was one of the most important and best assured advances made in theoretical chemistry. When we compare the formula of water with that of hydrochloric acid, we find that there is twice as much hydrogen combined with one atom of oxygen as there is combined with one atom of chlorine; and in a great many other instances, we find that we can replace two atoms of chlorine by one atom of oxygen, so that we get an idea of the exchangeable value of these elements, and we say that one atom of oxygen is worth two of chlorine, or is bivalent; similarly, nitrogen is said to be trivalent. The meaning attached to the word "valency," is simply one of interchangeability, just as we say a penny is worth two halfpennies or four farthings. The question next arises, is the valency of an element fixed or variable? If the word be defined as above, it is absolutely certain that the valency varies. Thus, tin may be trivalent, SnCl, or tetravalent, SnCl. Accordingly elements have been classed as monads, dyads, triads, etc. The lecturer objected most strongly to the word "atomicity;" he could not conceive of one atom being more atomic than another; he could understand the atomicity of a molecule or the equivalency of an atom, but not the atomicity of an atom; the expression seemed to him complete nonsense. He next considered the possibility of assigning a fixed limit to this valency or adicity of an atom, and concluded that the adicity was not absolutely fixed, but was fixed in relation to certain elements, e.g., C never combines with more than four atoms of H; O never more than two atoms of H, etc. The adicity of an element when combined with two or more elements is usually higher than when combined with only one, e.g., NH, NHCl. The term "capacity of saturation," may be used as a synonym for adicity, if care be taken to distinguish it from other kinds of saturation, such as an acid with an alkali, etc. Adicity is, however, quite distinct from combining force; the latter is indicated by the amount of heat evolved in the combination.

The lecturer then proceeded to criticise a statement commonly found in text books, that chemical combination suppresses altogether the properties of the combining bodies. The reverse of this statement is probably true. To take the case commonly given of the combination of copper and sulphur when heated; this is good as far as it goes, but there are numerous instances, as ClI, SSe, etc., where the original properties and characters of the combining elements do not completely disappear. The real statement is that the original properties of the elements disappear more or less, and least when the combination is weak and attended with the evolution of a slight amount of heat, and in every case some properties are left which can be recognized. So with reference to the question of atomic and molecular combination, as atomic combination does not necessarily produce change, it does not differ in this respect from what is usually called molecular combination.

The lecturer then referred to an important difference in the adicity of chlorine and oxygen. Chlorine can combine with methyl or ethyl singly. Oxygen can combine with both and hold them together in one molecule. The recognition of this fundamental difference between chlorine and oxygen, this formation of double oxides as opposed to single chlorides, marks an epoch in scientific chemistry.

The lecturer then considered the subject of chemical formulæ; it is the bounden duty of every formula to express clearly the number of atoms of each kind of elementary matter which enters into the constitution of the molecule of the substance. A formula may do much more than this. If we attempt to express too much by a complex formula we may veil the number of atoms contained in it. This difficulty may be avoided by using two formulæ, a synoptic formula giving the number of atoms present, and a complex formula perhaps covering half a page, giving the constitution of the molecule. But between the purely synoptic formula and the very elaborate formula there are others--contracted formulæ--which labor under the disadvantage, as a rule, of being one-sided, and so create a false impression as to the nature of the substance. Thus, for instance, to take the formula of sulphuric acid, HSO. This suggests that all the oxygen is united to the S; (HO)SO suggests that two atoms of hydroxyl exist in the molecule; then, again, we might write the formula HSOOH, or HOSO. All of these are justifiable, and each might be useful to explain certain reactions of sulphuric acid, but to use one only creates a false impression. The only plan is to use them variously and capriciously, according to the reaction to be explained. Again, ethyl acetate may be written--

 H_{3}C\

H_{2}C/

\

O

/

OC\

H_{3}C/ 

Or condensed--

 H_{5}C_{2} \

}O

H_{3}C_{2}O/ 

Or HCO.CHO, or HC.CHO. Now each of these two latter formulæ is a partial formula, each represents a one-sided view; it is justifiable if you use both, but unfair if you use only one.

We now come to the question as to the existence or non-existence of two distinct classes of compounds, one in which the atoms are combined directly or indirectly with each other, and the other in which a group of atoms is combined as an integer with some other group of atoms, without any atomic connection by so-called molecular combination. These two modes of combination are essentially distinct. The question is not one of degree. Are there any facts to support this theory that one set of compounds is formed in one way, another in a different way? Take the case of the sulphates: Starting with SO, we can replace one atom of O by HO, and obtain SO(HO) or HSO; replacing a second atom, we get SO(HO) or HSO, glacial sulphuric acid, a perfectly definite body corresponding to a definite class of sulphates, e.g., HMgSO, ZnSO, etc. By replacing the third atom of O we get S(HO) or HSOH; this corresponds to a class of salts, gypsum, HCaSO, etc. These are admitted without dispute to be atomic compounds. Are we to stop here? We may write the above compounds thus: HSO, HSOHO, HSO2HO. If we measure the heat evolved in the formation of the two latter compounds, it is, for HSO+HO, 6.272; HSO+2HO, 3.092. But if we now take the compound HSO+3HO we have heat evolved 1.744; so we can have HSO4HO, etc. Where are we to draw the line between atomic and molecular combination, and why? It comes to this: All compounds which you can explain on your views of atomicity are atomic, and all that you cannot thus explain are molecular. Similarly with phosphates, arsenates, etc. In all these compounds it is impossible to lay one's finger on any distinction as regards chemical behavior between the compounds called atomic and those usually called molecular.

Two points remain to be mentioned: The first is the relationship between alteration of adicity and two series (ous and ic) of compounds. Tin is usually said to be dyad in stannous compounds and a tetrad in stannic compounds, but in a compound like SnClAmCl, is not tin really a tetrad?

 {Cl

{Cl

Sn {Cl

{NH_{4} 

and yet it is a stannous compound, and gives a black precipitate with HS; so that valency does not necessarily go with the series. The second point is that an objection may be urged, as, for example, in ammonium chloride (the lecturer stated above that here N was a pentad, the addition of the chlorine having caused the N to assume the pentadic character), it may be said, why should you not suppose that it is the chlorine "which has altered its valency, and that the compound should be written:

 {H

{H

N { \

{H--Cl

{H/ 

There is something to be said for this view, but on the whole the balance of the evidence is in favor of nitrogen being a pentad.

In conclusion the lecturer stated that his principal object was to direct the attention of chemists, and especially of young chemists, to the question: Is there or is there not any evidence derived from the properties, the decompositions, or the relative stabilities of substances to warrant us in believing that two classes of compounds exist: one class in which there is interatomic connection alone, and another in which the connection is molecular?