204 
Journal of Agricultural Research 
Vol. XXXI, No. 3 
As a routine procedure each rat on test is cared for daily, ex¬ 
cept Sunday, as follows: The self-feeder is examined and any feed 
that has been pulled into the catch pan or base is returned to the 
hopper, as well as any feed that has been scattered on the bottom of 
the cage. When nearly empty the feeder is removed and weighed, 
and it is replaced by a clean feeder filled with feed. The blotting 
paper is changed daily, except Sunday, and the drinking vessels are 
filled as often as necessary. The cages, feeders, and drinking vessels 
are cleaned and sterilized once a week. 
A complete record for each rat is kept. Growth curves for the 
several rats that are fed the same ration are platted on one sheet. 
All rations tested are numbered serially. 
Unless otherwise noted, the rations fed in the vitamin A tests were 
made up according to the following standard: 
Standard ration 
Protein (N x 6.25) 
Fat_ 
Ash mixture_ 
Dried yeast_ 
Starch, etc_ 
Per cent 
__ 20 
__ * 10 
__ 4 
__ 10 
__ 56 
100 
The 20 per cent of protein comprises that present in the dried meat 
which is being tested for vitamin A plus sufficient protein from dried 
vitamin-A-free muscle tissue or casern to make up the difference, the 
yeast protein not being included. The 10 per cent of fat is the sum 
of the fat in the dried meat and sufficient hardened cottonseed ol to 
make the required amount. The ash mixture is made up according 
to a formula of Drummond and Watson (1, p. 237). Dried baker’s 
yeast and cassava starch comprise the balance of the ration. The 
proportion of protein in the ration, as well as that of dried yeast, is 
considerably higher than is necessarv for normal growth; but this is 
a distinct advantage, since it guards against a deficiency of either 
protein or vitamin B becoming a limiting factor in the growth of the 
rats in case of reduced food intake due to a deficiency of vitamin A. 
In some rations in which relatively large proportions of dried meat 
were incorporated the percentage of protein, as well as that of fat, 
frequently exceeded the proportion stated in the standard ration. 
Ash mixture 
Grams 
Sodium chloride_ 5. 2 
Magnesium sulphate_ 8. 0 
Sodium dihydrogen phosphate, 10. 4 
Dipotassium hydrogen phos¬ 
phate_ 28.6 
Grams 
Calcium superphosphate_ 16. 2 
Calcium lactate_ 39. 0 
Ferric citrate_ 3. 5 
Potassium iodide_ Trace. 
Manganese sulphate_ Trace. 
Purification of Ingredients in Ration 
The greatest difficulty encountered in the estimation of the vitamin 
A content of a food product is the preparation of a basal ration free 
from vitamin A or containing a very small amount of it. After con¬ 
siderable experimentation the desired result has been accomplished. 
It was found that the cassava starch, dried yeast, and hardened cot¬ 
tonseed oil contained no vitamin A and so did not need purification. 
