1815.] in the Physical Sciences. 9 



become almost universal. That substances always enter into che- 

 mical combination^ in determinate proportions which never vary, 

 has been known ever since chemists acquired the art of analysing 

 bodies. Thus carbonate of lime, wherever, or in whatever state, it 

 occurs, is always a compound of 43-2 carbonic acid and 57*8 lime; 

 and sulphate of barytes, of 34*5 sulphuric acid and 65*5 barytes. 

 In like manner, the yellow oxide of lead is always a compound of 

 100 lead and J-J oxygen; and red oxide of mercury, of 100 mer- 

 cury and 8 oxygen. Sulphuric acid is always composed of three 

 parts of oxygen and two parts of sulphur ; and carbonic acid, of 

 i'OOO oxygen and 751 carbon. This law is universally admitted by- 

 chemists ; and, indeed, the more rigorously it has been examined, 

 the more conspicuous and decided have become the proofs in its 

 favour. Even BerthoUet, who seems to be an enemy to the atomic 

 theory in the abstract, has admitted that all known compounds 

 unite in determinate ]jropurtions ; and has endeavoured to reconcile 

 this fact to his own opinions by several highly ingenious, and some 

 rather whimsical, arguments. The few exceptions which he was 

 able to muster up against the law have all disappeared before the 

 more rigid and exact examination of modern analysts, 



Mr. Dalton was the first person who ventured to account for this 

 fixedness in chemical proportions. According to him, it is the 

 atoms of bodies tliat unite together. One atom of a body, a, unites 

 with one atom of a body, ^, or with two atoms of it, or with three, 

 four, &c. atoms of it. 'I'he union of one atom of a with one atom 

 of I' produces one compound, the union of one atom of a with two 

 atoms of /• produces another compound, and so on. Each of these 

 compounds, of course, must consist of the same proportions, be- 

 cause the weight of every atom of the same body must of necessity 

 be the same. 



We have no means of demonstrating the number of atoms which 

 unite together in tliis manner in every comj)Ound; we must, there- 

 fore, have recourse to conjecture. If two bodies unite only in one 

 proportion, it is reasonable to conclude that they unite atom to 

 atom Her.ce it is most likely that water is composed of one atom 

 of oxygen and one atom of hydrogen ; oxide of silver, of one atom 

 silver and one atom oxygen ; and oxide of zinc, of one atom zinc 

 and one atom oxygen. 



When a body has the property of uniting with various doses of 

 oxygen, we can tlien determine the number of atoms which con- 

 stitute the compounds. Thus manganese unites with four doses of 

 oxygen; and supposing the manganese to be repicsented by 100, 

 the oxygen of each respective oxide is represented by the numbers 

 H, I'S, 4J, 5C> ; but these numliers are to each other as the 

 numbers one, two, three, four. Hence the first oxide is composed 

 of one atom manganese and one atom oxygen; the second, of one 

 atom manganese and two atoms oxygen ; the third, of one atom 

 manganese and three atoms oxygen ; and the fourth, of one atom 

 mangauesc and four atoms oxygen. In like manner, as mercury 



