44 Mr. W. Thomson on the 



fluorides. On breaking this vessel I found the inner surface 

 to be very irregularly etched, shewing what appeared to be 

 irregular crystalline forms of an average of a quarter of an 

 inch across and yi to j^ inch deep from apex to bottom of 

 rough crystals, but on carefully examining these by the 

 naked eye, by a pocket lens, and by the microscope, no 

 distinct and definite crystalline forms could anywhere be 

 discovered. The observations which strike one regarding 

 these are : — -First, if glass possess that absolutely homo- 

 geneous or colloid or gelatine structure which it is generally 

 supposed to have, why does it develope these curious 

 irregularities when submitted to a slow solvent action. One 

 would expect it to dissolve like a surface of gelatine when 

 slowly acted upon by water if it were so absolutely colloid 

 in its structure. On the other hand, if it be presumed to 

 have a crystalline structure, one would expect that the 

 surface would present such irregularities as it actually gives 

 when the surface is thus removed. 



With regard to the distinct crystalline forms produced 

 on glass by the action of the alkali acid fluorides, Tessie de 

 Mothay, Marechal, and F. Reinitzer seem satisfied that the 

 crystals \\dMQ:\iQQ.\\ produced by the solvent itself combining 

 with some of the constituents of the glass and depositing 

 crystals therefrom. The following is an extract from 

 Reinitzer's paper : — 



" Fig. I represents the edge of an etched plate. The 

 " crystals are hexagonal, and agree with those of silicon- 

 " sodium fluoride. There are also a few of a longish shape, 

 " which are very like those of silicon-calcium fluoride. It is 

 " believed that alkali fluoride and hydro-fluoric acid act on 

 " the glass, forming sodium-silicon fluoride and silicon- 

 " calcium fluoride which are set free in a crystalline form ; 

 " whereas, hydro-fluoric acid etches the spaces between the 

 " crystals. Silicon and calcium are derived from the glass, 

 " sodium partly from the etching bath and partly from the 



