TRANSACTIONS OF SECTION B. 665 



ibecotue inditipensable, and in inorganic chemistry I believe such a sj'stem is very 



\ useful. 



Recently this system has been found insufficient for the requirements 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. 



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 

 founded, 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 N.^O^ at low and NO^ at 

 liigh temperatures, it seemed not impossible that at the ordinary atmospheric 

 temperature nitric oxide was a simplified or dissociated molecule, and that if the 

 temperature were sufficiently reduced it would be found that its molecule would be 

 NjOo, 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 VCl^. Again, 

 the molecule of peroxide of chlorine is CIO.,, 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-detined crystalline salts containing what is called water of crystal- 

 lisation. 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 + ONaH and 

 OH2 + 0(NO,)j = OHCNO^) + 0(NO,)H. 



In the combination of water with an anhydrous salt, a phenomenon often accom- 

 panied by great rise of temperature, there doe^ not appear to be a double decompo- 

 sition. That tliere 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 

 different from 'atomic combination,' in which the atoms are directly united accord- 

 ing to their valencies. Another explanation has been sugge-sted 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 words to express — not to explain 



