302 On the Change ofArragonite into Calcareous Spar. 



will be interesting to prove this by direct experiments. The 

 conditions under which carbonate of lime crystallizes, sometimes 

 as calcareous spar, sometimes as arragonite, have not been fully 

 developed. Calcareous spar is formed, not only when the car- 

 bonate of lime crystallizes from an aqueous fluid, as is the case 

 with calc-sinter, but also when it is melted, as is observed 

 in the masses of limestone which have been enveloped in, and 

 melted by, the lava of Vesuvius. So also the carbonate of lime, 

 in the form of arragonite, is deposited from the hot springs of 

 Carlsbad, and occurs also as arragonite in rock formations, which 

 have undoubtedly been in a state of fusion. It is probable that 

 the small dose of carbonate of strontites is the cause of the car- 

 bonate of lime crystallizing in a second, the prismatic form, as 

 similar examples occur in oxide of copper, &c. Only one ex- 

 ample, says Poggendorf, is known to him of the change of an 

 arragonite crystal into calcareous spar. It is frequently the case, 

 he says, that fragments of ihe walls of Vesuvius fall into the fluid 

 lava, by which the minerals of which they are composed are 

 more or less changed. This was the case, amongst many other 

 minerals, with a crystal of arragonite. The rock in which it is con- 

 tained has not been fused, but the arragonite was so strongly 

 heated, that the outer part of it is changed into calcareous spar, 

 while the internal part remained as arragonite, so that the whole 

 arragonite crystal retained its original form. The heat had acted 

 so long on it, that the parts changed into calcareous spar assumed 

 the form of that substance, so that the crust of the arragonite 

 crystal consists of a great many crystals of calcareous spar, in 

 which the rhomboidal planes are distinctly visible, and which, 

 before the blowpipe, exhibits the same characters as calcareous 

 spar. — Poggendorf s Annalen, 1831. 



3. Chemical Examination of the Parmelia esculenta, a substance said 

 to have been rained from the sky in Persia, by Fr. Gobel in Dorpat. 



Dr Parrot gave me this lichen for analysis, with a note stating, 

 " he had brought a substance with him from his journey to 

 Ararat, which, in the beginning of the year 1828, rained in se- 

 veral districts in Persia to a depth of five or six inches, and was 

 eaten by the natives ; it appeared to him to be of organic ori- 

 gin." 



