CHAPTER V THE ATOMIC THEORY THE atomic view as to the constitution of matter, briefly sketched out in Chapter I., forms a part of what is to-day known as the atomic theory. When chemical changes were carefully studied from a quantitative standpoint, four laws were discovered in obedience to which chemical action takes place. These laws are distinguished as the laws of chemical combination. Three of these generalisations refer to quantitative relations as respects weight; while one expresses quantitative relations with regard to volume, and only relates to matter in the gaseous state. I. Law of Constant Proportion.-The same compound always contains the same elements combined together in the same proportion by weight; or expressed in other words, The weights of the constituent elements of every compound bear an unalterable ratio to each other, and to the weight of the compound formed. II. Law of Multiple Proportions. When the same two elements combine together to form more than one compound, the different weights of one of the elements which unite with a constant weight of the other bear a simple ratio to one another; or this law may be stated thus: When one element unites with another in two or more different proportions by weight, these proportions are simple multiples of a common factor. III. Law of Reciprocal Proportions, or Law of Equivalent Proportions.--The weights of different elements which combine separately with one and the same weight of another element, are either the same as, or are simple multiples of, the weights of these different elements which combine with each other; or in other words, The relative proportions by weight in which the elements, A, B, C, D, &c., combine with a constant weight of another element, X, are the same for their combinations with any other element, Y. IV. Law of Gaseous Volumes, or The Law of Gay-Lussac. -When chemical action takes place between gases, either elements or compounds, the volume of the gaseous product bears a simple relation to the volumes of the reacting gases. These four laws are the foundations upon which the whole superstructure of modern chemistry rests. (1.) The Law of Constant Proportions.-When two substances are mingled together, and remain as a mere mechanical mixture, they may obviously be present in any proportion, and it was at one time thought that when two substances entered into chemical combination with each other, they could do so also in any proportion, and that the composition of the resulting compound would vary from this cause. This belief was finally disproved, and the law of constant proportions definitely established by Proust in the year 1806. The same compound, therefore, 'however made, and from whatever source obtained, is always found to contain the same elements united together in the same proportion by weight. Thus, common salt, or, to adopt its systematic name, sodium chloride, which is a compound of the two elements sodium and chlorine, may be made by bringing the metal sodium into contact with chlorine gas, when the two elements unite and form this compound. It can also be made by the action of hydrochloric acid upon the metal sodium, or by adding hydrochloric acid to sodium carbonate, and by a variety of other chemical reactions. When the sodium chloride obtained by any or all of these processes is analysed, it is invariably found to contain the elements chlorine and sodium in the proportion by weight of 1:0.6479, or, expressed centesimally and when this is compared with the sodium chloride as found in nature, obtained either from the salt-mines of Cheshire, or the celebrated mines in Galicia, or by evaporating sea-water, it is found that the composition of the compound in all cases is exactly the same. In the same way the compound water, consisting of the two elements hydrogen and oxygen, whether it be prepared synthetically by causing the two elements to unite directly, or obtained from any natural source, as rain, or spring, or river, is found to contain its constituent elements hydrogen and oxygen in the ratio by weight of 1:8, or, If in the formation of sodium chloride by the direct combination of its constituent elements, an excess of either one or other be present beyond the proportions 39.32 per cent. of sodium and 60.68 per cent. of chlorine, that excess will simply remain unacted upon. If eight parts by weight of hydrogen and eight parts by weight of oxygen be brought together under conditions that will cause chemical action, the eight parts of oxygen will unite with one part of hydrogen, and the other seven parts of hydrogen merely remain unchanged. This fact, that elements are only capable of uniting with each other in certain definite proportions, marks one of the most characteristic differences between chemical affinity and those other forces, such as gravitation, that are usually distinguished as physical forces; for although there are many instances known in which the extent to which a chemical action may proceed (that is, the particular proportion of the reacting bodies which will undergo the permutation that results in the formation of different molecules) is influenced by the mass of the acting substances, it never governs the proportion in which the elements combine in these compounds. It follows from the law of constant composition that the sum of the weights of the products of a chemical action will be equal to that of the interacting bodies; and upon the validity of this law depend all processes of quantitative analyses. (2.) The Law of Multiple Proportions was first recognised by Dalton, who investigated certain cases where the same two elements combine together in different proportions, giving rise to as many totally distinct compounds. These proportions, however, were always found to be constant for each compound so produced, so that this law formed no contradiction to the law of constant composition. The simple numerical relation existing between the numbers representing the composition of such compounds will be evident from the following examples. The two compounds of * * In Dalton's day these two substances were the only known compounds of carbon with hydrogen. carbon with hydrogen, known as marsh gas and ethylene, are found to contain these elements in the proportions- The two compounds of carbon with oxygen contain these elements in the proportion Carbon monoxide. I part of carbon with 1.334 parts of oxygen by weight. Carbon dioxide I 2.667 " The elements nitrogen and oxygen form as many as five different compounds, in which the two elements are present in the propor I part of nitrogen with o. 571 parts of oxygen by weight. I Ι Nitrogen peroxide I The relative proportions of carbon combining with a constant weight of hydrogen in the two first compounds are as 1 : 2. Those of oxygen uniting with a constant weight of carbon in the second example are also as I 2, while in the nitrogen series the relative proportions of oxygen in combination with a constant weight of nitrogen are as I: 2:34:5 (3.) Law of Reciprocal Proportions.-Known also as the law of proportionality, or the law of equivalent proportions. When the weights of various elements, which were capable of uniting separately with a given mass of another element, were compared together, it was seen that these weights bore a simple relation to the proportions in which these elements combined amongst themselves. For example, the elements chlorine and hydrogen each separately combine with the same weight of phosphorus, the proportions being The elements chlorine and hydrogen can combine together, and they do so in the proportion Chlorine hydrogen 35.5 I but : 35:1 = 3.43 0.097 Therefore the proportions by weight in which chlorine and hydrogen separately combine with phosphorus is a measure of the proportion in which they will unite together. Again, the two elements carbon and sulphur each separately combine with the same weight of oxygen, the proportion being But the elements carbon and sulphur themselves unite together, and in the proportion Therefore the proportion by weight in which carbon and sulphur separately unite with the same mass of oxygen is a simple multiple of that in which these two elements combine together. These remarkable numerical relations will be rendered still more evident by comparing the proportions in which the members of a series of elements combine with a constant weight of various other elements : thus Chlorine. Hydrogen. Sodium. Potassium. Silver. Mercury. 0.02817 0.6479 1.02 3.04 2.816 unite separately with 1 part. It will be seen that the proportion in which these numbers stand to each other is as I : 23 : 39 : 107 : 100 35.5 Let us now compare these proportions with those in which the same elements unite with a constant weight of the element bromine Hydrogen. Sodium. Potassium. Silver. Mercury. 0.0125 0.2875 0.4875 1.34 1.25 Bromine. unite with 1 part, or as I : 23 : 39 : 107 : 100 80 Each of these five elements in like manner combines with oxygen, and the weights which are found to unite with a constant mass of oxygen are— |