232 Dr Charles Daubeny on the 



But this definition gives scope for a considerable diversity 

 of chemical composition, since silica is capable of entering 

 into chemical union with bases in the proportion of one, two, 

 and three atoms ; and, accordingly, I may enumerate the fol- 

 lowing species of the felspar family, as distinguished by Rose 

 and his disciples. In this enumeration, however, I will en- 

 deavour to simplify the matter as much as possible, by point- 

 ing out, in the first place, merely the relation which the silica 

 bears to the base which contains three atoms of oxygen to 

 two of radical ; and, in order to avoid an inconvenient cir- 

 cumlocution, will speak of this base as though it were in all 

 cases alumina, that being the negative element possessing the 

 above composition, which is most commonly found in union 

 with silica in this class of minerals. 



In anorthite, then, and in labradorite, the silica is to the 

 alumina in the proportion of atom to atom, the difference be- 

 tween these two minerals consisting, in the former being com- 

 posed of three proportionals of this combination ; in the lat- 

 ter of only one. 



In the andesin, or the felspar from the Andes, as well as 

 in oligoklase, the silica is to the alumina in the proportion of 

 two to one ; the difference between the two consisting in the 

 proportion w^hich the silica bears to the base with one atom 

 of oxygen. 



Lastly, in pericline, albite, glassy felspar, adularia, and or- 

 thoklase, the proportion of silica to alumina is as three to 

 one, the largest amount in which it is capable of entering into 

 chemical union with any base whatsoever. 



The chemical differences between these four minerals con- 

 sists in the nature of the base with one atom of oxygen united 

 to silica, which it contains ; this in orthoklase is chiefly potass ; 

 in albite wholly soda ; in pericline partly potass, but chiefly 

 soda ; and in adularia chiefly potass, though a small amount 

 of soda is also present. 



Thus we have, in the felspar family, three several grada- 

 tions in the proportion which the silica bears to the alumina 

 with which it is combined, namely, one, two, and three atoms 

 to one of base, as may be seen by the following table, for 

 which I am indebted to Abich. 



