The same applies to all elementary substances. Thus hydrogen, two atoms of which combine with one of oxygen to form water, combines one atom to one with chlorine to form the molecule of hydrochloric acid, which weighs 36·5 mc., being the united weights of one atom of chlorine, 35·5 mc., and one of hydrogen, 1 mc. These, with hundreds of similar instances, are the results not of theories as to molecules and atoms, but of actual facts, ascertained by innumerable experiments made independently by careful observers over long periods of years, many of them dating back to the labours of the alchemists of the middle ages in pursuit of gold. The atomic theory is the child and not the parent of the facts, and is indeed nothing but the summary of the vast variety of experiments which led up to it, as Newton’s law of gravity is of the facts known to us with regard to the attractions and motions of matter in the mass. But as Newton’s law enables us to predict new facts, to calculate eclipses and the return of comets beforehand, and to compile nautical almanacs; so the new chemistry, based on the atomic theory, affords the same conclusive proof of its truth by enabling us in many cases to predict phenomena which are subsequently verified by experiment, and to infer beforehand what combinations are possible, and what will be their nature. The actual existence, therefore, of molecules and atoms is as well-ascertained a fact, as that of cwts. and lbs., or of planets and stars, of solar systems and nebulæ.