The Molecular Volume of Solids. 



461 



C' + N' = 32 -|-24=56 respectively. The meaning of this is that the 

 components of these radicles always undergo a like condensation. 

 Water of crystallisation has a volume H' 4 0' 2 = 14, whilst that of 

 ammonia in ammonia-compounds is N' 2 H' 4 = 18. 



The tables which accompany this paper will be found to contain 

 pretty strong evidence of the truth of a conjecture first made by 

 Kopp with regard to oxygen, viz., that an element in one and the same 

 compound may undergo different condensations if it enters into the 

 composition of two distinct radicles. Hydrated ammonium sulphate, 

 (NH 4 ) 2 S0 4 .H 2 0, affords a good illustration of this, for its molecular 

 volume is (NH 4 )' 3 S / 6 0' 4 B['40' 2 , and it will be observed that the 

 hydrogen in the ammonium radicle is condensed to one- third, whilst 

 in the water of crystallisation it is condensed to one-fourth, and again 

 the oxygen in the acid radicle is condensed to one- fourth, whilst in 

 the water it is only one-half. 



The following circumstance is well worth consideration. Many 

 substances, having the same chemical composition, appear to possess 

 two distinct specific gravities, and therefore different molecular 

 volumes ; a good instance of this is to be found in mercuric sulphide, 

 (HgS), which, as cinnabar, has a specific gravity of about 9"0, but in 

 its amorphous state, a specific gravity of about 7 '6, corresponding to 

 molecular volumes Hg'gS'g and Hg^S'g respectively : (HgS) has also 

 sometimes a specific gravity intermediate between these limits, indi- 

 cating an admixture of the two states. These mixtures are rather 

 puzzling to any theory of molecular volumes, just as the densities of 

 gases at temperatures when they are undergoing dissociation appear to 

 be anomalous according to Avogadro's law. Perhaps these substances 

 might not inappropriately be called bivolumetric or disteric bodies ; a 

 few examples of such compounds are given in the following table : — 



Substance. 



Molecular 

 weight. 



Molecular 

 volume. 



Calculated 

 sp. gr. 



Observed 

 sp. gr. 



Silicic dioxide (crys- 













Si0 2 



Si' 4 0' 8 



2-688 



2-690 



Silicic dioxide (arnor- 













Zr0 2 



Si / 4 , 4 



2 -200 



2-200 



Zirconium dioxide .... 



Zr' 4 0' 8 



5-655 



5 -624 





Zr' 4 0' 4 



4-588 



4 35—4 -90 



Titanium dioxide (ru- 











tile) 



Ti0 2 



T' 2 0' 8 



4-256 



4-250 



Titanium dioxide (an- 















Ti' 4 0' 6 



3 -875 



3 -890 



Cobalt sesquioxide. . . . 



Co"o 3 



)3 



Co' 4 0' 4 



5 -627 



5 -600 





Co' 4 0' 3 



4*811 



4-814 



There is one more circumstance deserving mention, and that is, the 



