yune 27, 1 872 J 



NATURE 



171 



ATOMS AND MOLECULES* 



'l^IIE atomic composition of ponderable matter is a fiindtmental 

 pistulaie in the theory of chemical equivalency. By llie 

 application of the principles of experimental research, and by 

 melliods essentially modern, resulting in the discovery of many 

 elementary bodies and their modes of combination, a conception 

 of very great antiquity has been rendered more distinct and worthy 

 of credence. When this conception took definite form is not 

 known. Indeed, it is one of the many speculations naturally 

 elicited in discussing those subtle questions pertaining to tlie 

 existence of matter and its relations to mind or spirit, the solu- 

 tion of which has always baflled, and will continue to baffle, the 

 most profound thinkers. In attempting to unfold the mysteries 

 of nature by the deductive process, the ancient teachers of 

 Cosmogony were brought into direct conflict of opinion regard- 

 ing the ultimate conduion of matter. That it is composed of 

 indestructible atoms which admit of no division, seems to have 

 been the notion of some Oriental sages. Under the genius of the 

 Greek philosophy this notion assumed the form and consistency 

 of a theory. 



Among those who held tlie doctrine, while immatured, were 

 Ecphantus, Leucippus, and Democritus. Subsequently Epicurus 

 introduced such modifications and improvements as were essential 

 to iis complete development, t The Latin poet Lucretius, in his 

 " De Rerum Natura," lias given a full exposition of the Epicu- 

 rean pliilosophy ; from this, as well as from the writings of 

 Plutarch, it will be seen that the most prominent atomic tenets 

 did not differ essentially from the opinions entertained by eminent 

 scientists of modern times. it Newton admitted the creation of 

 primitive particles, extremely minute, but permanent. Descartes, 

 on the other hand, held with Aristotle, Plato, and Pythagorae 

 that the division of matter has no assignable limit. Leibnitz at- 

 tempted to reconcile the conflicting opinions of metaphysicians 

 and mathematicians, by supposing that matter, in its ultimate 

 condition, consists of unextended points which he denominated 

 monads, a term borron'ed from Pythagoras. At a later day, 

 Boscovich published his celebrated dynamic theory, in which 

 centres of force are substituted for monads. Neither of these 

 ingenious theories however, reaches the real points of perplexity. 



It is obvious that the science wliicli treats of the ultimate com- 

 position of bodies would lead to more correct conceptions regard- 

 ing minute combinations of ponderable matter. Analysis lias 

 sliown that nearly all tlie bodies formed in the grert laboratory 

 of nature are compounds. Thus far, sixty-three different kinds 

 of matter have resisted every effort to resolve them into simpler 

 constituents. Theie substances, distinguished as chemical ele- 

 ments, unite in exceedingly minute quantities according to ths 

 well-known laws of Stoichiometry. In the year 1789, Higgins, 

 a piofessor in the University of Dublin, advanced the idea tliat 

 certain compounds are formed by the combination of ultimate 

 particles or atoms of different elements. Dalton, in iSoj, in- 

 dependently aoived at a similar conclusion, which he generalised, 

 to explain the composition of all compounds, and made it tlie 

 basis of his " New System of Chemical Philosophy," published 

 five years later. The doctrine of Dalton has undergone, since 

 his day, such modifications as render it more acceptable ; but 

 that part of it which ascribes the union of indestructiljle atoms to 

 chemical affinity may be regarded as the first successful attempt 

 to explain that primordial action which the ancient atomists could 



* Reprinted from the American Chemist ^ corrected by the author. 



t Plutarch's " Morals." Edited by Prof. Goodwin, of H.-iri'arJ Univer. 

 sity. (Linle, Brown, and Co., Boston, Mass.) Vol. viii. pp, 111-112. Vol. 

 V. p. 3.15. • i 



I A full e.\-position of the ancient atomic philosophy would be foreign to 

 the purpo.=e of this paper. Many of the prevailing erroneous impressions 

 concerning it would, however, be corrected by an examination of tiie third 

 cluipter of Dr. Good's "Book of Nature," in which Epicurus is ably de- 

 fended against the charge of atheism. Evidently the Epicureans were op- 

 posed to Mythology: but while ignoiing the power of its gods, they were 

 naturally led to the recognition of a higher Power, an Intelligent Cause, Self- 

 existent, and Supreme. This deduction was reached by the earnest believers 

 in the atomic doctrine. According to Stobaeus, Ecphantus supposed the 

 material world to consist of atoms, but yet to be ordered and governed by a 



Divine providence. 'E«#avTo: U tiiv rCv dro^wv ff<^e<rrdviu rdif xoT^iv, ^lo^erudat ii 



airi iTfMvoU:. Eclog. Physic. lib. i. cap. xxv. And as evidence of the belief 

 prevalent among wise men several centuri'^s later, Berzelius, in his paper on 

 '* Proportions, determinate," quotes from Philo, who, in his collection of the 

 choicest phJosophical ideas of his time {" Libri Sapiential," cap. xi. v. 22), 

 says: — nd^ia biC'; ^liTp^ xai dpis^ui not <7T,iS/xu, SiiraU ("God uiadc all things by 

 measure, number, and weight ' ). This remarkable statement, as far as it 

 relates to things terrestrial, modern chemical investigatioirs have fully con- 

 firmed. 



not account for, and which the Latin poet above named describes 

 as irregular and fortuitous. * 



Chemists of the atomic school happily avoid the ve.xed ques- 

 tion concerning the indivisibility of matter, by defining an atom 

 as the smallest quantity of an element which can enter into the 

 composition of a ponderable molecule ; and the molecule, whe- 

 ther made up of one, two, or more elements, as the smallest 

 quantity which can exist in a free state. However, a certain 

 individuality must be assigned to the single atom, for a chemical 

 decomposition requiring its transfer from one molecule to another 

 involves its isolation in tiansitn. The absolute weight of 

 the sixty-three different atoms cannot be ascertained ; never- 

 theless, their relative weights have been determined with great 

 care. 



It is difficult to arrive at any clear notions concerning the size 

 of an object so minute as to be for ever invisible under the most 

 powerful magnifier. As an example of the conclusions regard- 

 ing molecules, founded on microscopic scrutiny, that of the 

 celebrated Ehrenberg may be cited, t Without attempting to 

 make a close approximation towards its actual dimensions, his 

 researches led him to infer that the diameter of an atom (the 

 molecule of tlie chemist) was considerably less than six millionths 

 of aline. Quite recently Sir W. Thomson, in a paper ' ' On the Size 

 of Atoms,"! presented four lines of argument founded on experi- 

 ments of physicists, which all lead to substantially the same 

 estimate of the dimensions of molecular structure. He 

 says : — 



"Jointly they establish, with what we cannot but regard as a 

 very high degree of probability, the conclusion that, in any ordi- 

 nary liquid, transparent solid, or seemingly opaque solid, the 

 mean distance between the centres of contiguous molecules is less 

 than the hundred-millionth and greater than the two thousand- 

 millionth of a centimetre. To form some conception of the de- 

 gree of cross-grainedness indicated by this conclusion, imagine a 

 rain-drop, or a globe of glass as large as a pea, to be magnified 

 up to the size of the earth, each constituent molecule being mag- 

 nified in the same proportion. The magnified structure would 

 be coarser grained than a heap of small shot, but probably less 

 coarse grained than a heap of cricket balls." 



From these deductions of Thomson some idea may be formed 

 of minute molecular grouping ; and I venture the suggestion 

 that, in regard to size, the smallest bullet would probably stand 

 about mid- way between the gloiiicramcii minimum and " the 

 great globe itself." 



Beyond this point of extreme tenuity, where matter first ex- 

 hibits that property which is revealed in visible forms, we are 

 forced to consider it in a still more expanded state, as the 

 universally diffused medium of light, heat, and actinism ; con- 

 sequently this conception of the minute ponderable globule does 

 not bring us very near the tiiiiiiiiia nalurtc, for a difference in size 

 cannot be less marked between ponderable atoms and those in- 

 finitesimal particles forming the luminiferous ether or a;th which 

 fills the interstellar space.s, and which, in a more condensed state, 

 probably forms the interatomic medium. Assuming that all 

 forces generating wave motions in elastic fluids follow the same 

 law of propagation, I endeavoured some years ago to estimate 

 the density of this inconceivably attenuated substance. § 



In that calculation the density of air is the unit of measure. 

 If instead hydrogen be taken as the unit, the density of lumini- 

 ferous ether is expressed by the decimal '00000000001653. 

 Whatever may be its actual density, its reality must be admitted, 

 until the positions established by the investigations of Huyghens, 



* Omnimodis coire, atque omnia pertentare, 



Qua;cunque inter se possint congressa creare. 



L^icrei. lib, v. ver. igi, 



+ Pogg. Aniialcn. xxiv., 35. J Nature, No. 22, vol. i., p. 551. 



§ Sound would be propagated, with exactly the velocity of light through 

 a fluid, under the standard pressure, 874,094,10.^.900 times rarer than air. 

 Therefore, if the density of air be i, the density of asth is represented by the 

 decimal •ooo,ooo,oooj8«4,i44-l-. 



It will not be infeted from this view that the aim has been to reach 

 "The first of things, quintessence pure," 

 for the elastic quality of xth involves the hj^jothesis of a still more subtle 

 fluid. We have raised one curtain only to find another to be raised. As the 

 unfathomed vaults of Heaven recede before the sweep of a more powerful 

 refractor, and nebulas resolved reveal nebulje beyond, so the most diminu- 

 tive germ that springs from the Creator's touch discloses through the lens of 

 higher power new signs of more wonderful mechanism within. Each nucleus 

 has its uuclei ! Each entoblast is but the boundary of a microcosm ! Each 

 particle a gala.fy of atoms revolving in the all-pervading ffith 1 Thus, before 

 every far-reaching human advance. Circumference and Centre will for ever 

 retreat. — Transactions of tlie American Institute^ 1864, p. 539. (*'Clydo- 

 nics," No. I ) 



