262 Prof. G. Rose on the Isomorphism of Stannic Acid, 



crust is not, What is its precise value ? but, Whether that value 

 is a veiy small or a very large fraction of the radius ? I am 

 strongly inclined to regard Mr. Hopkins's investigations as con- 

 clusive on this point, as I have already stated. I do not regard 

 his calculation as an exact one, nor do I conceive that he does so 

 himself: it is rather, I may say, a calculation of tendencies; or 

 perhaps it may be called the proximate solution of a hypothetical 

 problem, which he applies with the necessary modifications to 

 the case of nature, in order to ascertain which way the evidence 

 tends — towards the thickness being very small or being very 

 great. No other calculation that I have seen, conducted on 

 physical principles, grapples with this subject. 



8. The evidence which Mr. Hopkins's solution gives in favour 

 of a great thickness receives, I conceive, additional force from 

 the arguments drawn from the circumstances I pointed out in 

 this part of the world, where we have an enormous mass of 

 mountain-ground pressing downwards, and a vast ocean coopera- 

 ting with it by its deficiency of weight, in tending to break 

 through the crust with a force which it could not resist if it were 

 not of considerable thickness. 



Calcutta, July 25, 1859. J. H. PrATT. 



XXXIX. On the Isomorphism of Stannic Acid, Silicic Acid, and 

 Zirconic Acid [Zirconia). By Gustav Rose*. 



SINCE Marignac found that the fluostannates and the fluosi- 

 licates have the same crystalline form and are isomorphous, 

 the reasons for assuming that silicic acid, like stannic acid, con- 

 tains 2 equivs. of oxygen preponderate so greatly that there can 

 be no longer any doubt as to the admission of this assumption. 

 It would also be no subject for surprise to find silicic acid in the 

 form of tinstone, or stannic acid in the form of quartz. In fact 

 the first case is no longer a mere opinion, but has distinctly 

 occurred ; since it is known that tinstone and zircon exhibit such 

 an agreement in the angles of their crystals, in the development 

 of the system, and in the direction of the cleavage, that they 

 may be undoubtedly regarded as isomorphous. The crystalline 

 form in both is a quadratic octahedron, the terminal edges of 

 which form angles — in tinstone, of 131° 40' (Miller), and in 

 zircon of 123° 19' (Mohs). The first and second quadratic prism 

 occurs in both, and the straight terminal face is as infrequent in 

 one as in the other ; in both there is a cleavage parallel to the first, 

 and especially to the second quadratic prism. The agreement 

 in the angles is still greater when zircon is compared with rutile 



* Translated by Dr. Atkinson from PoggendorfF's Annalen, vol. cvii. 

 p. 602. 



