ture than water), in order to heat the apparatus to a constant and known temperature.

Let us suppose that the substance, whose molecular weight we now wish to find, is common ether.

C

We

begin by weighing our little bottle, first when empty, and then when filled with ether, thus determining, with great accuracy, the weight of the quantity of ether used. With a little dexterity we next pass the bottle under the mercury into the barometer-tube, when it at once rises into the vacuous space. We now pass free steam through the jacket, until we are sure that the temperature of the apparatus is constant at, say,

100° centigrade. The ether, expanding with the heat, soon forces out the glass stopper by which it was confined, and evaporates into the space above the mercury, depressing the column. At first the column oscillates violently, but it soon comes to rest, and we can then read on the graduated scale the volume of the vapor which the weight of ether taken has yielded. This vapor is evidently at the temperature of boiling water, or 100° centigrade; but what is its tension?

The method of measuring the tension will be obvious if you reflect that, in this apparatus, the pressure of the air on the surface of the mercury in the cistern is balanced by the mercury column in the tube and the tension of the vapor pressing on the upper surface of this column. Hence, the height of the column in the tube will be less than that of a true barometer in the neighborhood by just the amount of this tension. In order to find the tension, we have, therefore, only to observe the height of the barometer, and subtract from this the height of the column in our tube, which we must now measure with as much accuracy as possible. Omitting, as in the previous example, à few small corrections, our calculation will now appear thus:

Determination of the Molecular weight of Ether by Gay-Lussac's method, improved by Hofmann.

drogen gas at 100° and 57 centime-0.0686 of a crith.

tres, by calculation...

2.539 ÷ 0.0686 = 37 sp. gr. of ether.

37 x 2 = 74 molecular weight of ether.

LIMITATIONS OF OUR METHODS.

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As has been stated, the two methods of determining molecular weight, just described, apply only to those substances which can be readily volatilized by a moderate elevation of temperature. With some slight modifications, the first method may likewise be used for the permanent gases; and, by employing a globe of porcelain, St.-Claire Deville has succeeded in determining, in the same way, the molecular weight of several substances which do not volatilize under a red heat. But a great number of substances cannot be volatilized at all within any manageable limits of temperature, and a still larger number are so readily decomposed by heat as to be incapable of existing in the aëriform condition. The molecular weight of such bodies cannot, of course, be determined by direct weighing. In most cases, however, we are able to infer with considerable certainty the molecular weight of these non-volatile bodies from a knowledge of their composition and other chemical relations; but, nevertheless, there are numerous instances in which the conclusions thus drawn are very questionable, and a great deal of the uncertainty, which still obscures the philosophy of our science, arises from this circumstance.

LECTURE IV.

CHEMICAL COMPOSITION-ANALYSIS AND SYNTHESIS—THE ATOMIC THEORY.

In my previous lectures I have endeavored to give you a clear idea of the meaning which our modern science attaches to the word molecule. I must next attempt to convey, as far as I am able, the corresponding conception which the chemist expresses by the word atom. The terms molecule and atom are constantly confounded; indeed, have been frequently used as synonymous; but the new chemistry gives to these words wholly different meanings. We have already defined a molecule as the smallest mass into which a substance is capable of being subdivided without changing its chemical nature; but this definition, though precise, does not suggest the whole conception; for the molecule may be regarded from two very different points of view, according as we consider its physical or its chemical relations. To the physicist, the molecules are the points of application of those forces which determine or modify the physical condition of bodies, and he defines molecules as the small particles of matter which, under the influence of these forces, act as units. Or, limiting his regards to those phenomena from which our knowledge of molecular masses is chiefly derived, he may prefer to

CHEMICAL DEFINITION OF MOLECULES.

85

define molecules as those small particles of bodies which are not subdivided when the state of aggregation is changed by heat, and which move as units under the influence of this agent.

To the chemist, on the other hand, the molecules determine those differences which distinguish substances. Sugar, for example, has the qualities which we associate with that name, because it is an aggregate of molecules which have those qualities. Divide up a lump of sugar as much as you please. The smallest mass that you can recognize still has the qualities of sugar; and so it must be, if you continue the division down to the molecule. The molecule of sugar is simply a very small piece of sugar. Dissolve the sugar in water, and we obtain a far greater degree of subdivision than is possible by mechanical means; a subdivision which, we suppose, extends as far as the molecules. The particles are distributed through a great mass of liquid, and become invisible; still, the qualities of the sugar are preserved; and, on evaporating the water, we recover the sugar in its solid condition; and, according to the chemist, the qualities are preserved, because the molecules of sugar have remained all the while unchanged.

Consider, in the second place, a lump of salt. You do not alter its familiar qualities, however greatly you may subdivide it, and the molecules of salt must have all the saline properties which we associate with this substance. Dissolve the salt in water, and you simply divide the mass into molecules. Convert the salt into vapor, as you readily can, and again you isolate the molecules as before. But, through all these changes, the salt remains salt; it does not lose its savor, because the individuality of the molecules is preserved. So is