86 ATOMIC THEORY to produce a salt, which therefore has a fixed numerical constitution. A great number of ex- periments showed that chemical union always takes place in this manner, and thus was estab- lished the fundamental law of definite propor- tions. It was next discovered that' combina- tion may take place between the same sub- stances in different proportions, and that when this is the case these proportions have simple numerical relations to each other. Thus, if two elements A and B are capable of uniting in several proportions, they may be represented as A+ B, A+ 2B, A+ 3B, A + 4B, &c. The relations are not always so simple as this, but the principle is general, and is known as the law of multiple proportions. Again, it was found that if two elements which combine with each other combine also with a third, the proportions in the first combination are pre- served also in the second. If a body A unites with certain other bodies B, C, D, then the quantities B, 0, D, which combine with A, or certain simple multiples of them, represent for the most part the proportions in which they can unite among themselves. This is known as the law of equivalent proportions or chem- ical equivalence. It having thus been found that chemical actions follow strict numerical methods, and that each body has its fixed measure, it became important to determine ex- actly what these measures are. This resulted in the scale of combining numbers or equivalents, or, as they are now more commonly termed, atomic weights, which constitute the founda- tion of the science and are given in all text books. But if all kinds of matter in their chemical transformations are ruled by these numerical principles, we should expect that other material properties would be affected by them, and such is the fact. The combining weights of those elements which are known to exist in the state of gas or vapor are, with one or two exceptions, proportional to their specific gravities in the same state. Thus, the specific gravity of hydrogen being 1, that of oxygen is 16, sulphur vapor 32, chlorine 35-5, iodine vapor 127 ; but the figures represent also the combining numbers of these elements. Mr. Watts thus expresses the law of combination by volume : " If the smallest volume of a gase- ous element that can enter into combination be called the combining volume of that element, the law of combination may be expressed as follows : The combining volumes of all elemen- tary gases are equal, excepting those of phospho- rus and arsenic, which are only half those of the other elements in the gaseous state ; and those of mercury and cadmium, which are double those of the other elements." Gay-Lussac showed that combinations by volume take place in defi- nite and multiple proportions, and that the vol- ume of a compound gas always bears a simple ratio to the volumes of its elements, thus: 1 vol. hydrogen and 1 chlorine form 2 vols. hydrochloric acid. 2 vols. " 1 oxyiren " 2 " watery vapor. " 1 nitrogen " 2 " ammonia. Again, it is found that in many cases two or more compounds which are supposed to contain equal numbers of equivalents of their respective elements crystallize in the same or in very simi- lar forms, and such compounds are said to be isomorphous. Accordingly, these isoinorphoiis relations are often appealed to for the purpo-e of fixing the constitution of compounds, and thence deducing the atomic weights of their elements, in cases which would otherwise be doubtful. It has also been established that substances having different properties may have the same relative proportion of constitu- ents, and such are said to be isomeric. More- over, something analogous to this is seen among the elements themselves : they are capable of assuming different states, which capability is called allotropism. In both cases we are com- pelled to assume that their constituent parts are subject to differences of arrangement. Com- bining quantities are also intimately related to heat. This relation is thus stated by Mr. Watts : " The atomic weights of the elements, deter- mined according to their modes of combina- tion, are for the most part inversely propor- tional to their specific heats ; so that the pro- duct of the specific heat into the atomic weight is a constant quantity. The same quantity of heat is required to produce a given change of temperature in 7 grains of lithium, 56 of iron, 207 of lead, 108 of silver, 196-7 of gold." Final- ly, the law of combining proportions is impli- cated with the electrical relations of matter. Prof. Faraday proved that an equivalent of an element consumed in a battery gives rise to a definite quantity of electricity, which will pro- duce exactly an equivalent of chemical decom- position. For example, the consumption of 32 grains of zinc in a battery excites a current which will set free from combination 1 grain of hydrogen, 108 of silver, and 39 of potassium ; these being the combining numbers of the re- spective elements. The facts above stated are independent of all hypothesis, and are the re- sults of pure experiment. They demonstrate that in its ultimate and minutest form matter is in some way numerically constituted. How it is constituted was a question which the human mind could not escape. It was neces- sary to frame some clear conception of its ul- timate constitution that would connect and in- terpret the known facts. This was done by Dr. John Dalton of Manchester, England, in constructing the atomic theory. He was aware of the law of definite proportions, and he dis- covered the law of multiple proportions by in- vestigation of the compounds of carbon and hydrogen, of oxygen and carbon, and of nitro- gen and oxygen. To account for these laws, he assumed, first, that all matter consists of indivisible, unchangeable atoms of extreme minuteness ; second, that all the atoms of the same element have the same weight, but that in different elements they have different weights ; third, that these relative weights. correspond with the combining numbers.
