The values A, B, C, &c. are the fundamental constants of stachiometry. § 6. Atomic Hypothesis. The stachiometric laws are purely empirical, and were discovered by induction. They have been confirmed by thousands of experiments, and their validity is independent of any hypothesis. But man suspects a cause for every law, and is disinclined to acknowledge the existence of a law unless it can account for the cause of it. Consequently the stachiometric laws, which are now regarded as the most important ever discovered in natural science, were at first treated with neglect, until John Dalton investigated those laws and discovered a simple explanation of them. Dalton investigated two gaseous compounds of carbon and hydrogen, and found that the so-called heavy carburetted hydrogen now called ethylene contained exactly half as much hydrogen combined with one part by weight of carbon as is the case in light carburetted hydrogen or marsh-gas. To explain this and similar observations concerning the oxides of nitrogen Dalton made use of an old and much-disputed hypothesis. He assumed that all elements consist of very minute indivisible particles, having a definite weight, termed atoms, and that chemical compounds are produced by the union of these atoms. 1 This hypothesis was by no means new. More than two thousand years ago the Greek philosophers energetically debated the continuity of matter-whether matter completely fills the space it occupies, or whether it is composed of very minute individual particles separated from each other by spaces. These particles were termed atoms on account of their indivisibility. Democritus and many others based their system of natural philosophy on these hypothetical atoms, and attempted to explain the transformations of the universe as the result of their properties and rapid movements. Aristotle and his followers could not tolerate the idea of the existence of an empty space between the atoms, but maintained that the naтoμos, the indivisible. 1 whole space is completely filled with matter. This difference of opinion survived till recent times, but at the present day the truth of the atomic theory is no longer a matter of dispute. Dalton does not appear to have troubled himself about this discussion. He made use of the atomic theory because it enabled him to explain without difficulty the fact that the elements combine only in definite proportions by weight, and that if certain elements unite together in several different proportions these proportions bear a simple relation to one another. We assume that all the atoms of one and the same element have the same weight, but that this weight varies for different elements in the proportion of their stachiometric quantities. Let A be the weight of the atom of a certain element, and B that of another; then it is obvious that a compound of one atom of the first with one atom of the second, e.g. A + B, must contain half as much of the second as another compound A + 2B. As all particles of such a compound have the same composition, then any number of particles or any given quantity of this substance will contain the constituents in the same proportion as the individual particles, viz. in the proportion of the atomic weights or in a simple multiple of them. The atomic theory offers an exceedingly lucid explanation of the purely empirical law of combining proportions. It is clear we can only deduce from the stœchiometric values the relative, not the absolute, weight of the atoms, as we only know the relative number of atoms contained in a compound. Black oxide of copper contains one part by weight of oxygen to 3.959 parts of copper. If we can by any means prove that this oxide contains an equal number of copper and of oxygen atoms, then it must follow that the weights of the atoms of these two elements are in the same proportion to each other that the constituents are in the oxide -namely, as 1 : 3.959. The copper atom is 3.959 times. heavier than the atom of oxygen. This proportion by weight always remains the same, and is independent of the number of atoms of the elements entering into combination. § 7. Symbols. Dalton imagined the atoms to be small spheres, and represented the atoms of different elements by various symbols enclosed in a ring or circle, thus : 1 Atom . Oxygen Hydrogen Nitrogen Carbon Sulphur Phosphorus O Atoms of the metallic elements were represented by circles containing the initial letters of their names. Berzelius omitted the circle as inconvenient, and used the initial of the Latin name to represent the atom of any element. This system of notation is now universally adopted. Both Dalton and Berzelius placed two or more symbols side by side to indicate that the atoms had entered into combination. The number of atoms is indicated by prefixing a numeral or by the use of indices. The device of Berzelius for representing a double atom by drawing a bar through the symbol is no longer used. Two atoms of hydrogen may be represented by the following symbols, 2H, H,, H2, HH. The second of these symbols is that most frequently employed. § 8. Unit of Atomic Weights.-We have already seen (§ 6) that the weight of the atoms cannot be deduced directly from the combining proportions, and that it is only possible to decide how many times heavier or lighter one atom is than another. This, however, is all that is requisite for the development of chemical theory. It is not necessary to know the weight of individual atoms. The composition of any mixture of different substances can be equally well expressed in grams, ounces, or pounds, and in the same way the composition of any chemical compound can be expressed in terms of any unit of weight that may be selected. If we choose the weight of an atom of a given element as unity, we can by means of the stochiometric values express the atomic weights of the others in terms of this standard, so that a number is obtained for each element, which shows how many times heavier it is than the unit. Dalton's proposal to take the atom of hydrogen, the lightest of all the atoms, as unity is at the present time universally adopted. But for many years it was the custom to follow the example of Wollaston and Berzelius, who, for certain practical reasons, took the atom of oxygen as their standard. It is obvious that there will be a great difference in the atomic weights according to the standard selected. If hydrogen is taken as unity, it is clear that the atomic weights will be proportionally larger than is the case when the heavier atom of oxygen is taken as the standard. Just as in measuring distances, the numbers are larger if we reckon by feet instead of metres or by kilometres instead of miles. Now some atomic weights are smaller than that of oxygen, so in order to avoid fractions Wollaston took as his standard the tenth part, and Berzelius chose the hundredth part of an atom of oxygen, so that an atom of oxygen=10 or 100. This standard yielded atomic weights which were in some cases larger than 1000, and are now no longer used. § 9. Determination of Atomic Weights from Stochiometric Values. We have already seen (§ 6) that the relative values of the atomic weights can only be calculated from the composition of a chemical compound as determined by synthesis or analysis, when the number of atoms contained in the compound is known. But the number of atoms cannot be directly determined, and can only be deduced by the help of hypotheses varying in degree of probability. Water is composed of oxygen and hydrogen in the proportion by weight of 7.98 1. It does not follow that the atomic weights of these elements are in this ratio, but only that the weight of all the hydrogen atoms in a given quantity of water bears this proportion to the total weight of the oxygen atoms combined with them, so that n.Hm.0=1: 7.98 when n and m represent whole (unknown) numbers. The atomic theory only teaches us that a certain number of whole atoms of one substance has combined with a definite number of whole atoms of a second element, e.g. n H with m O. The values of m and n are not known. It is one of the most important problems in theoretical chemistry to determine the number of atoms which are united together in different compounds. The solution of this problem was attempted immediately 1 At the present time, oxygen = 16 is generally taken as the standard.TRANS. after the proposal of the atomic theory, but the possibility of a satisfactory solution was repeatedly and seriously contested. This dispute continued for upwards of half a century, and now at last we are in a position to maintain that the number of atoms in different compounds may be determined, if not with absolute certainty, at any rate with the greatest degree of probability. § 10. First Attempts to determine the Atomic Weights.-At first sight it would appear to be the simplest plan to regard the proportion by weight in which two elements unite together as identical with their atomic weights. But this is not possible, because many elements unite together in different proportions. In black oxide of copper one part by weight of oxygen is united to 3.959 parts by weight of copper; but in the red oxide, twice as much copper, viz. 7.918, is contained. Is the atomic weight of copper 3.959 or 7·918 times greater than that of oxygen? There does not appear to be any reason why one number should be selected in preference to the other. If we choose the first, then we have the formulæ and and Cu 0=3.959 +1, for the black oxide Cu2 0=7·918+1, for the red oxide. 2 Cu 0=7.918+1, for the red oxide. Berzelius selected the second value, but it has been replaced by the first, which is now universally regarded as correct. Dalton advocated the greatest simplicity; he assumed the existence of only one atom of each constituent in many compounds, in which, according to our present views, several atoms of one of the constituents are contained, e.g. |