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IOWA ACADEMY OF SCIENCE Voi.. XXVIII, 1921 
Fig. 19. Photomicrographic sections of five tungsten wires, drawn down successively 
from the same original wire. 
complete blanks, so it was, as usual, necessary to etch the surfaces. 
The most successful liquid was boiling hot hydrogen dioxide. A 
mixture of hydrofluoric and nitric acids served nearly as well. 
The accompanying photomicrographs (See Fig. 19) taken with 
monochromatic green light show, probably as well as any repro- 
duction can, the effect sought. However, it must be said that even 
the original negatives failed to show with anywhere the clearness 
the structure which one could observe directly through the eye- 
piece. In the present half tone the msagnification is 110 diameters. 
The actual sizes of the wires were respectively, 0.0227, 0.01762, 
0.01262, 0.00784, and 0.00240 cm., and the corresponding sections 
are shown in numbers 1 to 5 in the accompanying figure. The 
cracks shown in the wires developed during the process of embed- 
ding the wires in the copper, but these do not do any damage as 
