Part 18 (1/2)

FOOTNOTES:

[13] ”In reviewing once more the facts elicited by our inquiry, we find them arranged around a common centre, a group of atoms preserving intact its nature, amid the most varied a.s.sociations with other elements. This stability, this a.n.a.logy, pervading all the phenomena, has induced us to consider this group as a sort of compound element, and to designate it by the special name of _benzoyl_.”--Liebig and Wohler, 1832.

[14] ”Animal Chemistry, or Chemistry in its Applications to Physiology and Pathology,” 1842. ”Researches on the Chemistry of Food,” 1847. ”The Natural Laws of Husbandry,” 1862.

CHAPTER VII.

MODERN CHEMISTRY.

On p. 162 I referred to the work of the German chemist Richter, by which the _equivalents_ of certain acids and bases were established. Those quant.i.ties of various acids which severally neutralized one and the same quant.i.ty of a given base, or those quant.i.ties of various bases which severally neutralized one and the same quant.i.ty of a given acid, were said to be equivalent. These were the quant.i.ties capable of performing a certain definite action.

In considering the development of Dumas's subst.i.tution theory, we found that Laurent retained this conception of equivalency when he spoke of an equivalent of hydrogen being replaced by an equivalent of chlorine (see p.

272). A certain weight of chlorine was able to take the place and play the part of a certain weight of hydrogen in a compound; these weights, of hydrogen and chlorine, were therefore equivalent.

This conception has been much used since Laurent's time, but it has for the most part been applied to the atoms of the elements.

Hydrogen being taken as the standard substance, the elements have been divided into groups, in accordance with the number of hydrogen atoms with which one atom of each element is found to combine. Thus certain elements combine with hydrogen only in the proportion of one atom with one atom; others combine in the proportion of one atom with two atoms of hydrogen; others in the proportion of one atom with three atoms of hydrogen, and so on.

The adjective _monovalent_, _divalent_, _trivalent_, etc., is prefixed to an element to denote that the atom of this element combines with one, or two, or three, etc., atoms of hydrogen to form a compound molecule.

Let us consider what is implied in this statement--”The nitrogen atom is trivalent.” This statement, if amplified, would run thus: ”One atom of nitrogen combines with three atoms of hydrogen to form a compound molecule.” Now, this implies (1) that the atomic weight of nitrogen is known, and (2) that the molecular weight, and the number of nitrogen and hydrogen atoms in the molecule, of a compound of nitrogen and hydrogen are also known.

But before the atomic weight of an element can be determined, it is necessary (as we found on p. 146) to obtain, a.n.a.lyze, and take the specific gravities of a series of gaseous compounds of that element. The smallest amount of the element (referred to hydrogen as unity) in the molecule of any one of these gases will then be the atomic weight of the element.

When it is said that ”the molecular weight, and the number of nitrogen and hydrogen atoms in the molecule, of a compound of nitrogen and hydrogen are known,” the statement implies that the compound in question has been obtained in a pure state, has been a.n.a.lyzed carefully, has been gasefied, and that a known volume of the gas has been weighed. When therefore we say that ”the nitrogen atom is trivalent,” we sum up a large amount of knowledge which has been gained by laborious experiment.

This cla.s.sification of the elements into groups of equivalent atoms--which we owe to Frankland, Williamson, Odling, and especially to Kekule--has been of much service especially in advancing the systematic study of the compounds of carbon. It helps to render more precise the conception which has so long been gaining ground of the molecule as a definite structure.

A monovalent element is regarded as one the atom of which acts on and is acted on by only one atom of hydrogen in a molecule; a divalent as one, the atom of which acts on and is acted on by two atoms of hydrogen--or other monovalent element--in a molecule; a trivalent element as one, the atom of which acts on and is acted on by three atoms of hydrogen--or other monovalent element--in a molecule; and so on.

The fact that there often exist several compounds of carbon, the molecules of which are composed of the same numbers of the same atoms, finds a partial explanation by the aid of this conception of the elementary atom as a little particle of matter capable of binding to itself a certain limited number of other atoms to form a compound molecule. For if the observed properties of a compound are a.s.sociated with a certain definite arrangement of the elementary atoms within the molecules of that compound, it would seem that any alteration in this arrangement ought to be accompanied by an alteration in the properties of the compound; in other words, the existence of more than one compound of the same elements united in the same proportions becomes possible and probable.

I have said that such compounds exist: let me give a few examples.

The alchemists poured a stream of mercury on to molten sulphur, and obtained a black substance, which was changed by heat into a brilliantly red-coloured body. We now know that the black and the red compounds alike contain only mercury and sulphur, and contain these elements united in the same proportions.

Hydrogen, carbon, nitrogen and oxygen unite in certain proportions to produce a mobile, colourless, strongly acid liquid, which acts violently on the skin, causing blisters and producing great pain: if this liquid is allowed to stand for a little time in the air it becomes turbid, begins to boil, gets thicker, and at last explodes, throwing a white pasty substance about in all directions. This white solid is inodorous, is scarcely acid to the taste, and does not affect the skin; yet it contains the same elements, united in the same proportions, as were present in the strongly acid, limpid liquid from which it was produced.

Two substances are known each containing carbon and hydrogen united in the same proportions: one is a gas with strong and irritating odour, and exerting a most disagreeable action on the eyes; the other is a clear, limpid, pleasant-smelling liquid.

Phosphorus is a very poisonous substance: it readily takes fire in the air at ordinary temperatures, so that it must be kept under water; but a modification of phosphorus is known, containing no form of matter other than phosphorus, which is non-poisonous, does not take fire easily, and may be handled with safety.

Once more, there is a compound of nitrogen and oxygen which presents the appearance of a deep-red, almost black gas; there is also a compound of nitrogen and oxygen which is a clear, colourless gas; yet both contain the same elements united in the same proportions.

But a detailed consideration of _isomerism_, _i.e._ the existence of more than one compound built up of the same amounts of the same elements yet possessing different properties, would lead us too far from the main path of chemical advance which we wish to trace.

The chemist is to-day continually seeking to connect the properties of the bodies he studies with the molecular structures of these bodies; the former he can observe, a knowledge of the latter he must gain by reasoning on the results of operations and experiments. His guide--the guide of Lavoisier and his successors--is this: ”Similarity of properties is a.s.sociated with similarity of composition”--by ”composition” he generally means molecular composition.

Many facts have been ama.s.sed of late years which ill.u.s.trate the general statement that the properties of bodies are connected with the composition of those bodies. Thus a distinct connection has been traced between the tinctorial power and the molecular composition of certain dye-stuffs; in some cases it has even become possible to predict how a good dye-stuff may be made--to say that, inasmuch as this or that chemical reaction will probably give rise to the production of this or that compound, the atoms in the molecule of which we believe to have a certain arrangement relatively to one another, so this reaction or that will probably produce a dye possessed of strong tinctorial powers.