Peailiar Properties of Glass. 45 



" glass. On etching potash glass, tesseral crystals of silicon 

 " potassium fluoride can be observed, and this suggests a 

 " simple method for the detection of potash glass." 



There is, however, a simple method by which this theory 

 of Reinitzcr, and also of de Mothay and Marechal can be 

 tested, and that is, that sodium-silicon fluoride, calcium- 

 silicon fluoride, potassium-silicon fluoride, and also lead 

 and calcium fluorides are all easily acted upon by sulphuric 

 acid. If, then, these crystals be composed of the above- 

 named compounds, it is evident they should be dissolved and 

 removed, or destroyed by the action of sulphuric acid, which 

 attacks with facility those compounds. I have made the 

 experiment by boiling pieces of glass on which these crystals 

 had been developed in sulphuric acid of different strengths 

 up to prolonged boiling with strong vitriol, but on washing 

 the glasses after such treatment, none of the crystals were 

 destroyed or dissolved, and even their edges were not in the 

 faintest degree affected. Whatever, therefore, these crystals 

 may be, they are itot crystals of the sodium, calcium or 

 potassium silico fluorides, or of lead or calcium fluoride. 

 But assuming that they are so, then one would expect to 

 find them in the deeper parts of the etchings as well as near 

 the surface and edges ; which is not the case. 



I am of opinion that these crystals existed originally in 

 the glass, and that the action of the solvent developed them 

 just as hydrochloric acid developes the crystalline structure 

 on tin when a weak solution is washed over a bright and 

 smooth surface of it. It is not suggested that the hydro- 

 chloric acid combines with and produces the crystals, it 

 merely dissolves away the surface of the tin at some parts 

 more than at others, so as to develope the metallic crystals ; 

 and if the etching with the acid is continued, the crystals 

 which are at first developed disappear, which is just what 

 happens with the glass. 



I am of opinion that the crystals developed from the 



