data in regard to this binding and shown that if for ether we substitute
an ether-alcohol mixture the removal of the "A" with the fat is fairly
complete even from vegetable sources. They advance the idea that
preliminary treatment with alcohol is a process which will materially
assist in breaking the attachment of the vitamine and render its removal
with the fat solvent effective. Butter-fat rich in the "A" vitamine has
been conclusively shown to be free of nitrogen and phosphorus and it is
generally assumed that the "A" vitamine is a nitrogen-free and phosphorus
free compound. Further than that however we know nothing of its nature.

Concerning the "C" we know only that it is like the "B," water-soluble and
we know somewhat of its properties, but nothing of its chemical nature.

One of the greatest difficulties still encountered in the study of
chemical fractions is the delay in identification of the active portion.
For this purpose we must rely on tests that are far from delicate and
time-consuming to a degree. As a result the study of only a few fractions
must extend over long periods of time with all the cumulation of
difficulties in the way of change in material, etc. that this delay
implies. An idea of these difficulties can best be obtained by a review of
our present methods for vitamine testing and these methods constitute the
subject matter of the next chapter.



CHAPTER III


THE METHODS USED IN TESTING FOR VITAMINES