The same relation will appear in the case of the combination of these five elements with a constant weight of sulphur Hydrogen. Sodium. Potassium. Silver. Mercury. Sulphur. 0.0625 1.4375 2.4375 6.69 6.25 unite with I part. It is thus evident that the proportions in which the members of such a series combine with a constant weight of one element is the same as that in which they unite with a constant mass of another element. One part by weight of hydrogen combines with 35.5 parts of chlorine, 80 parts of bromine, 8 parts of oxygen, and 16 parts of sulphur-that is to say, these proportions of these four elements satisfy the chemical affinity of 1 part of hydrogen; they are therefore said to be equivalent. Twenty-three parts of sodium is likewise equivalent to 35.5 parts of chlorine, 80 parts of bromine, 8 parts of oxygen, and 16 parts of sulphur, and by the same reasoning it is also equivalent to I part of hydrogen, 39 parts of potassium, 107 parts of silver, and 100 parts of mercury. These numbers, therefore, are known as the equivalent weights of the elements, or their combining proportions, and the combining weight of an element may therefore be defined as the smallest weight of that element which will combine with 1 part by weight of hydrogen. This law of proportionality, or reciprocal proportion, was discovered by Richter, but it was left for Dalton to trace the connection between these three generalisations. Dalton adopted and adapted an ancient theory concerning the ultimate constitution of matter which was expounded by certain of the early Greek philosophers. The exponents of this theory held that matter is built up of vast numbers of minute indivisible particles, in opposition to the antagonistic theory believed by others, namely, that matter was absolutely homogeneous and capable of infinite subdivision. Dalton embraced the ancient doctrine of atoms, and extended it into the scientific theory which is to-day known as Dalton's atomic theory, and is accepted as a fundamental creed by modern chemists. According to this theory, matter consists of aggregations of minute particles, or atoms, which are indivisible. Dalton conceived that chemical combination takes place between atomsthat is to say, when chemical action takes place between two elements, it is due to the union of their atoms; the atoms, coming into juxtaposition with each other under the influence of chemical affinity, are held together by the operation of this force. He further assumed that the atoms of the various elements possessed different relative weights, and that the relations existing between these weights was the same as that between the weights in which experiment had shown the elements to be capable of combining together. In other words, he said that the numbers representing the combining proportion of the elements expressed also the relative weights of the atoms. Let us now see how this theory satisfies and explains the first three laws of chemical combination. (1.) The Law of Constant Composition.-It has already been shown (p. 26) that the compound sodium chloride, wheresoever and howsoever obtained, contains the elements chlorine and sodium in the proportion These numbers have been shown on p. 29 to represent the combining proportions— Now the atomic theory states, that sodium chloride is formed by the union of atoms of chlorine with atoms of sodium, and that the relative weights of these atoms is expressed by the combining weights of the elements, namely, 35.5 and 23. If therefore, sodium is to combine with chlorine, since atoms are indivisible masses, it follows that the compound produced by the union of one atom of each of these two elements must always have the same composition. (2.) The Law of Multiple Proportions.-The ratio in which oxygen combines with hydrogen to form the compound water is seen on p. 27 to be as 8: 1. This number 8, therefore, we will for the present argument regard as the relative weight of the atom of oxygen.* * Oxygen combines with carbon as already mentioned, forming two different compounds; in the first, the elements are present in the proportion * For reasons which will be explained later, chemists now regard the number 16 as representing (in round numbers) the relative weight of the atom of oxygen. that is to say, in the proportion of one atom of carbon to one atom of oxygen. According to the theory, if the atom of carbon unites with more oxygen than one atom, it must at least be with two atoms. It may be with three or with four, but as the compound must be formed by the accretion of these indivisible atoms, the increment of oxygen must take place by multiples of 8. When the second compound is examined it is found to contain its constituent elements in the proportion Carbon oxygen=1 : 2.667=6 : 16, that is to say, in the proportion of one atom of carbon to two atoms of oxygen. This information respecting the composition of these two compounds is conveyed both in their names and their formulæ. The first is termed carbon monoxide, and its formula is expressed by the symbol CO; while the second is distinguished as carbon dioxide, and has the formula CO2. The difference in the composition of the five compounds that nitrogen forms by union with oxygen will be made evident by the aid of this theory. The proportion of nitrogen to oxygen in these compounds is = = = 14: 8 14: 16 14:24 14:32 1= 14: 40 (1.) Nitrogen : oxygen = 1 : 0.571 (2.) Nitrogen: oxygen = I: 1.143 (3.) Nitrogen: oxygen = 1 : 1.714 (4) Nitrogen oxygen = 1 : 2.268 (5.) Nitrogen oxygen = 1 : 2.857 And it will be seen that the increase in the proportion of oxygen in the compounds takes place by the regular addition of a weight of that element equal to 8, which at the present stage of the argument we are regarding as representing the relative weight of the atom of oxygen. (3.) The Law of Reciprocal Proportions. If the illustrations given on p. 28 of the operation of this law be examined in the light of the atomic theory, their explanation will be evident: thus, the relative proportions in which hydrogen and chlorine separately combine with phosphorus is 0.097 : 3.43, and the ratio between these numbers is as 1 : 35.5, which is the proportion in which these two elements are known to unite together to form hydrochloric acid. These numbers, however, represent the relative weights of the atoms of these elements, therefore hydrochloric acid may be supposed to be formed by the union of one atom of hydrogen with one atom of chlorine. Again, the relative weights of carbon and sulphur which separately combine with a constant weight of oxygen are-carbon, 0.375; sulphur, 1 ; and the ratio between these numbers is as 6: 16. Carbon and sulphur, however, unite together in the relative proportion Therefore the compound they produce may be supposed to consist of one atom of carbon, having the relative weight 6, and two atoms of sulphur, each with the relative weight 16. CHAPTER VI ATOMIC WEIGHTS IN the third column of the table on page 22, the numbers are given which are at the present time generally accepted by chemists as representing the approximate atomic weights of the elements. These numbers depart, in many instances, from those arrived at by Dalton's methods: thus, the relative weights of carbon, oxygen, nitrogen, and sulphur, which were found to be equivalent to one part of hydrogen, are-carbon 6,* oxygen = 8, nitrogen sulphur 16; while the figures given as the approximate atomic weights of these elements in the table are-carbon 12, Oxygen 16, nitrogen 14, sulphur 32. We must now discuss some of the chief reasons for these departures. In the two compounds of carbon and hydrogen known to Dalton, namely, marsh gas and ethylene, the proportions of carbon to hydrogen are— = = = = = Carbon hydrogen = 6 : 1. 4.66, Dalton therefore concluded that ethylene was a compound containing I atom of carbon united with 1 atom of hydrogen, and to which, therefore, he gave the formula CH; and that marsh gas consisted of atom of carbon combined with 2 atoms of hydrogen, and which he accordingly represented by the formula CH There was, however, nothing to prove that the weight of carbon was constant in the two compounds, for it will be obvious that the same ratio between the weight of carbon and hydrogen will still be maintained by assuming that the hydrogen is constant, and that the carbon varies, thus * These are the numbers which Dalton ought to have obtained had his methods of determination been more exact. The figures he actually found for the combining weights of these four elements were respectively, 5, 7, 5, 13 |