546 



Mr. Richard Threlfall 



[March 19, 



kg./cm. at the temperature considered— a difference which corre- 

 sponds to • 2° C, the actual temperature possibly differing from the 

 thermostat temperature by • 1° C. 



We may now pass on to the consideration of some of the results 

 obtained, which refer not only to change of melting-points, but to 

 changes in the temperatm-es of transformation of isomorphic forms. 



4400 



4200 



4.000 

 E 



o 



& 



L. 3600 



o 

 a. 



i: 34-00 

 3200 



3800 



3000 



2800 



10 20 30 40 



Minute s 



Fig. 2. 



50 



As illustrations of such changes, I show here the transformation 

 of yellow to red mercuric iodide, which shows well in the projection 

 microscope ; also Mitscherlich's transformation of potassium bichro- 

 mate, and sulphur in two forms.* 



* Experimental Demonstration of a Transformation of Sulphur. — A micro- 

 scope slide is prepared by partially melting a fragment of monoclinic sulphur, 

 and inclosing some of the melt between the slide and cover-slip, well pressed 

 together. The presence of unmelted monoclinic sulphur insures the crystal- 

 lisation of this variety on lowering the temperature. By means of a hot 

 stage it is possible to preserve the crystallisation long enough to exhibit it by 

 means of a projection polarising microscope. The appearance is very charac- 



