328 



NA TURE 



[August 4, 1892 



their number and weights. A system either of arrangements 

 without iveights, or of weights without arrangements, he con- 

 sidered only half of what it should be," 



We can all sympathize with the members of the section of 

 1834 in their desire to obtain an uniform system of chemical 

 notation, for at that time several very different systems seem to 

 have been in use. Although the report is a short one, it proba- 

 bly directed the attention of chemists to the desirability of avoid- 

 ing confusion by the use of various systems, and since that period 

 many advances have been made. 



There is now little necessity for every chemist to "state 

 explicitly the exact quantities which he intends to represent by 

 his symbols" for the accurate determinations of atomic weights 

 by many chemists — and we must not omit to mention the work 

 of Stas (whose death we have had to deplore since the last meet- 

 ing of the British Association) — have given us a series of 

 numbers which are in the hands of all chemists, so that, except 

 in the cases where great refinement is requisite (or when the 

 atomic weight has not been universally accepted) there is no 

 need to state the values of the symbols. 



That great advances have been made in chemical notation is 

 well known to all ; even in my own short experience I have had 

 to learn several different methods. When I began to work at 

 chemistry I was told that sulphate of lead was to be expressed 

 by the formula PbO,S03. Hofmann taught me that it should 

 be PbS04 ; then Gerbardt doubled the atomic weights of oxygen 

 and sulphur and the formula became Pb2S04 ; Cannizzaro showed 

 that the atomic weight of lead should also be doubled, and the 

 formula again became PbS04, but representing twice as much as 

 formerly ; then Frankland taught me to write SOgPbo" as the 

 expression of the graphic formula — 



O, 



\/ 



Pb, 



which not only states that the compound contains 207 of lead, 

 32 of sulphur, and 64 of oxygen, but that the sulphur is hexad, 

 and is combined with two atoms of dyad oxygen, and with a 

 dyad compound radical containing one atom of lead and two of 

 oxygen ; and of all the formulae just given this is the only one 

 which satisfies the requirements which Dalton thought necessary 

 in 1835, namely, to indicate not only the weights of the elements 

 present, but also their arrangement. It may be objected that 

 we do not know that this formula really represents the arrange- 

 ments of the atoms in plumbic sulphate, but there can be very 

 little doubt that the four atoms of oxygen in the compound are 

 not all in the same condition, for if we examine the properties 

 of sulphuric acid (from which the sulphate of lead is derived by 

 the replacement of the hydrogen by lead), we find that two of 

 the atoms of oxygen are more closely associated with the hydro- 

 gen than are the other two, and, as there is some evidence, 

 although perhaps not very conclusive, that sulphur may be 

 capable of combining with six monad atoms, although no such 

 compound is yet known, it does not seem unreasonable to sup- 

 pose that sulphuric acid is really : — 



O^ /O— H 

 0^ \o— H 



What "the nature of the attraction that holds the atoms to- 

 gether may be is not known, but it is more probably of a charac- 

 ter similar to that of gravity which holds together sun and 

 planets, than of the nature of cohesion which would hold the 

 atoms rigidly together ; the atoms in each molecule are there- 

 fore most probably in a state of rotation around, or of vibration 

 to and from, the central atom which holds them together. The 

 pictorial representation in a plane does not therefore truly express 

 the position of the atoms, but merely the relations existing be- 

 tween them. In organic chemistry the use of formulae express- 

 ing such a relation has become indispensable, and in inorganic 

 chemistry I believe such a system is very useful. 



Recently this system has been found insufficient for the re- 

 quirements of organic chemistry, and recourse has been had 

 to the figure of a tetrahedron to represent the atom of carbon, 

 other atoms being attached to the solid angles ; in this way 

 the position of the atoms in space is more or less expressed. 



NO- II 88, VOL. 46] 



There are many cases, however, in which the atomicity theory 

 fails us. At first it seemed probable that the atomicity of an 

 element varied in pairs of attractions, that is, an element might 

 be monad, triad, or pentad, but not dyad or tetrad ; or it might 

 be dyad, tetrad, or hexad, but not triad or pentad ; but some 

 great difficulties have been encountered. Thus nitrogen, which 

 is pentad in ammonic chloride and triad in ammonia, forms the 

 compound nitric oxide, NO, in which it would appear to be 

 dyad ; it has been suggested, however, that in this body the 

 nitrogen is really triad, and that it possesses a "free bond." 

 Now the idea of a "free bond " seems contrary to the principles 

 of atomicity, since it is on the belief that such a free bond is 

 impossible that the explanation of the existence of elementary 

 molecules is formed, for it is said that when hydrogen is 

 liberated two atoms unite to form a molecule, so that their 

 mutual attractions may be satisfied. Nevertheless nitric oxide 

 is a very active body, uniting readily with other substances, so 

 the free bond seems to be on the look out for other kinds of 

 matter, but to have no attraction for the free bond of another 

 molecule of nitric oxide. As the molecule of nitric peroxide is 

 variable by alterations of temperature, being N2O4 at low and NO^ 

 at high temperatures, it seemed not impossible that at the ordinary 

 atmospheric temperature nitric oxide was a simplified or disso- 

 ciated molecule, and that if the temperature were sufficiently 

 reduced it would be found that its molecule would be N2O2, 

 and thus it would contain triad nitrogen without a free bond. 

 The density of the gas has, however, been determined at a 

 temperature as low as —73° and the molecule is still NO. 

 Another important exception to the variation of the atomicity 

 of an element in pairs was furnished by the investigations of 

 Sir Henry Roscoe on the chlorides of vanadium ; this element 

 which, from analogy, should be a triad or a pentad, appears to 

 form a chloride of the composition VCI4. Again, the mole- 

 cule of peroxide of chlorine is ClOj, which would make chlorine 

 a tetrad or the compound must have a free bond. 



Another set of phenomena which the atomicity theory will 

 not explain is the existence of well-defined crystalline salts 

 containing what is called water of crystallization. This water 

 is in many cases held with considerable pertinacity, the body 

 appearing to be a veritable chemical compound. But water 

 appears to be a saturated body, the attractions of the oxygen 

 being satisfied by those of the hydrogen. It is true that water 

 acts vigorously on other compounds, as on metallic oxides to 

 form hydrates, and on some anhydrides to form acids, but these 

 appear to be phenomena of double decomposition ; thus the 

 combination of water with sodic oxide and nitric anhydride 

 respectively may be expressed by the equations 



OH2 + ONa„ = OHNa -f- ONaH and 

 OH2 + 0(N62)2 = 0H(N02) 4- 0(N02)H, 

 In the combination of water with an anhydrous salt, a pheno- 

 menon often accompanied by great rise of temperature, there 

 does not appear to be a double decomposition. That there is a 

 chemical combination of some sort is shown by the changes of 

 properties produced, crystalline form and colour being both 

 sometimes altered. Compounds so produced have been called 

 •' molecular compounds" to imply that saturated molecules are 

 in some way or another combined, the combination being dif- 

 ferent from "atomic combination," in which the atoms are 

 directly united according to their valencies. Another explana- 

 tion has been suggested by assuming that there is some "residual 

 affinity " not saturated by the constituents of the body, and that 

 this residual affinity enables bodies to unite in a less stable 

 manner than in most compounds. But are not these terms — 

 " molecular combination " and "residual affinity" — analogous 

 to the term "catalysis," merely a/^ro'j- to express— not to explain 

 — what we do not understand? If "residual affinity" really 

 exists, it must reside in the oxygen of the water, or in the 

 hydrogen, or both ; if so, what will happen to some of the com- 

 plex constitutional formulae of the organic chemist in which the 

 carbon is tetrad, the oxygen dyad, and the hydrogen monad? 

 If any of these elements have a residual affinity should we not 

 expect to find additional unions between some of the atoms of 

 the same molecule over and above those represented by the 

 formula ? 



Oxygen may be tetrad, for which there is evidence in 0Ag4, 

 Under these circumstances water is by no means a saturated 

 compound, and there would be no difficulty in explaining the 

 combination of water with oxygen salts. Thus crystallized 



