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I. Molecular and Atomic Constitution. - The astronomer finds that the universe is made up of stars, or suns, each of which is probably, like our own sun, the centre of a solar system made up of planets. These solar systems are the larger units, or individuals, of the universe; and the planets are the smaller units of which these systems are composed. Some of these smaller units, as Venus and Mars, are simple; others, as the Earth and Jupiter, are complex, being made up of a planet and one or more moons. We now believe that all the bodies about us are made up of units analogous to these. The physicist finds matter to be made up of separate and distinct masses, or units, which he calls molecules. He cannot see these even with the aid of the microscope; but he is as certain of their existence as the astronomer is of his planets and solar systems. The chemist pulls these molecules in pieces, and finds that they are made up of smaller units, which he calls atoms or compound radicals, according as they are simple or complex. He believes that in most cases the molecules of the elements, as well as of compounds, are made up of two or more atoms; the only difference being, that in the one case the atoms are all alike, while in the other they are not. Thus a molecule of hydrogen is believed to be made up of two atoms of hydrogen; while a molecule of water is made up of two atoms of hydrogen and one of oxygen. When a molecule of water is broken up, it ceases to be water, and becomes hydrogen and oxygen. Hence we may

define a molecule as the smallest mass of a substance which can exist by itself; and an atom as the smallest mass of any substance which can exist in a molecule.

The molecules of a body are bound together by cohesion, and the atoms by affinity. The state of a body depends upon the strength of the cohesive force. When this force binds the molecules together firmly, they form a solid; when very weakly, a liquid; and when not at all, a gas. The molecules of a solid, as a general rule, tend to arrange themselves in regular order, so as to build up symmetrical forms called crystals. These crystals have usually the same form for the same substance, but very different forms for different substances.

It is probable that the atoms when bound together by affinity tend to arrange themselves with equal regularity; but as yet we are ignorant of the mode of arrangement. Indeed, the number of atoms in the molecules of many substances is as yet a matter of mere hypothesis.

2. Molecular Weight. When the molecules of any body are so far apart that the cohesive force exerts no constraint upon them, the substance is called a true gas. When the molecules are still somewhat under the restraint of cohesion, but not bound together by it, the substance is called an imperfect gas, or vapor. Now there are two characteristics of all true gases, which throw light upon their molecular constitution. “First, all true gases obey the same law of compressibility. Second!y, equal volumes of all true gases expand equally on the same increase of temperature.” The repulsive force, then, which acts among the molecules, and tends to keep them apart, or to separate them, is the same for all true gases. This has led to the conclusion that equal_volumes of all true gases have the same number of molecules. We can therefore readily find the molecular weight (that is, the relative weight of their molecules) of all those substances which can by heat be converted into true gases, by weighing equal volumes of these gases under the same pressure and temperature.

There are, however, many substances which cannot be brought into a gaseous state. To determine the molecular weights of these substances, we must find their atomic constitu

tion, and the atomic weights of their elements. Suppose, for instance, that we do not know the molecular weight of water, but do know that a molecule of water contains two atoms of hydrogen and one of oxygen, and that the atom of hydrogen weighs i and the atom.of oxygen 16. The molecular weight of water must then be 16+2 or 18.

In finding the molecular weight of different substances, they are all compared with hydrogen, whose molecular weight is assumed to be 2. The following table gives the molecular weights of several gases, all of which contain hydrogen; also the relative weight of the hydrogen in each molecule :

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3. Atomic Weights. — When any element forms a number of compounds which can be brought into the gaseous state, we find its atomic weight by first finding the molecular weight of these compounds, and then analyzing these compounds, so as to find the weight of the common element in each molecule, or, what amounts to the same thing, in equal volumes of the gases.

Thus, in the above table, it is found that when HCl is decomposed, it gives half its own volume, or, in other words, half its own number of molecules, of hydrogen. Hence a molecule of HCl contains half a molecule, or one part by weight, of hydrogen. The same is found to be true of HBr and HI. Water gas is found on analysis to give its own volume, or, in other words, its own number of molecules, of hydrogen. Hence each molecule of water contains a whole molecule, or two parts by weight, of hydrogen. The same is found to be true of H2S. Again, H2N gives one and a half its own volume of hydrogen. A molecule of ammonia, then, contains one and a half molecules, or three parts by weight, of hydrogen. The same is true of H P and HgAs. Again, H4C (marsh-gas) gives twice its own volume of H; and

each of its molecules must therefore contain two molecules, or four parts by weight, of H.

In this way, it is found that one part by weight is the smallest quantity of hydrogen found in a molecule of any of its compounds. Hence i is taken as the atomic weight of hydrogen; and since the molecular weight of hydrogen has been assumed as 2, each of its molecules inust contain 2 atoms.

In a similar way, it has been found that the molecular weight of nitrogen is 28, and its atomic weight 14. 6. This method of investigation can be extended to a large number of the chemical elements, and the conclusions to which it leads are evidently legitimate, and cannot be set aside until it can be shown that some substance exists whose molecule contains a smaller mass of any element than that hitherto assumed as the atomic weight, or, in other words, until the old atom has been divided."

This method is, however, applicable only to those elements which form a number of gaseous or volatile compounds. In other cases, the number of atoms in a molecule is a matter of inference merely. Whenever analogous elements, as iodine, bromine, fluorine, and chlorine, combine with the same element, we infer that the molecules formed have the same atomic constitution, especially if the compounds have the same crystalline form and similar chemical relations. Of course, if we know the atomic constitution of a compound, and the atomic weight of one of its elements, we can find the atomic weight of its other elements.

Suppose, for instance, we know that the molecule of baric chloride contains one atom of barium and two of chlorine, and that the atomic weight of chlorine is 35.5, we find on analysis that baric chloride contains 137 parts by weight of barium to 71 of chlorine, 71 =35.5 X 2= - the weight of two atoms of chlorine. Hence the weight of the one atom of barium must be 137.

“Nevertheless, it is true in very many cases that our conclusion in regard to the number of atoms which a molecule may contain is more or less hypothetical, and hence liable to error and subject to change. This uncertainty, moreover, must extend to the atomic weights of the elements, so far as they rest on such hypothetical conclusions."

Experiment, however, has shown that it is generally true that the elementary atoms have the same specific heat; that is, if we take of different elements quantities proportionate to their atomic weights, and which therefore contain the same number of atoms, the same amount of heat will raise them all the same number of degrees in temperature; in other words, it takes just as much heat to raise i atom of hydrogen i degree in temperature as it does to raise i atom of nitrogen, oxygen, or any other element i degree.

Hence, when we are in doubt as to the atomic weight of any. element, we can sometimes settle the question by its specific heat.

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4. Quantivalence. — When the atoms of different elements replace one another in a compound molecule, it is found that while in some cases i atom of an element may replace i of another, in other cases it may replace 2, 3, or 4 atoms of another. Thus, when hydric chloride and argentic nitrate act upon each other, we have the following change:

AgNO3 + HCI= AgCl + HNO3.

Here i atom of Ag changes place with 1 of H.
When Zn acts upon H2SO4 we have

H2SO4 + Zn=ZnSO4+H2.

Here i atom of Zn replaces, and is equivalent to, 2 atoms of H.

Again, in the reaction between water and phosphorous chloride, we have

3H, or H2H,03+ PC1,=H PO3 + 3HCI.

Here i atom of P replaces, and is equivalent to, 3 atoms of H.

“This relation of the elements to each other is called by Hofmann quantivalence; and selecting here, as in the system of atomic weights, the hydrogen atom as our standard of inference, the atoms of different elements are called univalent, bivalent, trivalent, or quadrivalent, according as they are, in the sense already indicated, equivalent to 1, 2, 3, or 4 atoms of hydrogen.

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