ATOMIC THEORY 2G9 



every ultimate atom of matter may be regarded as tho centre of such a set of physical 

 forces surrounding it as an atmosphere. 



In modern chemistry an atom is defined to be the smallest particle of any clement 

 which can exist in combination. The atom is thus distinguished from the niolecule, 

 the latter term being now applied to the smallest quantity of the substance capable 

 of existing in a free state. The molecule and the atom may coincide, or the molecule 

 may be made up of two or more atoms. Thus, an atom of chlorine, or 35'5 parts by 

 weight, is represented by the symbol Cl, since this denotes the smallest quantity of 

 chlorine capable of existing in any of its compounds hydrochloric acid (HC1) for 

 example ; -while the molecule of chlorine, or smallest quantity set free in any reaction, 

 will be represented by two atoms, or Cl 2 . 



ATOMICITY, or EQUIVALENCE. Terms employed by modern chemical 

 writers to denote the combining capacity of an element, or a radical, as determined by 

 the number of atoms of hydrogen, or other monatomic element, with which it can 

 combine. Thus chlorine, oxygen, boron, carbon, and phosphorus (using the atomic 

 weights, Cl = 35'5,0 = 16, B = ll, C = 12, and P = 31) may be said to be respectively 

 monatomic, diatomic, triatomic, tetratomic, and pentatomic elements ; or, to use equi- 

 valent expressions, they may be described as univalent, bivalent, tervalent, quadri- 

 valent, and quinquivalent : in other words, the elements just cited may be termed 

 respectively a monad, diad, triad, tetrad, and pentad. The atomicity, or equivalence, 

 is often indicated by dashes or by Koman numerals, placed at the upper right-hand 

 side of the symbol ; thus, B'" indicates the triad boron, P' the pentad phosphorus, and 

 so forth. It should be remembered, however, that mineralogists were formerly in the 

 habit of employing dashes in this way to represent so many atoms of sulphur, just 

 as they used, and still use, dots placed above a symbol to denote so many atoms of 

 oxygen. Hence, to a minoralogical reader, such expressions have now become am- 

 biguous ; Sb'", for instance, may either represent tersulphide of antimony, or merely 

 indicate that the metal antimony is a triad. 



Elements of equal atomicity are termed artiads ; and those of unequal atomicity 

 perissads. For a full discussion of the modern doctrine of atomicity, see Watts's 

 ' Dictionary of Chemistry,' and the supplementary volume. 



ATOMIC THEORY. The question as to whether matter be or be not infinitely 

 divisible, has been debated from tho earliest times, and is probably as far from a 

 settlement as ever ; we can, however, scarcely conceive of the existence of matter at 

 all, if there be no limit to its divisibility. It is easy to demonstrate that a mathe- 

 matical line is infinitely divisible, but a mathematical line is only an ideal thing; 

 having only one dimension, it can have no physical existence. We have, therefore, no 

 hesitation in admitting the existence of atoms of matter of particles infinitely small, 

 it is true, as regards pur perceptions, far exceeding in minuteness the finest sub- 

 division to which we can submit a body, but yet incapable of further subdivision. To 

 such insectible molecules the term atom, has been applied. 



If we take any substance chemically complex, we may suppose the existence of 

 atoms in this body, held together by the force of cohesion, which are themselves 

 heterogeneous, being made up, in fact, of atoms of the elementary chemical con- 

 stituents. 



Dr. Dalton suggested the happy idea, which has been most fruitful in its results, of 

 accounting for the constancy of chemical combinations by assuming that they were 

 composed of one or more atoms of the several elements, the weight of which atoms is 

 represented by the combining proportions ; that carbonic oxide, for instance, contains 

 single atoms of carbon and oxygen, whilst carbonic acid is composed of one atom of 

 carbon and two of oxygen. 



It must always be remembered that the combining proportions are purely the 

 results of experiment, and, therefore, incontestable, whatever may be the fate of this 

 theory, which, however, has now stood its ground for many years, and done excellent 

 service to science. 



This theory offers a most satisfactory explanation of the different laws of chemical 

 combination. 



The fact of bodies uniting only in certain proportions, or multiples of those pro- 

 portions, is a necessary consequence of the assumption that the weight of the 

 elementary atoms is represented by the combining proportions ; for, if they united in 

 any other ratio, it would involve the splitting up of these atoms, which are assumed 

 to be indivisible. 



And, of course, the combining proportion of a compound must be the sum of tho 

 combining proportions of the constituents, since it contains within itself one or more 

 atoms of the several constituents. 



The term atom is, therefore, very often used instead of combining proportion, a body 

 being said to contain so many atoms of its elements. 



