Now we choose mass as a unit of measure of the quantity of material; and we are justified in doing so, because experiments have shown that material, confined in a closed space, does not appreciably alter its mass. The mass is proportional to the weight, generally measured as the force exerted at some definite latitude on the earth’s surface, tending to pull the body towards the centre of the earth. This force is equal to about 981 dynes at London. There is no known reason why mass and weight should be proportional to each other; for the cause of the attraction of the earth has never been satisfactorily elucidated. We therefore use weight, or the attraction of the earth, as a convenient means of determining the relative masses of two objects. Hence it appears rational to prefer the expression “atomic weight” to “atomic mass,” seeing that the former represents the actual result of experiment, and also because we are dealing in atomic weights with relative numbers. But this is really a matter of choice.
The atomic weights therefore represent the relative masses in which the elements generally unite. They often, however, unite in multiples of these weights, as formulated by Dalton’s second law. The weights, arranged in numerical order in columns, give us the periodic table.
Now energy can be measured in other units besides those of force and mass. Heat is one form of energy, and it is measured by an interval of temperature, and by a property which we term specific heat. It happens that the latter property varies, not with the mass of the substance heated, but with its atom, so that all elements have approximately the same atomic heat; that is, quantities of elements proportional in mass to their atomic weights require approximately equal increments of heat to raise their temperature through an equal interval, say 1°. This is the formulation of Dulong and Petit’s law previously alluded to. But here we meet with irregularities, which have up till now defied classification. The heat imparted to an aggregation of atoms is not expended solely in raising their temperature; other work is done also, as is generally supposed, in the nature of expansion, or separation of parts, as in overcoming the attraction between the atoms in the molecule, or in imparting special motion to the atoms; such work, however, involves an expenditure of energy which is either very small in proportion to the total energy imparted as heat, or is nearly the same for all elements. At present we cannot decide between these alternatives, owing to the lack of knowledge of the nature of liquids and solids. The main fact, however, is incontestable: that the heat energy required to raise different elements through the same interval of temperature is the same, not for equal masses or weights of the substances, but for their atomic masses.
Again, many compounds when dissolved in water conduct an electric current, while they themselves are decomposed; and the different ingredients of the compound are often deposited at the points where the electric current enters or leaves the liquid. Where they are not so deposited, it is usually because of their action on the solvent water. Now Faraday found that when elements are deposited, equal quantities of electricity are conveyed either by equal numbers of atoms or by some simple fraction of these numbers. To this fraction we apply the term valency. Thus we say that an atom of oxygen is bivalent, or a dyad, because it conveys twice as much electricity through the liquid in which it is present as an atom of hydrogen, which is termed univalent, or a monad. Here we see a direct connection between the conveyance of an electric charge and the atomic weight. The electric unit of quantity is in fact defined as that which can be conveyed by a certain weight of hydrogen, by 8 times that weight of oxygen, by 108 times that weight of silver, and so on. Therefore the electrical unit is connected, not with unit of mass, or with the gravitational unit, but with the atomic unit.
We have therefore a number of systems, each capable of being equated to a unit of energy, but of which the terms are in some as yet unknown way related to each other, and often more directly than they can be related to mass and weight. This relation is only an approximate one in the case of specific heats; it appears to be an absolute one in the conveyance of electrical energy. The arrangement of the elements in the periodic table must therefore be considered by the light of such general views.
I would venture to suggest, as a tentative method of solving this problem, that it be considered whether mass or weight are such invariable properties of matter as have generally been taken for granted. That the relative weights or masses in which elements combine always retain their invariable proportion is true, so far as we can determine; but it must be remembered that we cannot cause them to combine except under a very limited series of conditions. For example, the act of combination equalises the temperature of two combining atoms; it also, in all probability, equalises their electrical charges. It is a legitimate speculation whether, could we maintain a difference in their temperature or in their electric potential, their atomic weights might not also change. Indeed, we are ignorant whether mass is changed by alteration of temperature. Experiments made by Sir John Airy, and interpreted by Professor Hicks, appear to show that variation of temperature is not without some influence on gravitational attraction. Others by Professor Landolt point in the same direction.
It therefore appears to me not impossible that the mass of the atoms may be affected by the various and different properties which they possess, some to a greater, some to a lesser extent. It must be admitted that atoms differ from each other in the readiness with which they combine with those of the same kind to form molecules; and that molecules of different elements differ from each other in their capacity to form molecular aggregates. Take, for example, such cases as caesium and fluorine, each intensely active, but towards different objects: caesium the most electro-positive of the metals, and fluorine the most electro-negative of elements. Surely their activity must be due to some cause which cannot but exert influence on their other properties, such as their mass and their gravitational attraction, as it doubtless has influence on their specific heats, and on many of their other physical properties. And contrast these instances with helium and with argon, the most indifferent of substances, the atoms of which are unwilling, and apparently unable, to pair even with themselves; it is hardly conceivable that these peculiarities should leave their other, and, as we are in the habit of thinking, invariable, properties unaffected. I venture to suggest that these powers of combination, due to some configuration or to some attractive force, tend to lessen the gravitational attraction by which we measure their atomic weights; that helium and argon, which possess little, if any, of such power to combine, show what may be termed the normal atomic weights, inasmuch as their gravitational attraction is subject to no deduction attributable to their reacting powers.
I cannot but think that, when some numerical values are assigned to this combinational power, it will be found that they will so increase their atomic weights as to display that regularity which, so far as we can see at present, is conspicuous by its absence.
I am aware that these suggestions are of a wholly speculative character; and yet I venture to put them forward in the firm conviction that no true progress in knowledge has ever been made without such speculations. It was the speculative phlogistic theory which combined phenomena apparently so distinct as the burning of a candle and the rusting of iron. It is true that that theory is now a phantom of the past, yet it served its purpose in directing attention to phenomena of a similar character. It would be easy to multiply instances of the kind; in almost every case some useful object has been served by speculation preceding exact knowledge. The object of science, as indeed of inquiry in all departments of human interest, is to reconcile the world of man with the world of nature, and to endeavour to know in part that of which we hope one day to attain to a perfect knowledge.
THE END