16 OC. M. Wetherill on the Crystalline Nature of Glass. 
Arr. IV.—On the Crystalline Nature of og of ’ by Cuarues M. 
WETHERILL, Ph.D., 
THE usual explanation given for the different appearance of 
etchings by hydrofluoric acid in the gaseous and in the liquid 
state, is that by employing gas, the products of decomposition 
of the glass remain in the corroded cavities cominunicating a 
ground-glass appearance. This does not obtain by. the use of 
the liquid acid, since in this case the said products are removed 
from the cavities. 
n examination of this subject by the aid of the microscope 
at once showed that the ordinary explanation is erroneous. 
Ground glass is seen, under the microscope, to be covered with 
irregular cavities of ‘uniform size, which act by the dispersion of 
light to produce the characteristic appearance of glass in this 
condition. 
When glass is exposed to the vapor of hydrofluoric acid, the 
corrodent is deposited in the condition of minute globules, each 
of which attacks the surface to which it is attached. Articles of 
glass placed near the apparatus in which the gas is generated are 
thus coated with a delicate film of the vapor, and are etched, so 
that the microscope exhibits extremely minute and shallow cav- 
ities in which, after cleansing the surface by water, no trace o 
other substance than glass is perceptible. When the exposure 
to the acid fumes is more prolonged, the cavities are deeper and 
more irregular. A still greater irregularity is effected by a more 
lengthened action of oP es vapor; the acid acts more in- 
tensely upon the spots first attacked, and the holes are extended 
with ragged margins and deepened by the action. 
On the other hand, when the glass is immersed in liquid hy- 
drofluoric acid, or if a drop of the same be suffered to fall upon 
the plate, the whole surface is corroded with a certain degree of 
fe tie it; ; Mag ak are no minute Sib “el pita: as in the case 
- an 2 Woelsendort 
| antozone. 
_ In observing the specimens etched by the liquid acid, the erys- 
oo tin nature ‘of glass was disco vered and witnessed in every 
TT ccrations of which an account is h made i labora- 
‘ay of the Smithsonian Institute, rome Je “= hes a * ie : 
