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 [30] These considerations would hold on the assumption that no combination takes place between chlorine and bromine.