276

277

New System of Chemical Philosophy, gives Dalton's pictorial

presentment of his conception of the atoms of these three

gases.

Dalton's application of the term atom to compounds, such as water, carbonic acid, &c., shews that he did not use the word atom in its strict etymological meaning as 'that which cannot be cut' but rather as signifying the smallest portion of a body which could exhibit the properties of the body. The atom of water, for instance, could be separated into atoms of hydrogen and oxygen, but this act of separation into parts produced kinds of matter wholly different from the water. The properties of the parts of the atom of water were quite unlike those of the atom itself; whereas the properties of a quantity of water were garded by Dalton as the same as those of the atom of water.

re

The reason of the regularities in the compositions of the two oxides of carbon, or the two compounds of carbon and hydrogen, examined by Dalton, was to be found, according to him, in the nature of the atoms of carbon, hydrogen, and oxygen. It is only necessary to assume that the atoms of these elements do not separate into parts in chemical reactions, then the facts find a simple explanation. One atom of hydrogen combines with one atom of carbon to form one atom of a certain hydride of carbon; if another compound of these elements is formed containing more hydrogen, relatively to the same

mass of carbon, the next smallest quantity of hydrogen which can combine with the atom of carbon is two atoms. Similarly with the oxides of carbon. One atom of carbon combines with one atom of oxygen to form an atom of an oxide of carbon: this is the simplest possible compound of the two elements. The next compound which can be formed by combining more oxygen with the same mass of carbon, must be that the atom of which is composed of one atom of carbon united with two atoms of oxygen.

Chemical action was thus conceived by Dalton to be an 278 action between atoms. A mass of any element, or compound, was regarded as constituted of a vast number of very small particles of matter, all alike, all having the same weight, but all unlike the atoms of any other element, or compound. The atoms of elements were supposed to be capable of combining together to form atoms of compounds. In some cases the atoms of compounds might combine together to form atoms of more complex compounds. An atom of one element might combine with one, two, three, &c. atoms of another element to form one, two, three, or more, distinct compounds; but the atoms of elements could not separate into parts. The atoms of compounds on the other hand separated into parts when compounds interacted to produce new kinds of matter, and these parts, which were elementary atoms, rearranged themselves to form atoms of the new compounds produced in the interactions. Dalton pictured to himself an atom of a compound as a structure, or building, formed of a definite number of elementary atoms arranged in a more or less definite manner. He used symbols to represent elementary atoms, and he grouped these symbols together to represent compound atoms. Thus the symbol O represented an atom of oxygen; atom of hydrogen; D, an atom of nitrogen; , an atom of sulphur;, an atom of aluminium; D, an atom of potassium; and so on. In Fig. 21 are given a few of Dalton's symbols for the atoms of compounds. A represents an atom of potash alum; B an atom of aluminium nitrate; C an atom of barium chloride; and D an atom of barium nitrate.

an

Thus did Dalton's conception of the atom throw light on the laws of fixity of composition, multiple proportions, and reciprocal proportions.

"It is one great object of this work" says Dalton in his 279 New System of Chemical Philosophy, "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

A O

Fig. 21.

compound particle, and the number of less compound particles which enter into the formation of one more compound particle." How then did he determine the relative weights of the ultimate particles of simple bodies?

Let us take the case of oxygen. The atom of oxygen, said Dalton, is 8 times heavier than the atom of hydrogen: let us call the weight of the atom of hydrogen, or the atomic weight of hydrogen, one; then the atomic weight of oxygen is asserted to be 8. Masses of hydrogen and oxygen combine in the ratio 1 : 8, to form water; but an atom of water is formed by the union of atoms of hydrogen and oxygen; if it is assumed that an atom of water is formed by the union of one atom of hydrogen with one atom of oxygen, then the atomic weight of oxygen must be 8. In saying that the atomic weight of oxygen was 8, Dalton implicitly made this assumption. But it might be assumed that an atom of water is composed of one atom of oxygen united with two atoms of hydrogen; as an atom of hydrogen is the unit in terms of which the weights of the atoms of other elements are stated, it follows from this assumption that the atomic weight of oxygen is 16; because 2 : 161: 8. Or it might be assumed that an

atom of water is composed of three atoms of hydrogen united with one atom of oxygen; in this case the atomic weight of oxygen must be 24. Or it might be assumed that an atom of water is composed of two atoms of oxygen united with one atom of hydrogen; in this case the atomic weight of oxygen is 4.

Before the atomic weight of oxygen could be determined 280 from the data of the composition of water, it was necessary to determine the number of atoms of oxygen and hydrogen which united to form an atom of water. Dalton's conception of the atom supplied no method whereby this could be done. To get over this difficulty if possible, Dalton framed several empirical rules regarding the compositions of the atoms of binary compounds.

He classified compound atoms formed by the union of two elements into binary, ternary, quaternary, &c., atoms. Calling the two elements A and B, he said that a binary atom is formed by the union of one atom of A with one atom of B; a ternary atom, by the union of, either one atom of A with two of B, or two atoms of A with one of B; a quaternary atom, by the union of, either one atom of A with three of B, or three of A with one of B. He then laid down the following rules:

"I. When only one combination of two bodies [elements] can be obtained, it must be presumed to be a binary one, unless some cause appears to the contrary.

II. When two combinations are observed, they must be presumed to be a binary and a ternary.

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

IV. When four combinations are observed, we should expect one binary, two ternary, and one quaternary, &c. &c."

By applying these rules to water, which was the only compound of hydrogen and oxygen known at the time, Dalton concluded that the atom of water was a binary atom; but his own analyses had convinced him that hydrogen and oxygen combine nearly in the ratio 17 to produce water [we now know that the ratio is 1 : 8]; therefore he concluded that the atomic weight of oxygen was approximately 7.

Dalton's rules for determining the compositions of compound atoms were not based on any general principle deduced from his fundamental conception of the atom; this conception could not indeed supply such a principle. If only one com

pound of two specified elements was known, the simplest assumption to make was certainly that embodied in Dalton's first rule. But the simplest assumption is not always the best.

281 The law of combination by volumes of gaseous elements enunciated by Gay-Lussac in 1809 (s. Chap. VI. par. 87) may be stated as follows. The gaseous elements combine in the ratios of their combining volumes, or in ratios which bear a simple relation to these.

282

By combining volume is here meant the smallest volume of a gaseous element which combines with unit volume of hydrogen; and unit volume of hydrogen is defined to be the volume occupied by unit mass of hydrogen. All measurements of volumes are assumed to be made at the same temperature and pressure.

Gay-Lussac interpreted his law, in the light of the Daltonian theory, to mean that the ratios of the masses of the combining volumes of gaseous elements are also the ratios of the masses of the atoms of these elements. Thus; 2 volumes of hydrogen combine with 1 volume of oxygen to form water; but a volume of oxygen weighs 16 times as much as an equal volume of hydrogen; therefore an atom of oxygen is 16 times heavier than an atom of hydrogen; but the atomic weight of hydrogen is 1, therefore the atomic weight of oxygen is 16.

Again, 1 volume of chlorine combines with 1 volume of hydrogen to form hydrogen chloride; but chlorine is 35.5 times heavier than hydrogen, bulk for bulk; therefore, the atomic weight of chlorine is 35.5.

If this interpretation of Gay-Lussac's law is admitted, the law supplies a means for determining the atomic weights of gaseous elements. But Gay-Lussac ventured on the further generalisation that equal volumes of gases (measured at the same temperature and pressure) contain equal numbers of

atoms.

Dalton shewed that this generalisation was inadmissible. Thus, consider the combination of hydrogen and chlorine. One volume of hydrogen combines with one volume of chlorine to form 2 vols. of hydrogen chloride; therefore, by GayLussac's generalisation, x atoms of hydrogen combine with x atoms of chlorine, to form 2x atoms of hydrogen chloride. To make the statement more definite, let us assume that x = = 1; then, a single atom of hydrogen by combining with a single atom of chlorine has produced 2 atoms of hydrogen chloride.