162 BRIDGMAN. 



Arsenic, again, is near phosphorus in the periodic table^ and I 

 thought that there might be another form like black phosphorus, but 

 none was found. The substance used was distilled arsenic; pressure 

 was transmitted to it by mercury, so as to avoid the possibility of 

 poisonous compounds with the kerosene. After the run, the surface 

 of the arsenic was found wet with mercury, and the appearance was 

 that of amalgamation. The mouth of the steel shell was also amal- 

 gamated, a thing which I have never observed before under similar 

 conditions of pressure and temperature. 



Iodine was tried at 30° and 200° to 12000 kgm. without result. 

 At 200° a much rounded melting point was found near 5000. Because 

 of the great chemical acti^dty of Iodine it was placed in a steel shell 

 beneath water, instead of being allowed to come in contact with 

 kerosene or mercury, as usual. In this way chemical action was 

 largely reduced, but at 200° the Iodine apparently goes slowly into 

 solution. 



Substances existing as Minerals ix two Forms. — Pb(N03)2; 

 HgS, trigonal, cubic; CaCOa, rhombic, trigonal. 



Pb (N03)2 is said by Morel ^'^ to have two mineral forms. HgS 

 crystallizes in a black and a red form; the black is very unstable. 

 Both of these substances have been commented on in previous sec- 

 tions. 



The CaCOa was investigated in the form of calcite. In nature 

 CaCOa occurs in two forms, as calcite and arragonite, the latter being 

 the more dense. There has been considerable speculation as to the 

 relation of these two forms, but it seems to have been finally settled 

 that at ordinary temperatures calcite is the stable form, the reversible 

 transition from calcite to arragonite running at fairly high tempera- 

 tures. This means that the phase diagram must be of the ice type. 

 There is, therefore, a possibility that a permanent change from calcite 

 to arragonite might be brought about by increase of pressure, if the 

 pressure could be carried far enough into the region of stability of the 

 arragonite to force the reaction from calcite to run, and the pressure 

 then released in a region where the reverse transition from arragonite 

 to calcite does not run because of viscosity. The experiment was tried 

 of maintaining pressure on calcite at 12500 kgm. for six hours at 200°, 

 cooling it, and then releasing pressure, but there was no permanent 

 change of density. Since the reaction from arragonite to calcite does 

 not run at atmospheric pressure and room temperature, it would run 



37 J. Morel, BuU. Soc. Min. France, 13, 337 (1890). 



