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in series, so as to be able to etch different slides at the same time. 
To my opinion it will be possible with careful treatment (regulating 
the power of the current and using different acids) in this way, from 
all cast-metals and alloys, to detach the crystals, thus making it 
possible to find out the quality and formation of the materials. 
After I had concluded these preliminary experiments, it came to 
my knowledge that the electric current had already been used as an 
etching medium in the work called “Contribution a Vétude des 
Alliages” edited by the Société d’ Encouragement de l'Industrie Nationale, 
Mr. Crarey describes a method, successfully made use of by him 
and which according to the added photograms, shows great resemblance 
with our method. He namely used an ordinary Danrent-cell to 
furnish the electric current and in it replaced zine by the alloy, 
that has to be etched, made a short circuit of the cell and obtained 
an etched design after having submitted the previously polished plane 
for half an hour to electrolysis. 
To my opinion however there are great objections here. For to 
obtain a somewhat powerful current it is necessary to reduce the 
inward resistance to a minimum, either by using larger electrodes or 
by considerably increasing the acid-percentage of the electrolyte. 
Especially with the etching of iron and steel the high percentage of 
acid will be an unsurmountable obstacle. A second objection is the 
impossibility, on account of that acid, to go on etching deep enough 
to detach crystals, for the angles and ribs which will be laid open 
by the etching-process will very soon dissolve again. 
Before I conclude, I will here openly thank Prof. SCHROEDER VAN 
DER Kork for his readiness in furnishing me all [ wanted, to render 
this investigation possible. 
The Hague, June 1902. 
Physics. — “On the conditions for the occurrence of a minimum 
critical temperature for a ternary system.” By Prof. J. D. 
VAN DER WAALS. 
Already in my “Molecular theory” [ have derived the condition 
on which a binary system presents a minimum critical temperature 
(Cont. II, p. 20). Starting from the form of the equation of state I 
have assumed there, we namely find: 
Biya = 
