14 On the Action of Hydrofluoric Acid on Glass. 



crystal is to be measured, the stage is moved till the apex of the 

 angle coincides with the centre of the goniometer and the diameter 

 with one side. The eye-piece is now turned till the cobweb crossing 

 the diameter at the centre coincides with the other side of the 

 angle. Now the number of degrees of the angle can be read at the 

 circumference. The advantage of this over the ordinary micro- 

 scopic goniometers is, that in ours the angles of the crystal and the 

 degrees of the goniometer are on the same hne of sight within the 

 tube of the microscope, while in the ordinary goniometer the degrees 

 are marked outside the tube. The photographic processes by 

 which the above are made can be learned by consulting any of the 

 standard works on photography, under the sections that treat of 

 copying engravings and taking transparent positives. — Sillimans 

 American Journal, Decemher, 1871. 



VI. — On the Action of Hydrofluoric Acid on Glass viewed 

 microscopically. By H. F. Smith, Esq. 



Having had occasion some time ago to use hydrofluoric acid (in 

 solution) for the purpose of etching on glass, I observed that in 

 some lights the varieties of colour presented while the action was 

 going on were very brilliant. This led me to examine it microscopi- 

 cally. The hydrofluoric acid was prepared in the ordinary method, 

 from calcium fluoride by the action of sulphuric acid. The solution 

 was then diluted and kept in a lead bottle for use when required. 



When the acid was first dropped upon the glass, no action was 

 evident, the appearance presented being simply that of a drop of 

 water on glass. In a very short time, however, the drop became a 

 little duller, but this almost immediately cleared away, and several 

 small particles, seemingly of glass, were seen floating in the drop. 

 These seemed to be undergoing a process of fusion ; the appear- 

 ance being similar to that seen when a small portion of metal is 

 thrown into some of the same substance in a state of fusion ; it is 

 tossed about for some time, and then finally disappears. This was 

 what evidently appeared to me to be going on here, the hydrofluoric 

 acid having apparently a solvent action on the glass. What 

 strengthened this opinion was the presence of magnificent colours, 

 changing every moment as these small portions of glass were 

 liberated from the larger piece, and were undergoing the process of 

 solution, thus leading one to suppose they consisted of small glass 

 prisms, the colours being more perfect than those obtained by 

 waier-]prisms simply. Some of these particles were completely 

 surrounded by a halo of colour, as if they had been thrown into a 



