use of on a large scale. In a special work on the subject: “Das 
mikroskopische Gefiige der Metalle und Legierungen,” this method 
and the practical use made of it, is treated exhaustively. The main 
substance of it is this: 
A piece of the material, which is to be submitted to microscopie 
examination is filed, till it is perfectly smooth, different numbers of 
carborundum-powder being used for the grinding, after which it is 
polished with tin-oxide or chrome-oxide, if a perfectly smooth slide 
is required; then, by means of the annealing colours, the ground plane 
will show a design, sharp-outlined. Since most metals and alloys 
have crystal-formations at their fractures, the annealing colours will 
produce a very strongly marked outline between the crystals and 
the ground-mass, because it is a known fact, that two substances, 
submitted to the same temperature, but of different formation (the 
erystals- and the surrounding mother-water) will not take the same 
tempering-colours. Next by making scratches on the surface, with 
needles of known hardness (analogous with the known hardness-scale 
of Mons) the hardness of the material may be fixed. 
A similar design, not so minutely detailed however, may be called 
forth by the corroding influence of acids, bases or salt-solutions, 
which the crystals and the encompassing matter are not equally 
proof against. For this purpose the rubbing and polishing need not 
be done so carefully. 
This method however has also its difficulties, which may often be 
very troublesome. As is always the case, when an acid operates 
upon a metal, also in this etching-process gas will be developed. 
The. microscopically small gas-bubbles which are formed on the 
slide will locally prevent the corroding process of the acid and be 
the cause of little holes and dots that have nothing to do with the 
design and may easily lead to faulty conclusions. The very long 
time the grinding and polishing sometimes takes (I state here the 
grinding of different species of iron and babbits) will keep many 
from applying this method. 
No satisfactory results, in some cases, being obtained oy this method, 
although the material showed distinct crystals at its fracture. Prof. 
SCHROEDER VAN DER KOLK was struck by the idea, whether it would not be 
possible to etch metal-planes in another way than by corroding 
through acids. It is a known fact that a metal in a galvanic cell 
will corrode at its negative pole. I need only state the equally known 
fact, that the zine of a bichromate cell, after the electric current is 
set working, shows a magnificent structure. To forestall the objec- 
tion, that the chromic-acid of the cell has been predominating here 
wy AS, 
