42 Mr. W. Thomson on the 



Notes on Some of the Peculiar Properties of Glass. 

 By William Thomson, F. R.S.Ed., F.I.C., F.C.S. 



(Received February 2.2nd, i88g.) 



I. — On the Crystalline Forms produced on Glass by the action 

 on it of Hydrofluoric Acid and the Acid Salts of the 

 Alkali Fluorides. 



At the Southport Meeting of the* British Association 

 (1883) I read a paper on this subject, and there shewed 

 pieces of glass on which very distinct hexagonal pyramids, 

 cubical, and other crystalline forms had been produced by 

 the action of solutions of the acid fluorides of potassium, 

 sodium, and ammonium, and anhydrous hydrofluoric acid 

 on the glass. Different crystals are produced on different 

 kinds of glass, depending on whether it contains potash, 

 soda, lime, or other base. Tessie de Mothay and Marechal 

 examined these crystals and mention that they are com- 

 posed of the fluorides of calcium and lead, by the separation 

 of which the surface is rendered more opaque. F. Reinitzer 

 in a paper on the same subject, 1886 {Dingl. Polyt. f. 262, 

 pp. 312-320) gives sketches of the same crystals, and offers 

 the explanation that they are the silico-fluorides of calcium 

 sodium or potassium. 



The Rev. Professor T. G. Bonney examined my speci- 

 mens, and, whilst he would not venture on any distinct 

 theory, suggested that they might possibly be due to the 

 crystallization of free silica produced by the action of the 

 fluorides on the glass. Professor Bonney microscopically 

 examined them, and both he and I failed by the ordinary 

 means to find that they polarized light, although they were 

 sufficiently large to be seen by an ordinary pocket lens. 



