40 PHYSIOGRAPHY. CLASS i. 



OBSERVATIONS. 



1. The octahedral Amnioniac-salt from Mount Vesuvius 

 consists of 



Muriate of Ammonia 99'5. 



Muriate of Soda 0-5. KLAPROTH. 



In its pure state it is N H 6 M 2 + Aq = 32-06 of Am- 

 monia, 51-16 Muriatic acid, and 16-78 Water. It is perfectly 

 volatile at a higher temperature, dissolves readily in water, 

 but does not attract moisture from the atmosphere. It emits 

 a pungent smell of ammonia, if rubbed wet with quicklime. 

 If a saturated solution of this salt be brought into a lower 

 temperatiire, its surface will very soon be covered with 

 feathery masses of aggregated crystals, which sink to the 

 bottom, when they have attained a certain weight. The 

 movement thus produced in the fluid gives rise to the for- 

 mation of numerous small crystals, terminated on all sides, 

 which appear to consist of three needles perpendicular to 

 each other, or as it were the pyramidal axes of the octahe- 

 dron, whenever they become large enough to be visible by 

 the assistance of a microscope. In the beginning they are 

 quite steady in the fluid, but they sink when they increase 

 in weight, during which their size also continually increases. 

 The continuation of this process, and the more rapid cool- 

 ing of the outer parts of the fluid, produce a regular move- 

 ment within the latter, since the colder particles sink, 

 while the warmer rise in its centre, and carry along with 

 them part of the crystals already formed. This movement 

 again accelerates the formation of the crystals, the fluid 

 appears now quite troubled, and does not become clear 

 again, till whatever had been dissolved in the fluid above 

 its capacity at that temperature, has fallen to the bottom 

 of the vessel in the shape of highly delicate flakes of snow. 

 MONGE seems to be the first who described this pheno- 

 menon. Crystals of considerable size are obtained by sub- 

 limation. 



It must be observed here, that the tessular form of the 

 crystals of this substance is given upon the authority of 



