Mr J. D. Dana on the Origin of Trap Minerals. 263 



keeping it below a dull red heat, a slight ammoniacal odour 

 is first exhaled, and next fumes of sulphate of ammonia. If 

 the heating process is stopped before the whole of the vola- 

 tile salt is sublimed, the residue is entirely soluble in the 

 water, seeming to prove that the proportions of the acid and 

 bases are the same as in the neutral salts. 



Some paradoxical appearances are connected with the for- 

 mation of this double salt. Thus, as is well known, when 

 magnesia is mixed with a solution of sulphate of ammonia, 

 a strong smell of the volatile alkali is evolved ; the mag- 

 nesia separating a portion of the alkali to combine with the 

 remainder of the acid. Thus again, even when the carbon- 

 ate of magnesia is similarly added, the same odour is pro- 

 duced in a less degree, and there is a slight effervescence. 

 Another instance of paradox may be mentioned. If a cer- 

 tain portion of sulphuric acid is added to a solution of sul- 

 phate of magnesia, ammonia, however largely added, will 

 occasion no precipitate. The explanation of this is obvious. 

 The same remark applies to the following : — When a precipi- 

 tate has been obtained by ammonia from sulphate of mag- 

 nesia, the volatile alkali being in great excess, if sulphuric 

 acid is added, and yet not in sufficient quantity to neutralize 

 the excess, the precipitate will be redissolved. The pheno- 

 mena are the same, substituting the nitrate or muriate of 

 magnesia for the sulphate, and using the nitric or muriatic 

 acid for the sulphuric, and owing to the same cause, viz., 

 the formation of soluble double salts. For purposes of ana- 

 lysis, it may be worth keeping in mind, that these last men- 

 tioned double salts are remarkably deliquescent. 



Barbadoes, May 1. 1846. 



Origin of the Co?istituent and Adventitious Minerals of Trap 

 and the Allied Rocks, By Jambs D. Dana. 



(Concluded from p. 203.) 



7. Bearing upon this subject, it should be observed, that 

 the constituents of amygdaloidal minerals are, in general, 

 those of the containing rock. Silica, potash, soda, alumina. 



