98 Mr. M. M. Pattison Muir on Chemical Claasijication. 



not discuss here, it does not follow that we are justified in 

 regarding nitrogen as a pentad, and in quoting the probabi- 

 lity of NH4 01, and not NH3 HCl, being the true formula of 

 sal ammoniac, as strong evidence in favour of the assertion — 

 because if we allow arguments as to the valency of nitrogen 

 to be drawn from the supposed composition of sal ammoniac, 

 there seems nothing to hinder our extending the argument 

 so as to include such bodies as those to which the empirical 

 formulae NH, ICI4, N2 H^ PtCl^, N2 H^ PtClg, N2H8HgCl4, 

 &c. are assigned ; and if we admit these compounds, and 

 others like them, we may make nitrogen of almost any va- 

 lency we choose. We may also extend the same kind of 

 reasoning to other elements : — Because KCIO3 and KNO3 are 

 isomorphous and show other points of analogy, chlorine is a 

 triad or it may be a pentad ; but KCIO4 and KMn04 are also 

 isomorphous ; chlorine appears to be a dyad or a tetrad ; and 

 so on. These and other similar contradictory results may be 

 obtained by admitting solid bodies (that is, bodies which are 

 either decomposed before passing into the gaseous state, or 

 which have never yet been volatilized) as evidence in argu- 

 ments concerning valency. 



That such discrepancies would be found might almost be 

 deduced a prio7'i from what we know of the nature of solids, 

 liquids, and gases. Whether we accept or reject the mole- 

 cular theory of matter, we cannot but admit that there is 

 generally a greater simplicity in the structure of gases, as 

 indicated by their properties, than in that of liquids and 

 solids. Two simple generalizations express, with very con- 

 siderable exactitude, the action of heat and of pressure re- 

 spectively upon the volumes of gases ; how complicated is the 

 expression which is required to indicate the relation between 

 the volume of a single solid or liquid body, and increase or 

 decrease of temperature or pressure ! How shall we hope 

 ever to frame a general expression which shall include these 

 relations for all solids or for all liquids ? 



It must be admitted that the molecule NH4 CI does not 

 exist in the gaseous state under any conditions hitherto realized. 

 Admitting that a substance which is a true atomic compound 

 of N, H, and CI in the proportion expressed by the formula 

 NH4 CI does exist in the solid state, we should a priori expect 

 that its molecular structure would be much more complicated 

 than this, NH4 CI ; but if it is more complicated than this, all 

 arguments as to the valency of nitrogen which are based on 

 the assumption that this is the true molecular formula are 

 valueless. 



But if we cannot admit, in default of further proof, the ex- 



