246 Intelligence and Miscellaneous Articles. 



covery, in his memoir on the composition of corals. Apatite and 

 chondrocyte require heat, as they are found only in granular lime- 

 stones. The chondrodite is not supposed to exist as such in coral, 

 hut to form from the mutual action of its elements (which are pre- 

 sent) during the slow action of the heat that gives the crystalline 

 character to the limestone. 



The magnesia of magnesian limestones is not attributable to the 

 corals, as the proportion obtained by the analyses is less than one 

 per cent*. It is derived probably from a foreign source; and this 

 may be true, in part at least, for the magnesia of the chondrodite, 

 although there is enough of this constituent present for a large 

 amount of this mineral. The silica may also be in part foreign, or 

 may proceed from the earthy impurities which were mixed with 

 the limestone at its formation. — Silliman's American Journal, July 

 1846. 



REMARKABLE DISCOVERIES IN ISOMORPHISM. 

 BY M. SCHEERER f. 



M. Scheerer has just found that in compounds containing mag- 

 nesia, protoxide of iron, oxide of nickel and other oxides isomorphous 

 with magnesia, a part of the base may be wanting without a change 

 of crystalline form, provided that this part be replaced by a quantity 

 of water which contains three times as much oxygen as this part of 



the base. For example, the compounds Mg 3 Si, Mg 2 Si + 3H, and 



Mg Si •+■ 6H in accordance with this principle, are isomorphous. 

 Thus chrysolite and serpentine may be isomorphous. The composi- 

 tion of the first, Mg 3 Si, is anhydrous and constant. Serpentine is 

 hydrated and has a varying composition, wherever found, not afford- 

 ing a chemical formula. But examined with reference to M. Scheerer's 

 views, we observe that in all the best analyses of serpentine, the 

 oxygen of the magnesia and of the protoxide of iron, added to one- 

 third the oxygen of the water, is equal to that of the silica ; and 

 consequently serpentine is a variable mixture of two isomorphous 



silicates, Mg 3 Si and Mg a Si -+• 3H. M. Scheerer has brought for- 

 ward numerous other examples from among silicates, sulphates, &c. 



M. Scheerer has also discovered that oxide of copper may be re- 

 placed in an isomorphous manner by two atoms of water. 



We may now see clearly why so many hydrated minerals have 

 never given uniform results, even with the most careful analyses. 



The memoir of Scheerer will appear in two or three months, in 

 Poggendorff's Annalen. The facts here briefly stated were communi- 

 cated by him to the Academy of Sciences at Stockholm, at its last 

 session. — Ibid. 



* Silliman's American Journal, New Series, i. 189, 198. 



f In a letter to B. Silliman,jun., from Berzelius, dated March 10, 1840. 



