
DIGHSTIBILITY OF FISH AND POULTRY. 12 
the nitrogen lag, z.e., the time elapsing between the ingestion 
of nitrogen in the food and its elimination in the urine, it was 
considered that this period could be taken as fairly representing 
the daily protein katabolism on the experimental diet. 
The weight and heat of combustion of the urine were deter- 
mined, and used in the computations of available energy, as 
explained beyond. ‘ 
Analyses of food, feces and urine.—The analyses of the food 
miaterials and feces were made according to the methods adopted 
by the Association of Official Agricultural Chemists.* In the 
_ duck-fat sample, only the water content was determined. The 
ash content of the butter samples was assumed to be 2.50 per 
cent. in each case. Fat, in both the duck-fat and butter sam- 
ples, was estimated by difference. Protein was taken through- 
out as equal to N X 6.25.T 
The heats of combustion were made in the usual way by 
means of the bomb calorimeter.{ The composition and heats 
of combustion of food materials and feces and the heats of com- 
bustion of the urine are given in Table 12. | 
From the table it will be noted that the most striking differ- 
ences between the fish and poultry samples were as regards 
fitout ie duck. contained 28.43 per cent., the chicken) and 
salmon nearly one-half, and the cod less than one-sixth of 
this amount. The amount of fat in fresh cod in fact has been 
shown by analyses elsewhere to be very small, averaging about 
.5 of one per cent. In this case, the higher fat content is due 
to the butter used in the frying. 
As regards protein, the poultry was somewhat richer, the 
difference being about 6 per cent.. The heat of combustion of 
the duck was highest, 4.244 calories per gram, that of the 
chicken and salmon about 2.6 calories per gram and that of the 
cod 1.616 calories per gram. The fact should not be lost sight 
of, however, that the composition of all these materials may be 
modified to a considerable extent, especially as regards water 

* U.S. Dept. Agr., Division of Chemistry, Bul. 46, revised. 
+ For strict accuracy, the protein of bread should be taken as N & 5.70, and that of 
total feces as the sum of the nitrogen from bread x 5.70and the nitrogen from other 
sources X 6.25. Sucha change, however, would not appreciably affect the coefficients 
of availability, and the use of the factor 6.25 secures uniformity with previous work. 
{ Journ. Amer. Chem. Soc. Vol. 25, No. 7 (1903), p. 659. 
9 
