normal growth. Assume that the addition of 5 grams of the unknown in 100
grams of the butter-free diet fails to produce normal growth but that by
adding 2 per cent of butter fat normal growth is reached. It is obvious
under these conditions that 5 grams of the unknown is equivalent in "A"
vitamine content to 5 minus 2 grams of butter fat, i.e., is equivalent to
3 grams of butter fat or expressed in per cents the substance contains 0.6
or 60 per cent of the "A" found in pure butter fat.

Experience has shown that it is dangerous to draw conclusions from
experiments of too short duration or to base them on too few animals. For
complete data the experiments should be carried through the complete life
cycle of the rat, including the reproductive period. Otherwise it may turn
out that the amount in the unknown while apparently sufficient for normal
growths is incapable of sustaining the drain made in reproduction. It is
this consideration that makes the accumulation of authoritative data on
vitamine contents of foodstuffs so slow and tedious and one of the reasons
why we lack satisfactory tables in this particular at present. Osborne and
Mendel raise another point of methodology and believe that more accurate
results will be obtained if the source of the vitamine is fed separately
than if mixed with the basal diet. It is easily possible that since one of
the effects of lack of vitamine, especially of the "B" type, is poor
appetite, the amount necessary to produce normal growth may be smaller
than would appear from results obtained by mixing it in the basal diet.
When so mixed the animals do not get enough to maintain appetite and
really decline because they do not eat enough rather than because the
amount of vitamine given is inadequate to growth. Details of this kind are
matters however that particularly concern the experimentalist and as our
purpose here is to merely describe the methodology we may perhaps turn now
to other types of testing. Before doing so it is perhaps unnecessary to
suggest that in all experiments it is important that the food intake