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 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 close together 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, H2, H2, HH. The second of these symbols is 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 stœchiometric 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 Stœchiometric 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.H:m.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. § 10. First Attempt 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 heavier 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 3.958 +1, for the black oxide Cu2 0=7·918+1, for the red oxide. If we take the second value, then we have Cu O=7·918+1, for the red oxide. DALTON'S ATOMIC WEIGHTS 18 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. According to Dalton's views, the atomic weights of oxygen, nitrogen, and carbon are to the atomic weight of hydrogen in the ratio of 0:N:C: H= 7·98 : 4·67 : 5·985 : 1; but according to Berzelius they stand to each other in the proportion of ON: CH 15.96: 14.01 11.97 1. = The weights which Dalton regarded as the atomic weights of oxygen and carbon are only half, and in the case of nitrogen only one-third, of the atomic weights accepted by Berzelius. § 11. Chemical Equivalents. At the beginning of the present century Wollaston proposed that the chemical symbols. should represent the equivalents as determined by experiment. In this way he hoped to avoid the want of uniformity resulting from the use of the hypothetical atomic weights. Those quantities of different substances which produce the same, or nearly the same, effect were regarded as equivalent. The expression was originally applied to those quantities of different acids which are required to neutralise a fixed quantity of a given base, and also to those quantities of different bases which are required to neutralise a certain weight of a given acid. The expression was afterwards used in a wider sense, and was applied to all kinds of substances, including the elements. It is obvious that no element can be strictly equivalent to another. It is only equivalent in certain respects—namely, in its capacity for uniting with a third substance, or displacing it in a compound. We regard as equivalent weights of the elements those quantities which have in this respect the same value, and we compare them as we do the atomic weights with one part by weight of hydrogen as unity. The equivalent weights of the elements are, therefore, those quantities of the elements which can enter into the same combinations as one part by weight or one atom of hydrogen, or can unite with one part by weight or one atom of hydrogen. The first definition holds good for the metals and semi-metals; the second is true of the non-metals, because all the latter can combine with hydrogen; but only a few of the metals are able to form hydrides. By means of this definition the equivalent weights can easily be determined by experiment. One part by weight of hydrogen unites with 19.06 parts by weight of fluorine One part by weight of hydrogen is replaced in its compounds by the following quantities of different metals. These quantities of the metals will consequently be able to unite with the non-metals in the proportions stated in the preceding table, e.g. |