knowledge so modified the notation of chemistry that the new formulæ, though differing but slightly from the old, represented more than composition by weight, namely, composition by volume also. Still later came the attempt, now being vigorously continued, to make every formula represent not only ultimate composition by weight and by volume, but also the probable arrangement of the atoms within the molecule. In other words, if we ignore the atomic hypothesis, a modern chemical formula aims to express some of the more important chemical relations and reactions of the body represented. In close connection with these purely chemical discoveries, we find a little physical work. Thanks to Kopp, we are able to calculate from the formula of almost any liquid its atomic volume, and thence its specific gravity at the boiling-point. Other investigators enable us to calculate the indices of refraction for different liquids, and, to a more limited extent, some other physical properties also. In short, a system of notation, originally based upon the properties of the atoms as regards weight, has been found to express also many of their other physical relations; and the list of facts thus expressed is continually lengthening. Evidently, then, the tendency of chemical investigation is to connect the physical properties of every substance directly with its composition.
Here we step over the border into physics. Plainly, if we have to deal with physical properties, we must study the forces represented by them. And, fortunately for the chemist, the tendency among physicists is entirely in his favor. Growing up contemporaneously with the development of chemical notation, we have had the grand ideas of the conservation of energy and the correlation of forces. We have learned that force is one, indestructible and uncreatable, and that all its manifestations are mutually convertible one into another. Either of the great modes of force may be active in affecting chemical composition; may cause chemical union or chemical separation; may be the motive of either analysis or synthesis. Now, in the direction here suggested, the main work of physics is being done. The chief object of the physicist to-day is to determine quantitatively the relations connecting all the different varieties of energy. Under what circumstances, and how, are forces transformed? Since these transformations are differently effected through the intervention of different forms of matter, it is clear that the physicist must take into account the chemical composition of the materials with which he deals. In short, then, the chemist must look to physics for a knowledge of the forces involved in chemical changes; while, on the other hand, the science of physics must needs throw from chemistry its information upon the nature of all the material agencies through which the transformations of force become apparent. Neither physics nor chemistry can work independently of the other; the more closely they become allied in the labor of investigation, the more rapidly will both progress. The
