CHEMISTRY. The doctrine of valency eolith! not have come into existence if not for the fact that toward the end of the fifties chem ists had learned the true atomic wei!drts of the (dements. Without a knowledge of the true relative weights of atoms, it would have been im possible to know their true number in molecules, impossible to know their true valet]. eies. Atomic weights were determined, calculat ed. and re-caleulated ever since Dalton first es tablished the atomic theory. Dalton him-elf, as stated in a previous section of this article, determined atomic weights on the basis of •er tain simple assumptions. Soon afterwards Ber zelins devoted himself to the problem with great assiduity. From the law of combining volumes, by (:ay-l.ussac in ISMS, Berzelius inferred that equal volumes of gaseous elements must contain equal numbers of particles. In Isl \lit cherlioli diseovered the principle of isomorphism. (See A•romtc Wmotirs.) Berze lius had carried out about two thousand analyses, and had thus determined the relative quantities of the (dements contained in a great variety of compounds. By combining the principle of iso morphi-in with that of equal gaseous volumes, was now able to calculate the atomic weights of the elements. Now. his principle of equal volumes was not quite correct. To him the par ticles of a gaseous element in the uncombined Mate were isolated atoms. While he distin guished between the particles of compounds and the atoms of elements. he failed to distinguish between the free partieles of elements and their atoms. That the particle of an element might be 1111 of two or more -ilcde atoms. it would have been impossible for him to admit; for, ac cording to his electro-chemical theory, only un like atoms could exist in combination with one another. Avogadro's memoir of 1St 1, in which more kw, on the subject, had been ad van•ed, therefore remained unnoticed, and Iter zelins's atomic weights were tor ye:inc clliployed by all. Nor were most of those figures wrong: for in ninny eases Berzelius's error eliminated itself. owing to the fact that the molecules of the ordinary gaseous elements are made up of equal numbers of atoms. Knowing the true atomic weights of the ordinary gaseous elements, 11erzelius was able to obtain correct figures for many other elements. with the aid of t he principle of isomorphism and certain other principles that need not be explained here. Thus, his figure for mercury was 21)(1, that for phosphorus 31, that for sulphur 32—ligures practically identical with those accepted at present. In 1S27. how ever. Da ma; invented his celebrated method of determining vapor densities, and undertook to apply Berzelius's principle of equal volumes to elements which are not ordinarily gaseous. Finding that the vapor of mercury is 101 times as heavy as an equal volume of hydrogen, the vapor of phosphorus i2.S times, and the vapor of sulphur 91; times, as heavy as hydrogen. Dumas concluded that the relative weights of their atoms must be, respectively. 101. (32.S•
and 9(1, and not 200, 31, and 32, as Berzelins thought. The error of Berzelins's principle thus emerged in the results of Dumas. But instead of rectifying the error of his prin ciple by introducing the concept of the mole cules of elements, Berzelius only concluded that the principle was unreliable. The result was that chemists began to disagree as to the true values of the atomic weights, and many even Med the hope of ever knowing atomic weights altogether. and decided to use nothing but equiralents. These represented the weights of elements that were capable of eombining with, or of being replaced by, unit. weight of hydrogen. For example, Berzelius•s view that an atom of oxygen was 10 times as heavy as an atom of hydrogen was abandoned, and as hydrogen com bined with S times its weight of oxygen, the lat ter was represented by its equivalent S. But the use of equivalents was not universal. many chemists using systems in which the figures were partly equivalents, partly atomic weights, and thus for years great confusion reigned in chem ical notation, the true purpose of which is to avoid confusion by exhibiting the composition of substances in the simplest and clearest pos sible manner. In the forties. Laurent and (ler hardt became eonvineed that the progress of kmiwledge in organic chemistry was seriously impeded by the lack of a consistent system of atomic weights. Their researches soon led them to distinguish (dearly between the atoms and molecules of elements, and to grasp the full %able of Avogadro's principle for determining relative weights of molecules. With the aid of this principle. Gerhardt found the true atomic weights of the elements; and, in the latter part of the fifties, his pupil Cannizzaro demonstrated clearly the consistency of the principle with all known facts. Thus was paved the way for the doctrine of valency. A few years later (in 1S1;91 Ilendeheff and Lothar .Meyer established A remarkable connection between the properties of tine elements and their atomic weights (see PEnumic and thus the correctness of the latter was confirmed in a very striking manner.
•h, further of general chemistry has been mainly in connection with the various sub of physical chemistry. brief historical account, of which may be found under 111.:Ac T1ers 1)issoct.vrtox; TrintmocteEmis TOY and ELECTROCHEMISTRY. See also the article LAIII•IUTORY.
CoNcLUsloN. In conclusion. we. could not do better than emphasize, with Dr. F. W. Clarke, the necessity of systematizatimi of chemical re seareb. The bi•th of general ideas may lie beyond control: but the sear•h of empirical Isnowledge, which must form the basis for all generalizations, can and enight, to be organized instead of being left entirely to the inclinations of individual investigators. Some organization of research has been realized at the German uni versities, with splendid results; and further or ganization is certain to yield an even more abun dant crop.