'particles or atoms of the bodies might have been inferred, 'from which their number and weight in various other 'compounds would appear, in order to assist and to guide 'future investigations, and to correct their results. Now it is one great object of this work, to shew the importance and advantage of ascertaining the relative weights of the ultimate 'particles both of simple and compound bodies, the number of 'simple elementary particles which constitute one compound 'particle, and the number of less compound particles which ' enter into the formation of one more compound particle.

'If there are two bodies, A and B, which are disposed to 'combine, the following is the order in which combination 'may take place, beginning with the most simple: namely—

'I atom of A + 1 atom of B = 1 atom of C, binary,

Dalton then states the following rules respecting chemical synthesis, which he employed in determining the relative weights of the smallest chemically indivisible parts of compound bodies'.

'Ist. When only one combination of two bodies can be 'obtained, it must be presumed to be a binary one, unless 'some cause appears to the contrary.

'2nd. When two combinations are observed they must be 'presumed to be a binary and a ternary.

'3rd. When three combinations are obtained, we may expect one to be a binary, and the other two ternary.

'4th. When four combinations are observed, we should expect one binary, two ternary, and one quaternary.' &c. &c. 'From the application of these rules,' Dalton says, 'to the 'chemical facts already well ascertained, we deduce the follow

1

1 By the 'smallest chemically indivisible part' of a substance is meant an amount such that, if divided, substances (or a substance) are produced different in properties from the original substance.

ing conclusions: Ist. That water is a binary compound of 'hydrogen and oxygen, and the relative weights of the two ' elementary atoms are as I : 7 nearly. 2nd. That ammonia 'is a binary compound of hydrogen and azote, and the relative 'weights of the two atoms are as 15 nearly....In all these 'cases the weights are expressed in atoms of hydrogen each 'of which is denoted by unity.'

Two oxides of carbon were known to Dalton, containing according to him, 5'4 parts by weight of carbon combined respectively with 7 and with 14 parts by weight of oxygen: the first of these bodies, in accordance with Dalton's second rule, was considered to be a binary, and the second a ternary compound; the formulæ given were CO and CO2 respectively. [C54, O=7.]

But Dalton's CO, might have been regarded as a compound of 27 parts by weight of carbon with 7 parts by weight of oxygen, in which case its formula would have been written CO [C=27]; Dalton's CO would then have become C,O [C1 = 5'4]. The atomic weight of carbon would be determined as 27 or 54 according as carbon monoxide or carbon dioxide was decided to be a binary compound.

At a later time it was said by some chemists that a binary compound is always more stable than a ternary; if this rule were applied to the case of the oxides of carbon, Dalton's number for the atomic weight of carbon would be confirmed'.

3. These examples illustrate the great shortcoming of the Daltonian theory: the atomic weights of Dalton are either multiples or submultiples of a certain number, but we cannot tell what multiple or what submultiple. Let the relative weights of two elements, hydrogen being taken as unity, which form a compound B, be Q and Q,, and let the atomic weights of these elements be A and A, respectively, then QQ, nA : n ̧A,, where n and n, are whole numbers. But inasmuch as the values of n, n1, A, and A, are unknown it is evident that analysis alone, aided by the Daltonian. theory, cannot determine the atomic weights of the elements which compose the substance B.

1 See especially Daubeny's Atomic Theory (2nd edition 1850), pp. 119–120.

This shortcoming in the theory could not be supplied without further data: Dalton distinctly states that in order to determine the number of elementary atoms in the atom of a compound a knowledge of the composition of many compounds of the given elements is required.

4. A few months after the announcement of Dalton's law of multiple proportions and atomic theory, GAY LUSSAC and HUMBOLDT' began their volumetric investigations which culminated three years later in the beautiful discovery of the former naturalist, that gaseous substances unite in fixed volumetric proportions which may be simply expressed.

There is a constant simple relation, said Gay Lussac, between the volume of a gaseous compound and the volumes of its constituent elements. Let one volume be defined as the volume occupied by one part by weight of hydrogen, then the combining volume of any gaseous element is always expressed by a whole number, e.g. one volume of nitrogen combines with one volume of oxygen to form two volumes of nitric oxide; two volumes of hydrogen and one volume of oxygen combine to form two volumes of water-gas; one volume of nitrogen and three volumes of hydrogen form two volumes of ammonia, &c. &c. Condensation sometimes occurs, sometimes the volume of the compound is equal to the sum of the volumes of the combining elements.

This discovery appeared to add fresh arguments to the theory of Dalton. The ratios of the weights of these combining volumes of the elements, hydrogen being taken as unity, represent, it was said, the relative weights of the atoms of these elements; and the conclusion was drawn, 'equal volumes of gaseous substances, measured at the same temperature and pressure, contain equal numbers of atoms.'

5. Dalton however refused to accept Gay Lussac's generalisation, and regarded his experimental methods as untrustworthy. We cannot, I think, fail to be struck with the justness of Dalton's objection to the statement ‘equal volumes contain equal numbers of atoms:' he argued somewhat as follows:-One volume of nitrogen and one volume. 2 Mém. de la Soc. d'Arcueil, 2. 207. Journal de Physique, 60. 129.

1

of oxygen form two volumes of nitric oxide; but one atom of nitrogen and one atom of oxygen form one atom of nitric oxide; therefore, had the above statement been correct, the volume of nitric oxide would have been equal to, not twice as great as the volume of oxygen or of nitrogen. So again, one atom of hydrogen and one atom of oxygen form one atom of water, according to Dalton's rules: but Gay Lussac shewed that two volumes of hydrogen combine with one volume of oxygen to produce two volumes of water-gas; hence the atom of hydrogen occupies twice the volume occupied by the atom of oxygen, and therefore the statement of Gay Lussac is incorrect. If Dalton's definition of atom and his rules regarding atomic synthesis are adopted Gay Lussac's statement that 'equal volumes contain equal numbers of atoms must be abandoned.

6. The difficulty was removed by AVOGADRO', who (in 1811) introduced the idea of two kinds of atoms:-' molécules 'intégrantes,' or as we should now say molecules; and 'molécules 'élémentaires, or as we should now say atoms.

The molecules of elements are decomposed in chemical processes, said Avogadro, and the atoms unite to form new compounds. Equal volumes of gases contain equal numbers 'of molecules. The reaction between nitrogen and oxygen inexplicable by Gay Lussac's law now becomes clear; each molecule of nitrogen and each molecule of oxygen divides into two parts, and these parts unite to form the new molecules of nitric oxide, hence there are twice as many molecules of nitric oxide produced as there were molecules of nitrogen or oxygen originally present.

By thus recognising a higher order of atoms, as it were, Avogadro reconciled Dalton's theory with Gay Lussac's results.

Ampère in 1814 drew prominent attention to the hypothesis of Avogadro, and attempted by its help to explain the structure of crystals. But the hypothesis had come before the times were fully ripe.

1 Journal de Physique, 73. 58: also Essai d'une manière de déterminer les masses relatives des molécules élémentaires des corps, &c.

2 Ann. Chim. Phys. 90. 43.

7. WOLLASTON' accepted Dalton's theory but proposed to use the word equivalent in place of atom. In his paper published in 1814 (loc. cit.) Wollaston drew up a table of equivalents which he thought would be serviceable to the practical chemist in determining the amount of an acid which would combine with a given weight of base, or the weight of precipitate obtainable in a given reaction, &c. He arranged his numbers on a scale with a slider attached, and adopted a mechanical contrivance for aiding the analyst in using the table. Although Wollaston employed the word equivalent in place. of atom, his scale and table must be regarded as helping to extend the use of the atomic theory". For the practical purpose which he had in view Wollaston did not deem it necessary to adopt any theory; at the same time he regarded the atomic weights of Dalton, especially the atomic weights of compounds, as too hypothetical, and he thought that equivalents were to be preferred for most purposes.

Wollaston referred his equivalent numbers to oxygen as 10 the amount by weight of any element which combined with 10 parts by weight of oxygen was regarded by him as the equivalent of that element. But the system of equivalents was liable to the same objection as had been urged against the system of atomic weights;-it was too vague.

(1) 75 parts by weight of carbon unite with 20 parts by weight of oxygen, said Wollaston, therefore the formula of the compound produced is CO,.

(2) Again 75 parts by weight of carbon unite with 10 parts by weight of oxygen, therefore the formula of the compound produced is CO.

But he might also have said

(1) 375 parts by weight of carbon unite with 10 parts by weight of oxygen, and the formula of the product is CO; and

(2) 75 of carbon unite with 10 of oxygen, therefore the formula of the compound is C,O.

1 Phil. Trans. for 1814, 1 et seq.

2 Wollaston appears to have first used this term in 1808 (Phil. Trans.).

3 See Cannizzaro, C. S. Journal [2], 10. 945.