K_2HPO_4 . . . . . . . . . . . . . . . . . . . . . . . 1.115
Ca lactate . . . . . . . . . . . . . . . . . . . . . . 0.289
Na_2HPO_4:l2H_2O . . . . . . . . . . . . . . . . . . . 0.526
Ca_2H_2(PO_4)_2:H_2O . . . . . . . . . . . . . . . . . 1.116
Fe citrate . . . . . . . . . . . . . . . . . . . . . . 0.138
No. 35 consisted of:
NaCl . . . . . . . . . . . . . . . . . . . . . . . . . 1.00
CaCO_3 . . . . . . . . . . . . . . . . . . . . . . . . 1.5

The very nature of these basal diets suggests their use. In general
however their utilization for testing purposes is based on the following
principles: Since the basal diet supplies all the requirements of a food
except the vitamine for which one is testing, it is simply necessary to
add the unknown substance as a given percent of the diet and observe the
results. If the amount added is small it is assumed that its addition will
not appreciably effect the optimum concentrations of nutrients, etc., and
for such experiments no allowances are made for the constituents in the
unknown. For example let us assume that we wish to test the value of a
yeast cake as a source of "B" vitamine. We first select a sufficient
member of rats of about thirty days age to insure protection from
individual variations in the animals. The age given is taken as an age
when the rats have been weaned and are capable of development away from
the mother and as furnishing the period of most active growth. These rats
are now placed on one of the basal diets which in this case supplies all
the requirements except the "B" vitamine. In this experiment any of the
diets of Osborne and Mendel or of McCollum will do that have been labelled
"A" _only_. After a week or so on this diet they will have cleared
the system of the influence of previous diets and their weight curves will
be either horizontal or declining. If now we make the diet consist of this
basal diet plus say 5 per cent of yeast cake, the weight curve for the