74 Chemical Examination of American Minerals. 



We must, therefore, admit that in the American variety, 

 there is a quantity of magnesia accidentally present, amount- 

 ing to 6*42 per cent. 



It has been long known to chemists, that there is a strong 

 affinity between magnesia and alumina. Now it is curious, 

 that when these bodies do combine, it is usually in the pro- 

 portions which constitute spinell. If into a solution of a 

 mineral in muriatic acid, which contains alumina and magne- 

 sia, you pour caustic ammonia to precipitate the alumina, 

 the precipitate will be found to contain a certain quantity of 

 magnesia. If this precipitate be thoroughly washed and igni- 

 ted, and then digested in muriatic acid, a white insoluble mat- 

 ter will remain, which proves when it is analyzed, to be a 

 compound of, 



1 atom magnesia, 

 6 atoms alumina. 



It is, therefore, essentially the same with spinell. The 

 silica and protoxide of iron found in spinell, are doubtless 

 foreign bodies. 



Note. The first variety of spinell alluded to by Dr. Thom- 

 son in this article, was from Franklin, in New-Jersey. It ap- 

 pears to be a green pleonaste, or ceylanite, but it may pos- 

 sibly constitute a new sub-species. The rock in which the 

 mineral is imbedded is not primitive, as it is supposed to be 

 by Dr. Thomson, but transition, as may be seen by referring 

 to Mr. Nuttall's memoir on the geology and mineralogy 

 of the valley of Sparta, first published in the New-York Med. 

 and Phys. Jour. vol. i. No. 1. and afterwards reprinted in 

 Silliman's Journal, vol. v. Also, a memoir on the same sub- 

 ject, by Professors Keating and Vanuxem, published in the 

 journal of the Academy of Natural Sciences, vol. ii. 



