LIMITATIONS OF OUR METHODS.

1

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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, the late St.-Claire Deville succeeded in determining, in the same way, the molecular weight of several substances which do not volatilize under a red heat. More recently Victor Meyer has devised a very ingenious method of determining the specific gravity of vapors, which, being independent of the temperature, can be used at the highest temperatures that the required vessels can be made to withstand; and by means of an apparatus of platinum, heated in a powerful furnace, he has been able to extend very considerably our knowledge in the same direction. 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.

1 For a description of this method see author's "Chemical Philosophy," revised edition, 1882, page 35.

LECTURE IV.

LAW OF CONSERVATION OF MASS, LAW OF DEFINITE PROPORTIONS, AND LAW OF GAY-LUSSAC.

ALL the processes we have studied thus far have not involved any change of substance in the materials employed. The liquid or crystalline films, which in some of our experiments produced such gorgeous phenomena of color, were not altered thereby. The bits of iron which became polarized by the influence of magnetism still remained metallic iron. The liquid which was crystallized in the snow-flakes, or converted into steam in our glass flask, remained the same familiar substance, water, in all these conditions. Such processes as these we call, in general, physical processes; and all modes of motion, and all mechanical processes of the arts by which various materials are converted into useful shapes without altering their substance, belong to this category.

There is another class of processes, however, and even a larger class, whose very essence consists in the change of one or more substances into wholly different substances. These processes are frequently accompanied by striking physical phenomena, such as the development of electricity, heat, or light; but the essence of the process is always a change of substance. Such processes we distinguish as chemical processes, and we speak

FACTORS AND PRODUCTS.

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of chemical changes, or of chemical phenomena, or of chemical qualities, understanding by the term chemical such processes, such changes, such phenomena, or such qualities, as cannot be manifested without a change of substance; that is, a change so fundamental that we always give to the products of the change different names from those of the materials, or factors, with which the change began.

The elements of every chemical change are these: 1. One or more substances called the factors, with which the change begins; 2. One or more substances called the products, with which the change ends. As just said, the chemical change may be accompanied with the manifestation of striking physical phenomena, as the burning of gunpowder, with the rush of a cannonball; the burning of coal, with the development of heat; or the solution of zinc in the acid of a voltaic battery, with the flow of an electrical current: but these are not the phenomena which it is the special province of the chemist to study-he leaves these to the physicist ; but, on his part, he inquires in every case what are the factors and what are the products of the change; and, when a new process is discovered, he is not content until he can clearly point out all the substances that enter into the process, as well as all the substances that are formed by it.

At first sight chemical processes are frequently very obscure, and one great reason is, that we live in an atmosphere which is a mixture of two invisible aëriform substances, named nitrogen gas and oxygen gas, and these substances, especially the last, are constantly entering as factors into chemical processes without our noticing the circumstance; and, again, the products of such processes, when aëriform, often escape notice by

mingling with the great volume of the air. Now, that we are on our guard, we are seldom deceived by the intervention of the atmosphere; but in former times, when the qualities and relations of aëriform bodies were little known, so great was the obscurity thus caused, that even the familiar processes of combustion have not been understood until within a century. These processes of combustion will furnish the best means of illustrating the general principles and facts we have just stated.

The burning of a log of wood is a chemical process, because, in burning, the material we call wood, together with a quantity of oxygen gas from the atmosphere disappear, and other substances, which in the aggregate we call smoke and ashes, appear in their place. Our fathers overlooked wholly the oxygen gas, and made a small account of the smoke, and it is no wonder that they misunderstood the process; but now we know all the factors and all the products. Here both the factors and the products are complex, for wood is an organism containing other substances than woodfibre, and smoke, although consisting chiefly of aqueous vapor and carbonic dioxide gas, carries various empyreumatic products. A burning candle, especially if it consists of some definite substance, will give a more apt illustration. Here the products are two definite substances, oxygen gas and the material of the candle; and the products two equally definite substances, aqueous vapor and carbonic dioxide. That these products are actually escaping from this candle-flame, I can easily show you. If I hold a cold glass bell over the flame, the inner surface soon becomes bedewed, and after a while drops of liquid water run down the sides. Carbonic-dioxide gas, or carbonic acid, as it is often called, may not be to every one as familiar a substance as water,

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but it is equally common. It is the gas which escapes from all effervescing drinks, and it has one characteristic property: it immediately renders a solution of lime (lime-water) turbid if brought in contact with the liquid. I uncork some bottled beer and pour it into a tall glass. As the effervescence subsides, a colorless gas collects in the upper part of the glass; for the gas is so heavy that it only quite slowly diffuses into the atmosphere. This heavy gas is carbonic dioxide, the substance of which I have been speaking. Into another tall glass I pour some clear lime-water, and now fill up the glass with carbonic dioxide, which I can readily pour off from the top of the beer; and notice that, when I shake up the carbonic dioxide gas with the lime-water, the last becomes very turbid, owing, in fact, to the formation of chalk. Now, let us test the products of the burning candle which we have been collecting in the glass bell. I invert the bell, pour into it some of the same clear lime-water, which at once becomes turbid as before.

What is true of the chemical process we have just studied is true of all chemical processes. There are always one or more factors and one or more products, and it is one of the great objects of chemical investigation to find out what are these factors and what are these products. Moreover, so great have been the advances in chemical knowledge during the last century that we actually do know what are the factors and what are the products in almost all the chemical processes which occur in Nature or can be produced by art. Furthermore, we have discovered two all-important and fundamental laws which govern chemical changes, and to these I wish to direct your special attention.

The first of those laws appears in the fact, univer