2-48 On the Composition of Arragonite. [Oct. 



other from the Hartz. The result was, that the proportion of car- 

 bonate of strontian varies in different varieties of arragonite ; but 

 that it is constant in the same variety. He lias found the same rule 

 to hold with respect to the magnesia in bitterspar, dolomite, and 

 litterkalk. The Arragonese and Bearne arragonite contains twice 

 as much strontian as that of Auvergne ; while this last contains one 

 and a half times as much as the varieties from the Hartz and from 

 Faroe. 



The author found (fully verifying his opinion respecting arrago- 

 nite) less carbonic acid in arragonite than in calcareous spar, in 

 whatever way that quantity was determined ; whether by exposing 

 the minerals to a strong heat, or by expelling the carbonic acid by 

 means of an acid, or by the quantity of carbonate of lime formed by 

 passing the gas, extricated, through lime water. 



There exists another striking difference between arragonite and 

 calcareous spar. Arragonite contains a small quantity of water che- 

 mically combined, as is obvious from the appearance of an enamel, 

 and the efflorescence which takes place when arragonite is exposed 

 to a slight heat. Rhomboidal calcareous spar contains no water 

 chemically combined with it ; and though exposed to the heat at 

 which arragonite is altered, still retains its lustre, its transparency, 

 and all its properties. It begins to alter only when the carbonic acid 

 makes its escape. The small quantity of water driven off by heat 

 from some varieties of calcareous spar is very various, and is only 

 mechanically contained in them. Hence those spars only give 

 water which decrepitate in water; and they yield the more the 

 stronger the decrepitation is. This, as all decrepitations, is owing 

 to the escape of water or air, which is mechanically confined between 

 the lamella? of the crystals, and therefore ought not to be consi- 

 dered as characteristic of certain bodies. The remarkable trans- 

 parency of Iceland crystal is owing, I conceive, to the total absence 

 of water in it : for it does not decrepitate, and it may be heated 

 almost to redness without undergoing any change. The efflores- 

 cence of arragonite, when exposed to a moderate heat, gives us, as 

 Haiiy has already remarked, an easy and sure mode of distinguishing 

 it from rhomboidal calcareous spar. 



The author found likewise evident traces of oxide of manganese 

 and oxide of iron in arragonite. The first is in all probability in 

 the state of a carbonate, and chemically combined with the other 

 two carbonates. The arragonite of Auvergne contains none of it. 

 Hence it is not an essential ingredient. The oxide of iron seems to 

 be in the state of an hydrate, and only to be mechanically mixed 

 between the lamella? of the mineral. This appears obvious from 

 the Spanish arragonite, which contains the most oxide of iron, and 

 owes its colour to it ; for when put into acids it becomes colourless 

 and transparent; while at the same time the hydrate of iron sepa- 

 rates from the plates, and subsides. This hydrate contains a mixture 

 of quartz, sand, and of gypsum, neither of which is to he considered 

 as a constituent of arragonite, 



