The third law is TILE LAW OF Alel•TIPLE PROPORTION, Which is, that when one sub stance combines with another in several proportions, the higher proportions are multiples of the first or lowest. Thus, hydrogen unites with oxygen in t wo proport ons: as 1 of hydrogen to 8 of oxygen, when ordinitty,pure water is the result of al.:tn.; and as 1 of hydrogen to 16 of oxygen, when peroxide of hydrogen, a powerful bleaching agent, is produced—the difference in the respective amonnts of the oxygen-8 and that the latter is a multiple of the former by 2. Again, carbon unites with oxygen in two proportions: as 6 of carbon to 8 of oxygen, when the inflammable gas, carbonic oxide, is formed; and as 6 of carbon to 16 of oxygen, when the noninflammable gas.' carbonic acid, is the result. The variation in this instance is, that the oxygen 1: present in the one case as 8, and in the other as a multiple of that number by 2, viz. 16. One of the best illustrations of this law occurs in the case of the union of nitrogen and oxygen: 14 parts of nitrogen can unite with 8 of oxygen, and thus form gas; but the same amount of nitrogen can combine with 16, 24, 32, or 40 of oxygen —in the latter case constituting anhydrous nitric acid—all of the higher numbers bein:: multiples of the first or lowest, viz. 8 by 2, 3, 4, and 5.
The fourth law is THE LAW OF coptrouxo puorounox, which teaches that the com billing proportion of a compound substance is the sum of the combining proportions of its components. Thus, the compound body, carbonic acid, which consists of 6 of car bon united with 16 of oxygen, has the combining proportion 22, which is the sum of the combining proportions of the carbon and oxygen composing it, viz. 6+16=22.
the compound substance. lime contains 20 of the metal calcium combined with 8 of oxygen. and has the combining proportion of 20+8 or 28. ItVhen carbonic acid and lime are linked together, as in marble, which is the carbonate of lime, then they are united in the proportion of 22 parts of carbonic acid and 28 of lime, Not only is :1:2 the proportion in which carbonic acid will combine with lime, but it is the proportion in which it will form compounds with every other substance of similar chemical constttu-, Lion.
The preceding laws regulating the union of substances by weight have been obtained by comparing together the results of numerous experiments, and every careful analysis serves to confirm their accuracy. But Dalton's theory was not limited to the statement of these laws; it was also an attempt to explain them. It assumes that each elementary substance consists of extremely small indivisible particles or atoms; that the atoms of any one element are all exactly alike, but differ from the atoms of every other element. Among other points of difference, they differ in weight, and although the absolute weight of an atom is unknown, the weights of two atoms, one of one element, the other of another element, are in the proportion of the combining weights of the elements they' belong to. Thus the combining weight of sulphur is twice that of oxygen: we do not
know the absolute weight of an Nom of either, but the A. T. assumes that each of sulphur is twice as heavy as an atom of oxygen. Further, Dalton's theory assumes that the ultimate particles of compound bodies contain a comparatively small number of atoms of the component elements. It is easy to see bow this theory explains the laws enunciated above. We must, however, remember that while the theory .s..tisfactorily explains the laws, the laws do not prove the theory. It is quite conceivable that suck laws might exist., although matterdid not consist of atoms. The A. T., however, rests not only on a chemical but also upon a physical foundation. According to the modern molecular theory, matter consists of small particles, each of which is in 'notion, and t his motion is the more rapid the hotter the substance is. These small particles or " mole cules" cannot be broken up without changing the character and properties of the sub stance. They are not, however, atoms. In the case of compounds, as the molecules of any one substance are all similar to one another, each molecule must contain all the compo nents: and in many elementary substances it can be proved, assuming the truth of the molecular theory, that each molecule consists of several similar atoms. A molecule, . then, is either a single atom, as in some elementary substances, or a group of atoms which remain together during those movements which depend on the temperature of the sub stance. Now, the velocity of these motions increases as the temperature is raised; when, therefore, the temperature is raised so high, and the velocity of the molecules so great that the collision' of the molecules with one another is sufficiently violent to break them up and separate their constituent atoms, the 'substance is decomposed, the atoms rearranging themselves into new groups (or molecules) capable of remaining' unbroken under the new conditions. This explains the decomposition of compounds by the action of heat.
When the temperature is not so high, and the violence of collision insufficient to break up the molecules, these are merely shaken, thrown into a state of vibration, thus the hold of the atoms upon each other is loosened. Now, if two substances' are mixed together, it may happen that some atoms in the one set of molecules are so. attracted by some atoms in the. other set, that, when a molecule of the one set meets one, of the other set in a vibrating or loosened condition, an exchange of atoms may take, place between them, or each may lose a part of its atoms, these going to form a new molecule. This gives an explanation of the action of one substance upon another, and further shows why, in general, a certain temperature is required in order that the action may take place.