790 



their symmetry, and at least in conformity with each otlier, with 

 rational proportions of their linear distances; and they innst also 

 remain so, if their aggregation shall be erystallographically a possible 

 one. For that reason the changes in symmetry of one of these space- 

 lattices, nuist be connected with the same changes in the other ones; 

 it can hardly be hazardons, to conclnde from the changes in the 

 Röntgenograni of one of them, with regard to the changes of the 

 other space-lattices. Besides it will seem somewhat improbable with 

 respect to the relatively slight change in molecular arrangement, thai 

 at the same time no further change should accompany it, which 

 takes place within the domain of the composing molecules themselves. 

 For the birefringence is, even just a little below the inv'^ersion- 

 temperature, again very strong, but disappears at 266° C. quite 

 suddenly. It is difticidt to believe, that so great a change could 

 only be attributed to the apparently not very great change in the 

 molecular arrangement. The conception, that the optical properties 

 partly, if not greatly, must be caused by the anisotropy of the 

 composing molecules themselves, more than by the structure of their 

 molecular aggregation, is often defended, just because it is able to 

 give a clear idea of the nature of optically-anomalous crystals. It 

 is true, our experiments have once more proved, that doubtlessly 

 the intluencte of the molecular arrangement is present; but perhaps 

 it is in this direction of research, that the cases are to be found, 

 which will allow a delinitive conclusion with respect to the one or 

 the other of those views. 



Experiments with leucite, in which the difficulties will be even 

 greater, because of the higher inversion-temperature and the much 

 slower transformation, are at present being made in our laboratories. 



Groningen, January 1914. 



Mineralogy. — ''On temperature-measurements of anisotropaus 

 bodies by means of radiation-pyrometers.'' By Prof. Dr. F. M. 

 Jaeger and I)i-. Ant, Simek. (Communicated l)y Prof. Haga). 



(Communicated in the meeting of January 31, 1914). 



§ i. In the study of the optical behaviour of white-hot silicates, it 

 accidentally happens that the temperature of the investigated objects 

 is measured by means of the now generally used radiation-pyrometers 

 of Wanner or of PIolborn — Kurlbalm. 



The temperature of the body, as determined in this way, generally 

 cannot coincide with its real temperature; for the mentioned pyro- 



