ATOMIC THEORY. 



ATOMIC THEORY. 



700 



edge melted it, thereby sealing the joint afresh. The cover d was 

 then allowed to fall into its place, and all was ready for the passage of 

 another train as soon as the piston had quitted the pipe so as to allow 

 of its being exhausted afresh. The end of the tube behind the train 

 was left open, to admit the air by which the piston was to be impelled ; 

 but the end in advance of the train was closed, and the air was pumped 

 out from the tube by a branch pipe near it, leading to the air-pumps. 

 These pumps could be worked either by a steam-engine or by any 

 other prime mover of sufficient power. The mode of action, then, 

 was as follows : the train of carriages was pushed forward by manual 

 labour until the piston had entered the tube ; which it might do, by 

 the aid of an ingeniously contrived valve, without impairing the 

 partial vacuum previously produced. It then advanced with a speed 

 proportionate to the rate at which air was abstracted from the tube 

 by the air-pumps until it reached the opposite end of the tube, where, 

 without the aid of an attendant, the valve which had closed the tube 

 sprang open, and the piston and train proceeded by momentum until 

 stopped by breaks. At Dalkey the inclination of the road was sufficient 

 to produce rapid motion in one direction by gravity alone ; so that the 

 atmospheric pressure was only required in ascending, while in 

 descending the piston was moved aside in such a way as to pass clear 

 of the tube. In longer and in level lines of railway there would 

 necessarily be a greater complexity of arrangement. It was this com- 

 plexity that led to the subsequent abandonment of all the schemes. 



The main arguments in favour of the atmospheric system of traction 

 are based upon the facilities which it affords for ascending steep 

 gradients, and consequently for constructing railways at less cost than 

 where heavy cuttings and embankments are necessary in order to 

 procure easy slopes for the locomotive ; the saving in the wear and 

 tear, and consequently in the necessary strength and cost of the rail- 

 way itself, in consequence of not having to convey the moving power 

 with the train ; and the security against collision, owing to the impos- 

 sibility of moving two trains on the same stage or engine-length of 

 railway at the same time. The objections raised to this apply to 

 every other mode of using stationary engines ; such are the necessity 

 of providing and constantly maintaining a power sufficient to conduct 

 the largest amount of traffic which can ever be conveyed, which would 

 render it as costly, as regards some large items of expense, to maintain 

 a railway for the passage of four or five trains per diem as one upon 

 which trains are constantly succeeding each other ; and the liability of 

 derangement to the whole system in consequence of the failure of a 

 single point in it. These are the principal grounds for objection to 

 what has been termed the MUximtity of the system, or, in other 

 . the comparative want of power to modify the mode of working 

 according to the fluctuations of a variable traffic or the exigencies 

 arising from accident. It is this inflexibility, together with the waste 

 of power which results from any defect in the valve, that led to the 

 abandonment of the system. It would be too much, however, to say, 

 that the system, though at present in abeyance, is unsuited for further 

 application. [RAILWAY.] 



ATOMIC THEORY, in chemistry, sometimes termed the dnflrine 

 of dtfinitt proportion* This very important theory, founded on 

 weji-ascertained facts, has bestowed on modern chemistry an almost 

 mathematical degree of precision. The hypothetical, which is to be 

 distinguished from the experimental part of the subject, supposes that 

 chemical compounds result from the combination of the ultimate 

 atoms of their constituent parts. It has been determined by experi- 

 ment, and the fact serves as the basis of the theory, that a compound 

 body, when pure, always contains the same proportions of its con- 

 stituents : thus, calcareous spar, and the pure part of marble, chalk, 

 and limestones, consist of carbonate of lime, composed of the same 

 proportions of carbonic acid and lime ; the carbonic acid always con- 

 tains the same quantity of carbon and oxygen, and the lime the same 

 proportions of calcium and oxygen. The same law also exists with 

 regard to all similarly-constituted oxides, sulphurets, and salts, and 

 indeed as to all chemical compounds whatever, whether presented to 

 us by nature or formed by art : this is a simple statement of the 

 fundamental facts upon which the superstructure of the atomic theory 

 has been raised. 



Before we proceed to detail the minutiae of the theory, it will be 

 proper to give a sketch, though a slight one, of the principal discoveries 

 connected with the subject. 



The earliest experiments which could have served as a basis for the 

 atomic theory are those of Wenzel, a German chemist, who published, 

 in 1777, a work ' On the Affinities of Bodies;' the experiments detailed 

 in it, though neglected at the time, are now acknowledged to possess a 

 very considerable degree of accuracy. The author showed that when 

 any two neutral salts decomposed each other, the resulting new com- 

 pounds were exactly neutral. " The very attempt," remarks Dr. 

 Thomson, " to analyse the salts was an acknowledgment that bodies 

 united with each other in definite proportions ; and these definite pro- 

 portions, had they been followed out, would have ultimately led to tho 

 doctrine of atoms." (' History of Chemistry,' vol. ii. p. 278.) 



With reference to this subject, it is observed by Sir H. Davy, that 

 " there may be found in the works of Dr. Bryan Biggins, Mr. William 

 Higjrins, and Professor Richter, hints or conclusions bearing directly on 

 this doctrine. Dr. Bryan Higgins, in his ' Experiments and Observa- 

 tions relating to Acetic Acid, Fixable Air, Dense Inflammable Air, &c.,' 



ABTS AND SCI. DIV. VOL. I. 



published in 1786, contends, that elastic fluids unite with each other in 

 limited proportions only ; and this depends upon the combination of 

 their particles or atoms with the matter of fire which surrounds them 

 as an atmosphere, and makes them repulsive of each other ; and he dis- 

 tinguishes between simple elastic fluids, as composed of particles of the 

 same kind, and compound elastic fluids, as consisting of two or more 

 particles combined, in what he calls molecules, definite in quantity 

 themselves, and surrounded by definite proportions of heat. Dr. Bryan 

 Higgins's notions have, I believe, never been referred to by any of the 

 writers on the atomic theory. Mr. William Higgins's claims have, on 

 the contrary, often been brought forward. Yet, when it is recollected 

 that this gentleman was a pupil and relation of Dr. Bryan Higgins, 

 and that his work, called the ' Comparative View,' was published some 

 years after the treatises I have just quoted, and that his notions are 

 almost identical (with the addition of this circumstance, that he men- 

 tions certain elastic fluids, such as the compounds of azote, consisting 

 of one, two, three, four, and five particles of oxygen to one of azote,) it 

 is difficult not to allow the merits of prior conception, as well as of very 

 ingenious illustration, to the elder writer." (' Discourse before the Royal 

 Society,' 1826.) 



In justice however to Mr. Higgiiis, it must be admitted that his 

 views were much more extended than those of Dr. Higgins ; for it 

 appears that he entertained precisely the same notion of the compo- 

 sition and atomic constitution of water as that now generally admitted, 

 in this country at least. In his ' Comparative View of the Phlogistic 

 and Antiphlogistic Theories,' published in 1790, p. 37, he says, "As two 

 cubic inches of light inflammable air require but one of dephlogisti- 

 cated air to condense them, we must suppose that they contain an equal 

 number of divisions, and that the difl'erence of their specific gravity 

 depends chiefly on the size of their ultimate particles ; or we must 

 suppose that the ultimate particles of light inflammable air require 

 two or three, or more, of dephlogisticated air to saturate them. If 

 this latter were the case, we might produce water in an intermediate 

 state, as well as the vitriolic or the nitrous acid, which appears to be 

 impossible ; for in whatever proportion we mix our airs, or under what- 

 soever circumstances we combine them, the result is invariably tho 

 same. This likewise may be observed with respect to the decompo- 

 sition of water. Hence we may justly conclude, that water is com- 

 posed of molecules formed by the union of a single particle of 

 dephlogisticated air to an ultimate particle of light inflammable air, 

 and that they are incapable of uniting to a third particle of either of 

 their constituent principles." 



It is a remarkable circumstance, that although Mr. Higgius's view of 

 the atomic constitution of the five compounds of oxygen and azote is 

 that which is even now very commonly admitted, he does not state 

 their composition ; and his idea of the comparative atomic constitution 

 of sulphurous and sulphuric acids is decidedly erroneous. " Indeed," 

 as remarked by Sir H. Davy in the discourses above quoted, " neither 

 of the Higginses attempted to express the quantities in which bodies 

 combine by numbers." 



In 1792, Richter, a Prussian chemist, published a work called ' Ele- 

 ments of Stochiometrie ; or the Mathematics of the Chemical Elements.' 

 This author treated the subject almost in the same way as Wenzel had 

 previously done, but extended it very considerably ; he endeavoured 

 to determine the capacity of saturation of each acid and base, and to 

 indicate by numbers the weights which mutually saturate each other. 

 He published a table of these, but though the attempt was new and 

 exceedingly ingenious, the results were far from accurate. 



The discoveries of Proust, a French chemist who was professor of 

 chemistry at Madrid, are well worthy of notice, he being the first 

 person who attempted an accurate analysis of metallic oxides. He 

 found that metals unite only with determinate proportions of oxygen, 

 and that the same law existed with sulphur and the metals, and that 

 these might be stated in numbers : his opinions were strenuously 

 opposed by Berthollet, but their accuracy is now generally admitted. 



In the year 1803, Mr. (afterwards Dr.) Dalton, of Manchester, com- 

 municated to the Literary and Philosophical Society of Manchester an 

 essay containing an outline of his speculations on the subject of the 

 composition of bodies (' Manchester Memoirs,' second series, vol. i. p. 

 286). The following year he explained his notions on the subject to Dr. 

 Thomson, and in 1808 he published the first volume of his ' New 

 System of Chemical Philosophy,' in which he gave an outline of his 

 views of the constitution of matter, and this without any acquaintance 

 with what had been previously done on the subject by Higgins. 



Dr. Dalton was unquestionably the first who laid down, clearly and 

 numerically, the doctrine of multiples, and endeavoured to express by 

 simple numbers the weights of bodies believed to be elementary. He 

 announced it as a general rule, that " when only one combination of 

 two bodies can be obtained, it must be presumed to be a binary one, 

 \mless some cause appear to the contrary." Consistently with this 

 law, and correctly at the time it was written, Dr. Dalton regarded 

 water as a binary compound of hydrogen and oxygen, and the relative 

 weights, since corrected, are considered as one to eight. As, then, 

 water consists of an atom of hydrogen and an atom of oxygen, either 

 of these elements may be selected as xinity, and, in fact, as we shall 

 hereafter notice, both have been occasionally employed as such. Dalton 

 fixed on hydrogen, because it is that body which unites with others in 

 the smallest proportion : thus, then, we have water composed of one of 



