next few weeks will show whether that amount supplies enough for normal
growth, comparison being made with the normal weight curve for a rat of
that age.

In this method it is assumed that the amount of yeast cake added will not
derange the proportions of protein fat, etc., in the basal diet enough to
affect optimum conditions in these respects. This is a curative type of
experiment. If we wish to develop a preventive experiment the yeast cake
may be incorporated in the diet from the first and the amount necessary to
prevent deviation from the normal curve determined. Both methods are
utilized, the one checking the other. If however the amount of the
substance necessary to supply the vitamine required for normal development
is large such addition would of course disturb the proportions of
nutrients in the normal diet and in that case analysis must be made of the
substance tested to determine its protein, fat, carbohydrate and salt
content and the basal diet corrected from this viewpoint so as to retain
the optimum proportions of these factors. McCollum's cereal testing
combinations are illustrative of such methods applied to cereals. Still
another method is to add a small per cent. of the unknown and then add
just enough of the vitamine tested to make sure that normal growth
results. Such a method gives the results in terms of a known vitamine
carrier. For example, if we add to a basal diet, sufficient in all but the
"A" vitamine (Steenbock's mixture for example), a small per cent of a
substance whose content in "A" is unknown and note that growth fails to
result we can then add butter fat until the amount just produces normal
growth. If now we know just what amount of butter fat suffices for this
purpose when used alone we can calculate the part of the butter which is
replaced by the per cent of unknown used. To put this in terms of figures
will perhaps make the idea clearer. Let us assume that 5 per cent of
butter fat in a given diet is sufficient to supply the "A" necessary for