2 BULLETIN 310, U. S. DEPARTMENT OF AGRICULTURE. 
mined it has not been possible to correlate melting point with digesti- 
bility in an exact way on the basis of experiments hitherto made. 
The practical importance of further investigations on the subject is 
indicated by the fact that the common domestic practices in the use 
of fats, as of other food materials, is at present based very largely on 
empirical knowledge, and quality in the resultant product depends 
more upon experience and acquired skill than it does upon knowledge 
of the physical and chemical properties of the ingredients used and 
of their relation to the desired object. Yet it is obvious that the 
whole matter is one which can not be definitely understood, stand- 
ardized, and controlled unless such knowledge is available. A full 
understanding of the materials, of the nature of household methods 
of handling them, and of the resulting effects is necessary as a basis 
for real economy as well as for more rational and satisfactory use of 
foodstuffs. The right use of food materials must be governed also 
by a knowledge of their digestibility and their value as sources of 
energy available to the body. Neither can one overlook the modi- 
fications which are due to the combination of ingredients into foods 
and to the combination of foods to form meals. 
It seemed desirable, therefore, from both the scientific and the 
practical standpoint to study the digestibility, of the more common 
culinary and table fats prepared in a comparable manner and incor- 
porated in a uniform basal ration. It was with this idea in mind 
that the digestion experiments with four of the more common animal 
fats reported in this bulletin were undertaken. These experiments 
form part of an extended study of the food value and household 
uses of animal and vegetable fats of different sorts, having to do with 
(1) the thoroughness of digestion of the fats, (2) the specific dynamic 
effect of a diet rich in fats of different kinds, (3) the relation of 
the available (net) to the total (gross) energy values of fats, and (4) 
the relation of domestic ways of using fat to the quality of foods. 
NATURE OF THE DIET. 
The experiments here reported were made with beef tallow, mutton 
tallow, lard, and butter, which were purchased in the open market. 
The butter was a good commercial grade and was used as purchased. 
The beef, mutton, and pork fats were kidney fats. A fairly large 
quantity of each was rendered in the laboratory in order to provide 
uniform material for use in each series of experiments. The fat was 
cut into small pieces, freed from any noticeable muscular tissue, and 
finely ground in a meat cutter such as is used in the home. It was 
then rendered in a double boiler, when it was found that the fat sep- 
arated readily from the inclosing tissues (the temperature during 
rendering did not quite reach 100° C). The fat was finally -strained 
through medium fine huckaback and heated, but not much abo^e 
