A NEW ASPECT OF AN OLD THEORY.

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lowers, to a great extent, ignored the more important phenomenon in magnifying the less, and it is only within a few years that the true relations of the two have been understood. All honor to these great pioneers of science, and let their experience teach us that, in science, as in religion, we see as through a glass darkly, and that we must not attach too much importance to the forms of thought, which, like all things human, are subject to limitations, and liable to change.

LECTURE XI,

QUANTIVALENCE AND METATHESIS-ALKALIES AND ACIDS.

BEFORE studying metathesis, the third, as you will remember, of the three classes into which we divided chemical reactions, I must ask your attention, at the beginning of my lecture this evening, to a most important general principle, to which a study of the results of analysis and synthesis has led, and which will greatly help to elucidate the metathetical processes we have yet to investigate.

The diagram on the curtain before us illustrates the truth we have to present. The story, indeed, is here told in our chemical hieroglyphics, but let us try to decipher them. In attacking our work, let us not fail to remember that these symbols really exhibit the constitution of the molecules of the definite substances

THE BASIS OF FACT.

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Our

they represent. The symbol H2O, for example, shows that a molecule of water consists of two atoms of hydrogen and one of oxygen. Remember that this symbol is not the expression of a mere hypothesis, but represents the results of actual experiment. In a former lecture we have dwelt at length on the evidence on which it is based. We cannot continually retrace our steps; but be sure that you recall this evidence, so that we may plant the ladder, on which we shall attempt to climb higher, on firm ground. Now, what is true of the symbol of water, is true of all the symbols on this diagram. There is not one of them in regard to which there is a shade of doubt. atoms may be mere fancies, I admit, but, like the magnitudes we call waves of light, the magnitudes we have measured and called atoms must be magnitudes of something, however greatly our conceptions in regard to that something may change. Our whole atomic theory may pass, the words molecule and atom may be forgotten; but it will never cease to be true that the magnitude which we now call a molecule of water consists of two of the magnitudes which, in the year 1872, were called atoms of hydrogen, and of one of the magnitudes which were called, at the same period, atoms of oxygen.

Look, now, at the first line of symbols, and see in what a remarkable relation the atoms there represented stand to each other. In a molecule of hydrochloric-acid gas (HCl), one atom of chlorine is united to one atom of hydrogen. In the molecule of water (H2O) one atom of oxygen is united to two of hydrogen. In the molecule of ammonia gas (NH) one atom of nitrogen is united to three atoms of hydrogen, and in the molecule of marsh gas (CH) the atom of carbon is

united to four atoms of hydrogen. It would appear, then, that the atoms of chlorine, oxygen, nitrogen, and carbon, have different powers of combination, uniting respectively with one, two, three, and four atoms of hydrogen. In order to assure yourselves that this relation is not an illusion, depending on the collocation of selected symbols, but results from a definite quality of the several atoms, examine the symbols of the second line, and you will see that, in a similar way, the atoms of sodium (Na), mercury (Hg), antimony (Sb), carbon (C), and phosphorus (P), unite respectively with one, two, three, four, and five atoms of chlorine. Moreover, on comparing the two lines, notice that the atom of chlorine, which combines with one atom of hydrogen, combines also with one atom of sodium. Again notice that the atom of carbon, which combines with four atoms of hydrogen, combines also with four atoms of chlorine. Further, observe on the third line that the atom of mercury, which combines with two atoms of chlorine, combines with only one of oxygen; and that the atom of carbon, which combines with either four atoms of chlorine or four atoms of hydrogen, combines with two atoms of oxygen; and compare with these facts those first noticed, that the atom of oxygen combines with two atoms of hydrogen, and the atom of chlorine with but one.

Relations so far reaching and so intricate as these cannot be accidental; and when you are told that the examples here given have been selected, on account of their simplicity, from a countless number of instances in which similar relations have been observed, you will not be satisfied until you find some explanation of the cause of these facts.

The explanation which our modern chemistry gives

is this: It is assumed that each of the elementary atoms has a certain definite number of bonds, and that by these alone it can be united to other atoms. If you wish to clothe this abstract idea in a material conception, picture these bonds as so many hooks, or, what is probably nearer the truth, regard them as poles like those of a magnet. If we have grasped this idea, let us turn back to our diagram and we shall find that the relations we had but. dimly seen have become clear and intelligible. The hydrogen sodium and chlorine atoms have only one bond or pole, and hence, in combining with each other, they can only unite in pairs. The oxygen-atom has two bonds or poles, and can combine, therefore, with two hydrogen-atoms, one at each pole. The mercury-atom has also two bonds, and takes, in a similar manner, two atoms of chlorine; but it can only combine with a single atom of oxygen, for the two poles of one just satisfy the two poles of the other. Again, the atom of carbon has four bonds, which may be satisfied by either four atoms of hydrogen, or four atoms of chlorine, or two atoms of oxygen, or one atom of oxygen and two of chlorine, or, lastly, one atom of oxygen and two of hydrogen. Further, the atom of phosphorus has five bonds, and holds five atoms of chlorine, or three atoms of chlorine and one of oxygen. Finally, the chromium atom binds six atoms of fluorine, or three of oxygen, or two of oxygen and two of chlorine. This quality of the atoms, which we endeavor to represent to our minds by the conception of hooks, bonds, or poles, we call, in our modern chemistry, quantivalence, and we use the Latin terms univalent, bivalent, trivalent, quadrivalent, quinquivalent, sexivalent, etc., to designate the atoms which have one, two, three, four, five, six, etc., hooks, bonds, or poles, respectively.