schaller: tlsemannite 417 



in the central portion of the cube were invariably normal to the 

 direction of the applied load in the direction of least resist- 

 ance, which is also the direction of flow. Further experiments 

 along this line with polymorphic crystals of low inversion tem- 

 peratures and of prismatic forms, are in progress. These prove 

 that external directive pressure is a factor which enters into the 

 crystal force system, and are in accord with the results cited 

 above. 



Summary. Experimental proof is offered in the foregoing 

 pages of the reversibility of the relations between a strained solid 

 and its liquid. The mechanism of this action has been found to 

 be exactly that postulated in 1862 by James Thomson from a 

 purely theoretical basis: namely, that, on crystallization, each 

 particle (atoms and groups of atoms) enters into the crystal state 

 in the condition of the crystal at the point to which it becomes 

 affixed; and that if the crystal be under a state of strain the 

 freshly deposited particle enters into the same state of strain. 

 This fact is essential if equilibrium relations are to exist between 

 a strained crystal and its liquid, because only under these condi- 

 tions can the relations be strictly reversible, and thermodynamic 

 reversibility is necessary if the thermodynamic equations are to 

 be valid. 



MINERALOGY. — Ilsemannite, hydrous sulphate of molybdenum. 

 Waldemar T. Schaller, Geological Survey. 



A secondary blue molybdenum mineral was described in 1871 

 and named ilsemannite; according to Dana, 1 it is cryptocrystalline 

 and blue-black to black, becoming blue on exposure. The min- 

 eral is readily soluble in cold water to a deep blue solution. It 

 has never been analyzed quantitatively and its composition 

 has been assumed to be Mo0 2 .4Mo0 3 . Recent analysis of 

 material from near Ouray, Utah, has shown that the amount of 

 molybdenum in a lower state of oxidation is insignificant and 

 that practically all of the molybdenum is present as Mo0 3 . 



1 Dana, E. S. System of Mineralogy, 6th ed., p. 202. 1S92. 



