By accepting the accuracy of this generalisation, and using it in every experiment, Lavoisier was able to form a clear mental picture of a chemical change as the separation and combination of homogeneous substances; for, by using the balance, he was able to follow each substance through the maze of changes, to determine when it united with other substances, and when it separated into substances simpler than itself.
CHAPTER XIII.
THE CHEMICAL ELEMENTS CONTRASTED WITH THE ALCHEMICAL PRINCIPLES.
It was known to many observers in the later years of the 17th century that the product of the calcination of a metal weighs more than the metal; but it was still possible, at that time, to assert that this fact is of no importance to one who is seeking to give an accurate description of the process of calcination. Weight, which measures mass or quantity of substance, was thought of, in these days, as a property like colour, taste, or smell, a property which was sometimes decreased, and sometimes increased, by adding one substance to another. Students of natural occurrences were, however, feeling their way towards the recognition of some property of substances which did not change in the haphazard way wherein most properties seemed to alter. Lavoisier reached this property at one bound. By his experimental investigations, he taught that, however greatly the properties of one substance may be masked, or altered, by adding another substance to it, yet the property we call mass, and measure by weight, is not affected by these changes; for Lavoisier showed, that the mass of the product of the union of two substances is always exactly the sum of the masses of these two substances, and the sum of the masses of the substances whereinto one substance is divided is always exactly equal to that mass of the substance which is divided.
For the undefined, ever-changing, protean essence, or soul, of a thing which the alchemists thought of as hidden by wrappings of properties, the exact investigations of Lavoisier, and those of others who worked on the same lines as he, substituted this definite, fixed, unmodifiable property of mass. Lavoisier, and those who followed in his footsteps, also did away with the alchemical notion of the existence of an essential substratum, independent of changes in those properties of a substance which can be observed by the senses. For the experimental researches of these men obliged naturalists to recognise, that a change in the properties of a definite, homogeneous substance, such as pure water, pure chalk, or pure sulphur, is accompanied (or, as we generally say, is caused) by the formation of a new substance or substances; and this formation, this apparent creation, of new material, is effected, either by the addition of something to the original substance, or by the separation of it into portions which are unlike it, and unlike one another. If the change is a combination, or coalescence, of two things into one, then the mass, and hence the weight, of the product is equal to the sum of those masses, and hence those weights, of the things which have united to form it; if the change is a separation of one distinct substance into several substances, then the sum of the masses, and hence the weights, of the products is equal to that mass, and hence that weight, of the substance which has been separated.
Consider the word water, and the substance represented by this word. In Chapter IV., I gave illustrations of the different meanings which have been given to this word; it is sometimes used to represent a material substance, sometimes a quality more or less characteristic of that substance, and sometimes a process to which that substance, and many others like it, may be subjected. But when the word water is used with a definite and exact meaning, it is a succinct expression for a certain group, or collocation, of measurable properties which are always found together, and is, therefore, thought of as a distinct substance. This substance can be separated into two other substances very unlike it, and can be formed by causing these to unite. One hundred parts, by weight, of pure water are always formed by the union of 11.11 parts of hydrogen, and 88.89 parts of oxygen, and can be separated into these quantities of those substances. When water is formed by the union of hydrogen and oxygen, in the ratio of 11.11 parts by weight of the former to 88.89 of the latter, the properties of the two substances which coalesce to form it disappear, except their masses. It is customary to say that water contains hydrogen and oxygen; but this expression is scarcely an accurate description of the facts. What we call substances are known to us only by their properties, that is, the ways wherein they act on our senses. Hydrogen has certain definite properties, oxygen has other definite properties, and the properties of water are perfectly distinct from those of either of the substances which it is said to contain. It is, therefore, somewhat misleading to say that water contains substances the properties whereof, except their masses, disappeared at the moment when they united and water was produced. Nevertheless we are forced to think of water as, in a sense, containing hydrogen and oxygen. For, one of the properties of hydrogen is its power to coalesce, or combine, with oxygen to form water, and one of the properties of oxygen is its ability to unite with hydrogen to form water; and these properties of those substances cannot be recognised, or even suspected, unless certain definite quantities of the two substances are brought together under certain definite conditions. The properties which characterise hydrogen, and those which characterise oxygen, when these things are separated from all other substances, can be determined and measured in terms of the similar properties of some other substance taken as a standard. These two distinct substances disappear when they are brought into contact, under the proper conditions, and something (water) is obtained whose properties are very unlike those of hydrogen or oxygen; this new thing can be caused to disappear, and hydrogen and oxygen are again produced. This cycle of changes can be repeated as often as we please; the quantities of hydrogen and oxygen which are obtained when we choose to stop the process are exactly the same as the quantities of those substances which disappeared in the first operation whereby water was produced. Hence, water is an intimate union of hydrogen and oxygen; and, in this sense, water may be said to contain hydrogen and oxygen.
The alchemist would have said, the properties of hydrogen and oxygen are destroyed when these things unite to form water, but the essence, or substratum, of each remains. The chemist says, you cannot discover all the properties of hydrogen and oxygen by examining these substances apart from one another, for one of the most important properties of either is manifested only when the two mutually react: the formation of water is not the destruction of the properties of hydrogen and oxygen and the revelation of their essential substrata, it is rather the manifestation of a property of each which cannot be discovered except by causing the union of both.
There was, then, a certain degree of accuracy in the alchemical description of the processes we now call chemical changes, as being the removal of the outer properties of the things which react, and the manifestation of their essential substance. But there is a vast difference between this description and the chemical presentment of these processes as reactions between definite and measurable quantities of elements, or compounds, or both, resulting in the re-distribution, of the elements, or the separation of the compounds into their elements, and the formation of new compounds by the re-combination of these elements.
Let us contrast the two descriptions somewhat more fully.