of the Metallic Oxides. 83 



the real relations of bodies. They create imaginary relation- 

 ships, removing apart those bodies which ought to be brought 

 together, and bringing those together which ought to be kept 

 apart. Our formulae should be like glasses of a perfect trans- 

 parency, through which the bodies should be seen as they are, 

 and not like screens to hide, or disguises to disfigure. 



Most formulae now in use refer to the perfect gaseous state. 

 These formulae are undoubtedly excellent for bodies of which 

 the normal state is gaseous ; but their exclusive use is insuf- 

 ficient, and creates a void to be regretted in the comparative 

 study of bodies, both from the individual and general point of 

 view. The number of bodies naturally gaseous is in fact very 

 limited ; and if the number of those capable of becoming so 

 is very considerable, still there are many which are not, and 

 with an absolute or relative fixity resist the highest tempera- 

 tures, or are decomposed. 



Our usual molecular formulae are therefore necessarily 

 insufficient, being inapplicable to numerous compounds, and 

 to the various states which the same body is capable of 

 assuming. These formulae, referring to the perfect gaseous 

 state, do not tell us anything of the molecular nature of these 

 same bodies when in the ordinary condition. Thus it is, to 

 appeal to classical examples, with the formulae of sulphur, and 

 of the fatty acids, or at least with the first terms of the latter 

 series. 



It is in all respects desirable that the molecular formulae of 

 bodies should refer to their natural state, and that the several 

 formulae should be determined for the various physical states 

 which the bodies can either naturally or artificially assume. 

 That is the only way to avoid misconceptions, artificial 

 difficulties, false analogies between unlike bodies, and to 

 retain for various compounds their own physiognomy. 



There is no teacher of Chemistry who has not frequently 

 noticed, both in mineral and in organic chemistry, differences 

 in the properties of bodies, which are often most profound, 

 and totally beyond theoretical prevision, notwithstanding that 

 the bodies are apparently related as regards their formulae 

 and the analogy of their composition. It is easy to say that 

 there are exceptions and anomalies. But in spite of these 

 words, the difficulties still exist in all their entirety, embar- 

 rassing the mind of both teacher and taught. The greatest 

 caution should be used in qualifying exceptional and anoma- 

 lous facts; otherwise confusion is created in the mind, and 

 place is given to chance in the occurrence of natural facts : 

 it is the ruin of law and general principles. It is well to 

 remember the old adage, applicable in the physical as well as 



G2 



