Chemists educated in recent years can hardly conceive the confusion that prevailed 60 years or so ago, while the principles that have been outlined above were struggling for recognition and universal adoption. There was no agreement as to what atomic weights or what formulas should he used. Mendeleeff says: "Some took 0 = 8 and others 0=16. Water in the first case would be HO and hydro gen peroxide HOB, and in the second case (as is now generally accepted) water would be H20 and hydrogen peroxide H202 or HO. Discus sion and confusion were reigning. In 1860 the chemists of the whole world met at Carlsruhe for the purpose of arriving at some agreement on the subject. There was great difference of opinion, and a conditional agreement (or com promise) was proposed and defended with the greatest acumen by the ranks of science. A conditional agreement was not arrived at, and ought not to have been; but instead of it, truth, in the form of the law of Avogadro-Gerhardt, received by means of the congress a wider de velopment, and soon afterward conquered all minds. Then the new so-called Gerhardt atomic weights established themselves, and in the seventies they had already become gen erally used. As soon as a few of the atomic weights had been determined with some little degree of precision, it became evident that they came nearer to exact integers than one would naturally expect them to, judging from the theory of probability. As early as 1815, Prout made the assumption (since known as 'Trout's Hypothesis))) that the true values of these atomic weights are really integral numbers; and he drew the inference that all elements are composed of hydrogen. Thus nitrogen, whose atomic weight is almost exactly 14, he believed to contain 14 times as many atoms to the molecule as hydrogen contains, and he be lieved the fundamental atom to be the same in each case. There is nothing about this assump tion which conflicts with what has been said above about nitrogen being a diatomic gas; for all that Avogadro's law positively shows is that when a molecule of that gas divides, it splits into halves, and therefore contains an even number of atoms. In the absence of any evidence to the contrary we assume it to be simply diatomic, although we must always re member that future research may require us to admit it to be tetratomic, hexatomic, or even more complex. Prout's hypothesis has provoked a great deal of discussion, and since it was first proposed it has been attacked and defended by many distinguished chemists; and although it is in decided disfavor at present, we can hardly yet say that it has been finally laid to rest. One can scarcely glance at a table of atomic weights (such as that here presented) without noting the tendency shown by these atomic weights to approach integral values. Of
course there are numerous conspicuous excep tions — chlorine, for example — but the fact that many of the atomic weights are nearly in tegral demands some sort of an explanation. What that explanation may ultimately prove to be, we cannot now guess; but it is possible that it will be found in the development of the re markable °corpuscular hypothesis)) of Prof. J. J. Thomson. (See ELECTRON.) The atomic weight of oxygen was long thought to be pre cisely 16; very careful experiments then indi cated that 15.96 is a closer approximation to the real fact; and it has recently been ascer tained that 15.88 is a still better approximation. It is a matter of choice what element is taken as having the atomic weight unity, hydrogen having been chosen for this purpose merely be cause it is the lightest element known. For many purposes it would be convenient if the atomic weight of oxygen were precisely 16; but this value is now known to be incompatible with the assumption that the atomic weight of hydrogen is 1. Chemists have therefore been in the habit, in recent years, of multiplying all the atomic weights, as deduced on the hypothe sis that H 1, by a constant number, so de termined as to make the atomic weight of oxygen come precisely 16. It happens that the multiplier required for this purpose is 1.008, which is therefore the atomic weight that must be assigned to hydrogen, if we are to adopt a scale on which the atomic weight of oxygen is to be precisely 16. A majority of the chemists of the world now use this modified scale, on which the atomic weight of hydrogen is taken as 1.008; and the scale so established is known as the °International') scale of atomic weights. The values of the atomic weights on this scale, according to the latest official revision (that of 1917) are given in the accompanying table.
The many questions that suggest themselves . as to the size and physical nature of atoms be long properly to the domain of physics, and are discussed under BROW NIA N MOVEMENT, ELECTRON, and MOLECULAR THEORY. The day will doubtless come when the physicist and the chemist will find some common ground for the discussion of the nature of atoms and mole cules; but at present these two sciences deal with such widely different classes of phenomena that no such common ground can be clearly dis cerned. The atom and the molecule of the physicist appear to be hardly capable of pos sessing the properties that the chemist demands; but this difficulty may one day be overcome. See CHEMISTRY; ELECTRON ; MOLECULAR THEORY; GASES, KINETIC THEORY OF; VALENCY; SPECTROSCOPE; PERIODIC LAW.