C. M. Wetherill on the Crystalline Nature of Glass. 23 
The following are the results of the experiments: 
_ When the vapor escaping from the crucible was condensed 
up@n plates of glass, a “ground glass” etching, with in some in- 
stances distinct traces of crystallization, resulted, 
The following is the action of the liquid acid upon different 
kinds of glass. 
1. Greenish window glass, free from lead. One drop of the 
acid acted energetically, coating the glass with a white sediment. 
When washed, the spot was found to be deeply etched, and pre- 
sented a roughened although transparent appearance. Under 
the microscope, with oblique, transmitted light, the surface was 
ound to be covered with a web of acicular crystals, crossing at 
all angles, and presenting exactly the appearance of sublimed 
caffeine. The average length of the needles was 0°08 of a 
millimeter; their thickness somewhat less than 0:006 mm. 
It was difficult at first to determine whether the crystals were 
elevated, or depressed below the surface of the plate, in which 
case they would have represented casts of crystals dissolved out 
by the acid; but by careful management of the light, studying 
the shadows and comparing them with caffeine crystals, they 
were judged to be in relief. Polarized light had no effect upon 
them. Beside these crystals, there were observed scattered over 
the field of view a few irregular etchings, in intaglio, which 
seemed to be casts of crystalline scales dissolved out by the hy- 
drofluoric acid. : 
_4. A piece of the same plate of glass was treated with succes- 
sive drops of the acid upon the same spot, waiting to add a drop 
until the reaction of the former one had ceased. 
more upon No, 5, the action was ter duration. In these 
examples the evidence of acicular crystallization was apparent 
as a shading upon the ground glass surface. rd the edges of 
@gement of the light to detect the crystals which were observed 
here and there in the form of scales or tablets, apparently bro- 
ken and very small. A few acicular crystals were also detected. 
