Part 10 (1/2)
If Lavoisier's memoirs are examined closely, it is seen that at the very beginning of his chemical inquiries he a.s.sumed the accuracy, and the universal application, of the generalisation ”nothing is created, either in the operations of art or in those of nature.” Naturalists had been feeling their way for centuries towards such a generalisation as this; it had been in the air for many generations; sometimes it was almost realised by this or that investigator, then it escaped for long periods. Lavoisier seems to have realised, by what we call intuition, that however great and astonis.h.i.+ng may be the changes in the properties of the substances which mutually react, there is no change in the total quant.i.ty of material.
Not only did Lavoisier realise and act on this principle, he also measured quant.i.ties of substances by the one practical method, namely, by weighing; and by doing this he showed chemists the only road along which they could advance towards a genuine knowledge of material changes.
The generalisation expressed by Lavoisier in the words I have quoted is now known as the _law of the conservation of ma.s.s_; it is generally stated in some such form as this:--the sum of the ma.s.ses of all the h.o.m.ogeneous substances which take part in a chemical (or physical) change does not itself change. The science of chemistry rests on this law; every quant.i.tative a.n.a.lysis a.s.sumes the accuracy, and is a proof of the validity, of it.[11]
[11] I have considered the law of the conservation of ma.s.s in some detail in Chapter IV. of _The Story of the Chemical Elements_.
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 h.o.m.ogeneous 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 a.s.sert 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 ma.s.s or quant.i.ty 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 ma.s.s, and measure by weight, is not affected by these changes; for Lavoisier showed, that the ma.s.s of the product of the union of two substances is always exactly the sum of the ma.s.ses of these two substances, and the sum of the ma.s.ses of the substances whereinto one substance is divided is always exactly equal to that ma.s.s 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, subst.i.tuted this definite, fixed, unmodifiable property of ma.s.s. 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, h.o.m.ogeneous 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 ma.s.s, and hence the weight, of the product is equal to the sum of those ma.s.ses, 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 ma.s.ses, and hence the weights, of the products is equal to that ma.s.s, 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 ill.u.s.trations 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 quant.i.ties 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 ma.s.ses. 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 ma.s.ses, 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 quant.i.ties 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 quant.i.ties of hydrogen and oxygen which are obtained when we choose to stop the process are exactly the same as the quant.i.ties 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 quant.i.ties 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.
The alchemist wished to effect the trans.m.u.tation of one substance into another; he despaired of the possibility of separating the Elements whereof the substance might be formed, but he thought he could manipulate what he called the _virtues_ of the Elements by a judicious use of some or all of the three Principles, which he named Sulphur, Salt, and Mercury. He could not state in definite language what he meant by these Principles; they were states, conditions, or qualities, of cla.s.ses of substances, which could not be defined. The directions the alchemist was able to give to those who sought to effect the change of one thing into another were these. Firstly, to remove those properties which characterised the thing to be changed, and leave only the properties which it shared with other things like it; secondly, to destroy the properties which the thing to be changed possessed in common with certain other things; thirdly, to commingle the Essence of the thing with the Essence of something else, in due proportion and under proper conditions; and, finally, to hope for the best, keep a clear head, and maintain a sense of virtue.
If he who was about to attempt the trans.m.u.tation inquired how he was to destroy the specific properties, and the cla.s.s properties, of the thing he proposed to change, and by what methods he was to obtain its Essence, and cause that Essence to produce the new thing, he would be told to travel along ”the road which was followed by the Great Architect of the Universe in the creation of the world.” And if he demanded more detailed directions, he would be informed that the substance wherewith his experiments began must first be mortified, then dissolved, then conjoined, then putrefied, then congealed, then cibated, then sublimed, and fermented, and, finally, exalted. He would, moreover, be warned that in all these operations he must use, not things which he could touch, handle, and weigh, but the _virtues_, the _lives_, the _souls_, of such things.
When the student of chemistry desires to effect the transformation of one definite substance into another, he is told to determine, by quant.i.tative experiments, what are the elements, and what the quant.i.ties of these elements, which compose the compound which he proposes to change, and the compound into which he proposes to change it; and he is given working definitions of the words _element_ and _compound_. If the compound he desires to produce is found to be composed of elements different from those which form the compound wherewith his operations begin, he is directed to bring about a reaction, or a series of reactions, between the compound which is to be changed, and some other collocation of elements the composition of which is known to be such that it can supply the new elements which are needed for the production of the new compound.
Since Lavoisier realised, for himself, and those who were to come after him, the meaning of the terms _element_ and _compound_, we may say that chemists have been able to form a mental picture of the change from one definite substance to another, which is clear, suggestive, and consistent, because it is an approximately accurate description of the facts discovered by careful and penetrative investigations. This presentment of the change has been subst.i.tuted for the alchemical conception, which was an attempt to express what introspection and reasoning on the results of superficial investigations, guided by specious a.n.a.logies, suggested ought to be the facts.
Lavoisier was the man who made possible the more accurate, and more far-reaching, description of the changes which result in the production of substances very unlike those which are changed; and he did this by experimentally a.n.a.lysing the conceptions of the element and the compound, giving definite and workable meanings to these conceptions, and establis.h.i.+ng, on an experimental foundation, the generalisation that the sum of the quant.i.ties of the substances which take part in any change is itself unchanged.
A chemical element was thought of by Lavoisier as ”the actual term whereat a.n.a.lysis has arrived,” a definite substance ”which we cannot subdivide with our present knowledge,” but not necessarily a substance which will never be divided. A compound was thought of by him as a definite substance which is always produced by the union of the same quant.i.ties of the same elements, and can be separated into the same quant.i.ties of the same elements.
These conceptions were amplified and made more full of meaning by the work of many who came after Lavoisier, notably by John Dalton, who was born in 1766 and died in 1844.
In Chapter I., I gave a sketch of the atomic theory of the Greek thinkers. The founder of that theory, who flourished about 500 B.C., said that every substance is a collocation of a vast number of minute particles, which are unchangeable, indestructible, and impenetrable, and are therefore properly called _atoms_; that the differences which are observed between the qualities of things are due to differences in the numbers, sizes, shapes, positions, and movements of atoms, and that the process which occurs when one substance is apparently destroyed and another is produced in its place, is nothing more than a rearrangement of atoms.
The supposition that changes in the properties of substances are connected with changes in the numbers, movements, and arrangements of different kinds of minute particles, was used in a general way by many naturalists of the 17th and 18th centuries; but Dalton was the first to show that the data obtained by the a.n.a.lyses of compounds make it possible to determine the relative weights of the atoms of the elements.
Dalton used the word _atom_ to denote the smallest particle of an element, or a compound, which exhibits the properties characteristic of that element or compound. He supposed that the atoms of an element are never divided in any of the reactions of that element, but the atoms of a compound are often separated into the atoms of the elements whereof the compound is composed. Apparently without knowing that the supposition had been made more than two thousand years before his time, Dalton was led by his study of the composition and properties of the atmosphere to a.s.sume that the atoms of different substances, whether elements or compounds, are of different sizes and have different weights. He a.s.sumed that when two elements unite to form only one compound, the atom of that compound has the simplest possible composition, is formed by the union of a single atom of each element. Dalton knew only one compound of hydrogen and nitrogen, namely, ammonia. a.n.a.lyses of this compound show that it is composed of one part by weight of hydrogen and 4.66 parts by weight of nitrogen.