THE usual explanation given for the different appearance of etcbin~8 by hydrofluoric acid in tbe gaseous and 10 the liquid state, IS that by employing gas, the products of decomposition of the glass remain in the corroded cavities comlnunicatiog a ground-gloss appearance. This does not obtain by the use of the liquid acid, since in this case the said products are removed from the cavities. An examination of this subject by the aid of the microscope at once showed that the ordinary explanation

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... erroneous. Ground glass is seen, under Lhe microscope, to be covered with irregular cavities of uniform size, which act by the dispersion of light to produce the oharaoteristic appearanoe of glass in this condition. When gla..qg is exposed to the vapor of hydrofluorio 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 tbe apparatus in which the gas is generated are thus coated with a delicate film of the vapor, and are etched, 80 that the microscope exhibits extremely mInute and shallow cavities in which, after cleansing the surface by water, no trace of other substance than glass is perceptible. When the exposure to the. ncid fumes is m.ore prolon~ed, the ~v~ties are deeper and more Irregular. A still greater IrregularIty IS effected by a more lengthened action of the corrosi ve vapor; the acid acts more intensely 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 hydrofluoric acid, or if a drop of tlie same be suffered to fall upon the plate, the whole surface is corroded with a certain dewee of uniformity. Tbere are no minute points of action as in tile case of the deposition of spherules of the acid vnpor. Hydrofluoric acid gas, 80 caUed, is thus sbown to be a vapor,