A History of Science, vol 4 by Henry Smith Williams (the two towers ebook .TXT) 📕

atom which has just flung itself into the circuit of two hydrogen

atoms, the next moment flings itself free again and seeks new

companions. It is for all the world like the incessant change of

partners in a rollicking dance. This incessant dissolution and

reformation of molecules in a substance which as a whole remains

apparently unchanged was first fully appreciated by Ste.-Claire

Deville, and by him named dissociation. It is a process which

goes on much more actively in some compounds than in others, and

very much more actively under some physical conditions (such as

increase of temperature) than under others. But apparently no

substances at ordinary temperatures, and no temperature above the

absolute zero, are absolutely free from its disturbing influence.

Hence it is that molecules having all the valency of their atoms

fully satisfied do not lose their chemical activity—since each

atom is momentarily free in the exchange of partners, and may

seize upon different atoms from its former partners, if those it

prefers are at hand.

While, however, an appreciation of this ceaseless activity of the

atom is essential to a proper understanding of its chemical

efficiency, yet from another point of view the “saturated”

molecule—that is, the molecule whose atoms have their valency

all satisfied—may be thought of as a relatively fixed or stable

organism. Even though it may presently be torn down, it is for

the time being a completed structure; and a consideration of the

valency of its atoms gives the best clew that has hitherto been

obtainable as to the character of its architecture. How

important this matter of architecture of the molecule—of space

relations of the atoms—may be was demonstrated as long ago as

1823, when Liebig and Wohler proved, to the utter bewilderment of

the chemical world, that two substances may have precisely the

same chemical constitution—the same number and kind of

atoms—and yet differ utterly in physical properties. The word

isomerism was coined by Berzelius to express this anomalous

condition of things, which seemed to negative the most

fundamental truths of chemistry. Naming the condition by no

means explained it, but the fact was made clear that something

besides the mere number and kind of atoms is important in the

architecture of a molecule. It became certain that atoms are not

thrown together haphazard to build a molecule, any more than

bricks are thrown together at random to form a house.

How delicate may be the gradations of architectural design in

building a molecule was well illustrated about 1850, when Pasteur

discovered that some carbon compounds—as certain sugars—can

only be distinguished from one another, when in solution, by the

fact of their twisting or polarizing a ray of light to the left

or to the right, respectively. But no inkling of an explanation

of these strange variations of molecular structure came until the

discovery of the law of valency. Then much of the mystery was

cleared away; for it was plain that since each atom in a molecule

can hold to itself only a fixed number of other atoms, complex

molecules must have their atoms linked in definite chains or

groups. And it is equally plain that where the atoms are

numerous, the exact plan of grouping may sometimes be susceptible

of change without doing violence to the law of valency. It is in

such cases that isomerism is observed to occur.

By paying constant heed to this matter of the affinities,

chemists are able to make diagrammatic pictures of the plan of

architecture of any molecule whose composition is known. In the

simple molecule of water (H2O), for example, the two hydrogen

atoms must have released each other before they could join the

oxygen, and the manner of linking must apparently be that

represented in the graphic formula H—O—H. With molecules

composed of a large number of atoms, such graphic representation

of the scheme of linking is of course increasingly difficult,

yet, with the affinities for a guide, it is always possible. Of

course no one supposes that such a formula, written in a single

plane, can possibly represent the true architecture of the

molecule: it is at best suggestive or diagrammatic rather than

pictorial. Nevertheless, it affords hints as to the structure of

the molecule such as the fathers of chemistry would not have

thought it possible ever to attain.

These utterly novel studies of molecular architecture may seem at

first sight to take from the atom much of its former prestige as

the all-important personage of the chemical world. Since so much

depends upon the mere position of the atoms, it may appear that

comparatively little depends upon the nature of the atoms

themselves. But such a view is incorrect, for on closer

consideration it will appear that at no time has the atom been

seen to renounce its peculiar personality. Within certain limits

the character of a molecule may be altered by changing the

positions of its atoms (just as different buildings may be

constructed of the same bricks), but these limits are sharply

defined, and it would be as impossible to exceed them as it would

be to build a stone building with bricks. From first to last the

brick remains a brick, whatever the style of architecture it

helps to construct; it never becomes a stone. And just as closely

does each atom retain its own peculiar properties, regardless of

its surroundings.

Thus, for example, the carbon atom may take part in the formation

at one time of a diamond, again of a piece of coal, and yet again

of a particle of sugar, of wood fibre, of animal tissue, or of a

gas in the atmosphere; but from first to last—from glass-cutting

gem to intangible gas—there is no demonstrable change whatever

in any single property of the atom itself. So far as we know, its

size, its weight, its capacity for vibration or rotation, and its

inherent affinities, remain absolutely unchanged throughout all

these varying fortunes of position and association. And the same

thing is true of every atom of all of the seventy-odd elementary

substances with which the modern chemist is acquainted. Every one

appears always to maintain its unique integrity, gaining nothing

and losing nothing.

All this being true, it would seem as if the position of the

Daltonian atom as a primordial bit of matter, indestructible and

non-transmutable, had been put to the test by the chemistry of

our century, and not found wanting. Since those early days of the

century when the electric battery performed its miracles and

seemingly reached its limitations in the hands of Davy, many new

elementary substances have been discovered, but no single element

has been displaced from its position as an undecomposable body.

Rather have the analyses of the chemist seemed to make it more

and more certain that all elementary atoms are in truth what John

Herschel called them, “manufactured articles”—primordial,

changeless, indestructible.

And yet, oddly enough, it has chanced that hand in hand with the

experiments leading to such a goal have gone other experiments

arid speculations of exactly the opposite tenor. In each

generation there have been chemists among the leaders of their

science who have refused to admit that the so-called elements are

really elements at all in any final sense, and who have sought

eagerly for proof which might warrant their scepticism. The first

bit of evidence tending to support this view was furnished by an

English physician, Dr. William Prout, who in 1815 called

attention to a curious relation to be observed between the atomic

weight of the various elements. Accepting the figures given by

the authorities of the time (notably Thomson and Berzelius), it

appeared that a strikingly large proportion of the atomic weights

were exact multiples of the weight of hydrogen, and that others

differed so slightly that errors of observation might explain the

discrepancy. Prout felt that it could not be accidental, and he

could think of no tenable explanation, unless it be that the

atoms of the various alleged elements are made up of different

fixed numbers of hydrogen atoms. Could it be that the one true

element—the one primal matter—is hydrogen, and that all other

forms of matter are but compounds of this original substance?

Prout advanced this startling idea at first tentatively, in an

anonymous publication; but afterwards he espoused it openly and

urged its tenability. Coming just after Davy’s dissociation of

some supposed elements, the idea proved alluring, and for a time

gained such popularity that chemists were disposed to round out

the observed atomic weights of all elements into whole numbers.

But presently renewed determinations of the atomic weights seemed

to discountenance this practice, and Prout’s alleged law fell

into disrepute. It was revived, however, about 1840, by Dumas,

whose great authority secured it a respectful hearing, and whose

careful redetermination of the weight of carbon, making it

exactly twelve times that of hydrogen, aided the cause.

Subsequently Stas, the pupil of Dumas, undertook a long series of

determinations of atomic weights, with the expectation of

confirming the Proutian hypothesis. But his results seemed to

disprove the hypothesis, for the atomic weights of many elements

differed from whole numbers by more, it was thought, than the

limits of error of the experiments. It was noteworthy, however,

that the confidence of Dumas was not shaken, though he was led to

modify the hypothesis, and, in accordance with previous

suggestions of Clark and of Marignac, to recognize as the

primordial element, not hydrogen itself, but an atom half the

weight, or even one-fourth the weight, of that of hydrogen, of

which primordial atom the hydrogen atom itself is compounded. But

even in this modified form the hypothesis found great opposition

from experimental observers.

In 1864, however, a novel relation between the weights of the

elements and their other characteristics was called to the

attention of chemists by Professor John A. R. Newlands, of

London, who had noticed that if the elements are arranged

serially in the numerical order of their atomic weights, there is

a curious recurrence of similar properties at intervals of eight

elements This so-called “law of octaves” attracted little

immediate attention, but the facts it connotes soon came under

the observation of other chemists, notably of Professors Gustav

Hinrichs in America, Dmitri Mendeleeff in Russia, and Lothar

Meyer in Germany. Mendeleeff gave the discovery fullest

expression, explicating it in 1869, under the title of “the

periodic law.”

Though this early exposition of what has since been admitted to

be a most important discovery was very fully outlined, the

generality of chemists gave it little heed till a decade or so

later, when three new elements, gallium, scandium, and germanium,

were discovered, which, on being analyzed, were quite

unexpectedly found to fit into three gaps which Mendeleeff had

left in his periodic scale. In effect the periodic law had

enabled Mendeleeff to predicate the existence of the new elements

years before they were discovered. Surely a system that leads to

such results is no mere vagary. So very soon the periodic law

took its place as one of the most important generalizations of

chemical science.

This law of periodicity was put forward as an expression of

observed relations independent of hypothesis; but of course the

theoretical bearings of these facts could not be overlooked. As

Professor J. H. Gladstone has said, it forces upon us “the

conviction that the elements are not separate bodies created

without reference to one another, but that they have been

originally fashioned, or have been built up, from one another,

according to some general plan.” It is but a short step from

that proposition to the Proutian hypothesis.

But the atomic weights are not alone in suggesting the compound

nature of the alleged elements. Evidence of a totally different

kind has