484 Prof. Odling on the Natural Groupings 



Chromate K^O . Cr^O^ 

 Bichromate K20.2Cr^03 

 Terchromate K^O . SCr^ 0^ 



SK^O. P^O^ Phosphate. 

 3K^0 . 2P2 0^ Biphosphate. 

 K^O. P'^O^ Metaphosphate. 



In a similar manner the formula for pyrophosphate of potash 

 will be 2K^0 . P^ O^ an undoubtedly quadribasic salt. 



The iodic is the only monobasic acid known to produce anhy- 

 drosalts. The anhydrosalts of the bibasic acids are very nume- 

 rous ; the members of the fourth group of elements more parti- 

 cularly are remarkable for the various ratios of oxide to anhy- 

 dride they present, which, however, may be arranged in a very 

 simple sequence. Among tribasic salts the anhydrophosphates 

 are very numerous. The sequence of ratios is not so simple as 

 in the iodic, chromic, silicic, &c. salts, owing to the greater com- 

 plexity of tri- than of mono- and bi-basic compounds*. 



4K2 0.2P2 05 



Pyrophosphate. 



SK'^O . P2 0^ 3K20 . 2P2 O^ 3K20 . SP^ O* 



Common phosphate. Fleitmann and Hen- Metaphosphate. 



neberg's phosphate. 



Group IV. Boron, Silicon, Titanium — Tin. 

 The analogy in properties between certain compounds of silicon 

 and boron is so great as to necessitate the accordance of similar 

 formulae to corresponding silicic and boric products. Whether 

 we represent the chloride of silicon by SiCF or by SiCl"*, we 

 must adopt a similar expression for the chloride of boron. The 

 formula representing silicic chloride as SiCl'* corresponds with 

 the vapour- density of this and of other volatile siliceous com- 

 pounds, considered as two-volume molecules. But, on the other 

 hand, the formula representing chloride of boron as BCF, corre- 

 sponds with the vapour-density of boric compounds, considered 

 as two- volume molecules ; so that to represent by our formulae 

 the general resemblance in properties between analogous boric 

 and silicic compounds, we must violate the correlation of formula 

 and vapour-density in one case or the other. Seeing that the 

 chloride of silicon manifests a general resemblance to the chlo- 

 rides of titanium and tin, and that the vapour-densities of the 

 last two chlorides correspond with the formulse TiCl^ and SnCl'' 

 respectively, I prefer to neglect the vapour-density rule in the 

 single case of boric, rather than in the triple case of the silicic, 



* That basic and anhydrosalts are of analogous constitution, and that 

 the pyro- and meta-jihosphates are strictly comparable in their properties 

 and modes of formation to ordinary anhydrosalts, was shown by the author 

 in a paper " On the Constitution of Salts," Quarterly Journal of the Che- 

 mical Society, vol. viii. p. 10. 



