Correspondence — The Rev. R. A. Bullen. 431 



of BIniodide of Mercury. Heat the scarlet crystals in the teaspoon 

 over the lamp ; they become pale yellow. Place them on the paper, 

 and rub them with the thumb-nail, and they become scarlet again. 

 They are now reduced to a red powder colouring the paper. Pass 

 the paper to and fro over the lamp, the scarlet instantly turns yellow. 

 Attempt to rub the powder oif by passing the paper over the carpet, 

 the paper instantly turns red again. 



Miller explains the change from yellow to red as follows : — 

 "Warrington has shown that this change of colour depends upon 

 a change in the molecular constitution of the salt, in consequence 

 of which the rhomboidal crystals are converted into octahedra with 

 a square base." This alone would be sufSciently perplexing, but 

 the changes seem capable of being produced over and over again, 

 and we have simply to alternate thermal and dynamic metaraorphic 

 processes to produce the yellow biniodide and the scarlet biniodide 

 of mercury at pleasure. If these things may be done in the 

 drawing-room, what may not thermal and dynamic metamorphism, 

 alone or combined, effect in the laboratory of nature ? 



One more possible case. In using the soldering iron (which iron 

 is always copper) a very moderate temperature is essential to keep 

 the solder on the face of the clean copper, a tempei'ature far below 

 that requisite to melt copper with tin to produce one of the copper- 

 tin alloys ; yet occasionally the soldering iron will show streaks of 

 yellow gun-metal. An amateur in his attempts at soldering is apt 

 to press the soldering iron heavily on his work, and it seems possible 

 that the alloy indicated on the iron may arise from insufficient heat 

 being reinforced by equally insufficient pressure ; the two combined 

 being, however, sufficient to produce an alloy of copper and tin. 

 If this be so we have in the soldering iron a case of thermal and 

 dynamic metamorphism in combination, just as in the biniodide of 

 mercury we have thermal and dynamic metamorphism in alternation. 



A friend employed on the Manchester Ship Canal once sent me a 

 specimen of red sandstone from a slickenside. At the plane of 

 contact and pressure the red colour is entirely discharged, and the 

 stone apparently porcellanized. The depth to which the meta- 

 morphic action extends is so very slight, sometimes less than ^th 

 of an inch, that the metamorphosis would seem due to pressure 

 rather than heat, as it is difficult to understand how pure thermal 

 action could be restricted to such a mere film. 



ToKQUAY, IQth June, 1894. A. E. HuNT. 



SHELLS FROM POETLAND EUBBLE DEIFT. 



Sir, — I had the pleasure of several hours work in June last at the 

 Eubble Drift at Portland, both at Chesilton and at the Bill. 



I was not able to obtain any recent shells from the Chesilton 

 Rubble ; but I would remark on the enormous angular and sub- 

 angular blocks of Portland oolite that occur in this section at various 

 horizons, the whole of the material being unsorted, and therefore 

 having been accumulated by more or less rapid aggregation of 

 material in a comparatively brief space of time. 



