CHAP. VI., 3.] 



HEAT (ATOMIC CHEMISTRY). DALTON. 



139 



which had chemically combined with the other ; and 

 if it happen that fresh compounds are formed pos- 

 sessing new qualities, then the varying ingredient 

 reaches 3, 4, or 5 times the amount which it had in 

 the first combination ; or more generally the propor- 

 tions by weight of the combining elements may be 

 exactly represented by whole numbers. 3. " If two 

 bodies combine in certain proportions with a third, 

 they combine in the very same proportions with 

 each other." This is called the law of reciprocal pro- 

 portion. Thus, suppose that one ounce of a body A, 

 saturates or forms a chemical combination with two 

 ounces of B, 5 of C, and 11 of D ; then if B, C, and D 

 are capable of combining, it will be in the exact pro- 



portions of the numbers 2, 5, and 11. Hence a single 



number being determined for each body, determines 

 all its possible combinations with other known bo- 

 dies. 4. It may perhaps be added as an indepen- 

 dent fact, that when complex bodies combine with 

 other bodies, no matter whether simple or compound, 

 the combining number of the complex body is the 

 sum of the numbers representing the constituents. 

 Thus the combining number of lime is 28 ; but lime 

 consists of calcium 20 and oxygen 8. So also the 

 combining number of water is 9 ; but water consists 

 of hydrogen 1 and oxygen 8 : and the combining 

 number of hydrate of lime is 37, the sum of the 28 

 parts of lime and 9 of water of which it is composed. 

 (622.) The discovery of these laws has been termed by 

 artial an- gj r John Herschel the most important, after the laws 

 I0n f of mechanics, which the study of nature has yet dis- 

 use laws, closed. No slight or transient reputation is due 

 to him who first clearly apprehended and taught them. 

 Nor must we be surprised to find several claimants to 

 a share of the honour. It is the invariable history of 

 all great generalizations, that they have been partly 

 anticipated; and it may serve to moderate the self- 

 esteem of even the greatest discoverers, that however 

 high may be their individual merits, they are in some 

 sense the mere exponents of the aggregate know- 

 ledge of their contemporaries. The laws of motion 

 were partially anticipated before the time of Galileo, 

 and could not have remained much longer undefined ; 

 and even the unparalleled discoveries of Newton 

 must, in all probability, have ere long been made 

 piecemeal by the united energy of his contemporaries 

 and immediate successors. The steam-engine was 

 not the sole creation of Watt, nor was Davy the first 

 to apply the voltaic battery to chemistry. In like 

 manner, Dalton's laws of chemical combination were 

 published at a happy moment, which gave them 

 speedy acceptance with the active chemists of his 

 day; whilst those who had seen with sufficient clear- 



ness portions of these laws twenty or thirty years 

 before, addressed a scientific public by no means pre- 

 pared to appreciate their value, or to feel a conviction 

 of their generality. Had Wenzel, 1 Higgins, 2 and Wenzel, 

 Richter 3 individually apprehended the great impor- Hi gg in8 

 tance of the definite and multiple combining pro- ter 

 portions which they announced, had they felt the 

 theory of them to constitute the very foundations of 

 chemistry, they would not have rested until they 

 had verified it in numerous details, and applied it to 

 the various purposes of speculation and practice, as 

 Dalton did. But whether from want of energy, or 

 from ill fortune, their ideas sunk into entire oblivion ; 

 and the ingenuity and social position of Berthollet 

 were giving a currency to opinions respecting chemi- 

 cal forces which tended to undo even the far more 

 elementary notions of the constancy of elective affi- 

 nities, at the time when Dalton's researches were 

 unostentatiously brought before the world. His first 

 insight into the theory of chemical combinations 

 dates from the year 1803. It was expounded by 

 him both in conversation and by lectures in 1804, 

 at which date Dr Thomas Thomson recorded the 

 results of a conversation held with him at Manches- 

 ter, which, three years later (evidently with Dalton's 

 approbation), he published in his work on Chemistry. 

 Finally, Dalton himself, in 1808, announced the prin- 

 ciples of his theory at no greater length than five 

 pages " on Chemical Synthesis" in his Chemical 

 Philosophy. 



Now, it is to be observed, that Dalton's views were (623.) 



all along expressed in the language of a strictly Ato- Importance 



^ r . 11 of the Ato- 



mic theory. Compounds are only chemically com- m5c T neory 



plete, when one or several atoms of an element com- to the pro- 

 bine with one, two, or more atoms of another. Any gress of 

 superfluity of either element remains uncombined, or c 

 mechanically mixed. All the other parts of the laws 

 of combination readily lead to the same idea, and, in 

 fact, find in it their simplest expression. There is 

 no wonder, then, that Dalton firmly believed in the 

 physical existence of his atoms, and that the new 

 properties of compounds are due to the peculiar mo- 

 dification of the most elementary parts into which 

 bodies can be divided without a loss of those proper- 

 ties, that is, without decomposition. He figured 

 these elementary molecules by uniting the symbols 

 of their constituents, and by so doing, may be said 

 to have laid the foundation of those algebraic sys- 

 tems of technical notation, which speak to the eye 

 only in another way from Dalton's diagrams, and 

 which have been of such eminent service to the 

 chemist. Nor must we think too lightly of a hypo- 

 thesis which served so materially to aid in realizing 



1 Lehre von der Vertuandschaft der Kdrper, 1777. He shows that in a double decomposition the new compounds are chemi- 

 cally perfect. 



2 A Comparative View of the Phlogistic and Antiphlogistic Theories, 1789, by Mr William Higgins, points out the multiple com- 

 bining proportions of sulphur and oxygen, and of nitrogen and oxygen, but only incidentally. Dr Bryan Higgins, a relative of 

 Mr W. H., had published, in 1786, a work said to contain the idea of definite atomic combinations. 



8 Anfangsgrtinde der Stochyometrie, 1792. He gave a series of numbers representing the combining proportions of different 

 elements. 



