AUTHORS' PREFACE. V
in this case the question of raw material for the preparation of
the individual compounds is of chief importance, and here the
characterization of the element in question is at present of more
importance than the type of combination.
The instructor will at once notice on glancing through these
pages that for a rapid and successful carrying out of the pro-
cesses certain requirements are to be met as regards apparatus
and laboratory facilities; these should at all times be available
for the student's use.
The experiments necessary for working out and testing the
methods prescribed were carried out during the last eight years
with the aid of students at the University of Kiel, and to some
extent at the Universities of Gottingen and Clausthal. Published
processes have frequently been changed or improved according to
our experience, though no doubt it will still be found that the
directions and yields can be further improved.