THE CENTURY’S PROGRESS IN CHEMISTRY. 749 THE CENTURY’S PROGRESS IN CHEMISTRY. HENRY SMITH WILLIAMS, M.D. tlie casual observer, it might .seem to beginnings have great end- have no alliance whatever. The won- SMALLings—sometimes. As a case in point, derful theory of atoms, on which the note what came of the small original ef- whole gigantic structure of modern chem- fort of a self-trained back-country Quak- istry is founded, was the logical out- er youth named John Dalton, who along growth, in the mind of John Dalton, of toward the close of the last century be- those early studies in meteorology. came interested in the weather, and was The way it happened was this; From led to construct and use a crude rain- studying the rainfall, Dalton turned nat- gauge to test the amount of the wa- urally to the complementary process of terfall. The simple experiments thus evaporation. He was soon led to believe inaugurated led to no fewer than two that vapor exists in the atmosphere as an hundred thousand recorded observations independent gas. But since two bodies regarding the weather, which formed the cannot occupy the same space at the same basis for some of the most epochal dis- time, this implies that the various atmos- coveries in meteorology, as we have seen. pheric gases are really composed of discrete But this w Tas only a beginning. The particles. These ultimate particles are so simple rain-gauge pointed the way to the small that we cannot see them—cannot, most important generalization of our cen- indeed, more than vaguely imagine them tury in a field of science with which, to —yet each particle of vapor, for example, 750 HARPER’S NEW MONTHLY MAGAZINE. is just as mucli a portion of water as if 1803. I wonder if Dalton himself, great it were a drop out of the ocean, or, for and acute intellect though he had, sus- that matter, the ocean itself. But again, pected, when he read that paper, that he water is a compound substance, for it was inaugurating one of the most fertile may be separated, as Cavendish had movements ever entered on in the whole shown, into the two elementary sub- history of science? stances hydrogen and oxygen. Hence the atom of water must be composed of two lesser atoms joined together. Im- Be that as it may, it is certain enough agine an atom of hydrogen and one of that Dalton’s contemporaries were at first oxygen. Unite them, and we have an little impressed witli the novel atomic atom of water; sever them, and the wa- theory. Just at this time, as it chanced, ter no longer exists; but whether united a dispute was waging in the field of chem- or separate the atoms of hydrogen and istry regarding a matter of empirical fact of oxygen remain hydrogen and oxygen which must necessarily be settled before and nothing else. Differently mixed to- such a theory as that of Dalton could gether or united, atoms produce different even hope for a hearing. This vras the gross substances; but the elementary question whether or not chemical ele- atoms never change their chemical nature ments unite with one another always in —their distinct personality. definite proportions. Berthollet, the great It was about the year 1803 that Dalton co-worker with Lavoisier, and now the first gained a full grasp of the conception most authoritative of living chemists, of the chemical atom. At once he saw contended that substances combine in that the hypothesis, if true, furnished a almost indefinitely graded proportions be- marvellous key to secrets of matter hith- tween fixed extremes. He held that solu- erto insoluble—questions relating to the tion is really a form of chemical combi- relative proportions of the atoms them- nation—a position which, if accepted, left selves. It is known, for example, that a no room for argument. certain bulk of hydrogen gas unites with But this contention of the master -was a certain bulk of oxygen gas to form most actively disputed, in particular by water. If it be true that this combi- Louis Joseph Proust, and all chemists of nation consists essentially of the union repute were obliged to take sides with one of atoms one with another (each single or the other. For a time the authority of atom of hydrogen united to a single atom Berthollet held out against the facts,but at' of oxygen), then the relative weights of last accumulated evidence told for Proust the original masses of hydrogen and of and his followers, and toward the close of oxygen must be also the relative weights the first decade of our century it came to of each of their respective atoms. If one be generally conceded that chemical ele- pound of hydrogen unites with five and ments combine with one another in fixed one-half pounds of oxygen (as, according and definite proportions. to Dalton’s experiments, it did), then the More than that. As the analysts were weight of the oxygen atom must be five led to weigh carefully the quantities of and one-half times that of the hydrogen combining elements, it was observed that atom. Other compounds may plainly be the proportions are not only definite, but tested in the same way. Dalton made that they bear a very curious relation to numerous tests before he published his one another. If element A combines theory. He found that hydrogen enters with two different proportions of element into compounds in smaller proportions B toform two compounds, it appeared that than any other element known to him, the weight of the larger quantity of B is and so, for convenience, determined to an exact multiple of that of the smaller take the weight of the hydrogen atom as quantity. This curious relation w7 as no- unity. The atomic weight of oxygen then ticed by Dr. Wollaston, one of the most becomes (as given in Dalton’s first table accurate of observers, and a little later it of 1803) 5.5; that of water (hydrogen plus was confirmed by Johan Jakob Berzelius, oxygen) being of course 6.5. The atomic the great Swedish chemist, who was to be weights of about a score of substances are a dominating influence in the chemical given in Dalton’s first paper, which was world for a generation to come. But this read befoi’e the Literary and Philosoph- combination of elements in numerical ical Society of Manchester, October 21, proportions was exactly what Dalton had THE CENTURY’S PROGRESS IN CHEMISTRY. 751 noticed as early as 1803, and what had as regards the amount of space that sep- led him directly to the atomic weights. arates it from its fellows under given con- So the confirmation of this essential point ditions of pressure and temperature. The by chemists of such authority gave the compound atom,composed of two or more strongest confirmation to the atomic elementary atoms, Avogadro proposed to theory. distinguish, for purposes of convenience, During these same years the rising au- by the name molecule. It is to the mole- thority of the French chemical world, cule, considered as the unit of physical Joseph Louis Gay-Lussac, was conduct- structure, that Avogadro’s law applies. ing experiments with gases, which he had This vastly important distinction be- undertaken at first in conjunction with tween atoms and molecules, implied in Humboldt, but which later on were con- the law just expressed, was published in ducted independently. In 1809, the next 1811. Four years later, the famous year after the publication of the first vol- French physicist Ampere outlined a sim- ume of Dalton's New System of Chemical ilar theory, and utilized the law in his Philosophy, Gay-Lussac published the re- mathematical calculations. And with sults of his observations, and among other that the law of Avogadro dropped out of things brought out the remarkable fact sight for a full generation. Little sus- that gases, under the same conditions as pecting that it was the very hey to the to temperature and pressure, combine al- inner mysteries of the atoms for which ways in definite numerical proportions they were seeking, the chemists of the as to volume. Exactly two volumes of time cast it aside, and let it fade from the hydrogen, for example, combine with one memory of their science. volume of oxygen to form water. More- This, however, was not strange, for of over, the resulting compound gas always course the law of Avogadro is based on bears a simple relation to the combining the atomic theory, and in 1811 the atom- volumes. In the case just cited the un- ic theory was itself still being weighed ion of two volumes of hydrogen and one in the balance. The law of multiple pro- of oxygen results in precisely two vol- portions found general acceptance as an umes of water vapor. empirical fact; but many of the leading Naturally enough the champions of lights of chemistry still looked askance at the atomic theory seized upon these ob- Dalton’s explanation of this law. Thus servations of Gay-Lussac as lending strong Wollaston, though from the first he in- support to their hypothesis—all of them, clined to acceptance of the Daltonian view, that is, but the curiously self-reliant and cautiously suggested that it wr ould be well self-sufficient author of the atomic theory to use the non-committal word “equiva- ” himself,who declined to accept the obser- lent instead of ‘ ‘ atom ”; and Davy, for a vations of the French chemist as valid.