10 DEPARTMENT BULLETIN 1492, U. S. DEPT. OF AGRICULTURE 
The last column (Ration Group D) in Table 1 relates to pigs 
slaughtered at weights under 125 pounds, and figures for the refrac- 
tive index only are given. Sufficient data on iodine numbers were 
not available to determine the limits, but so far as noted the values 
parallel those of the refractive index. The limits of grades for 
thf* refractive index are in most cases 2 points higher than in Ration 
Group B and about 3 points lower than in Ration Group A. Ex- 
planation of this behavior appears to center in the adipose tissue 
rather than in the fat itself. Young pigs usually have a relatively 
thin layer of adipose tissue which has not become filled with fat. 
The composition of the adipose tissue has been found to vary with 
the fatness and size of the animal. The protein and water content 
decrease while the fat content increases as the animal takes on 
weight and finish. 
The smaller quantity of fat in the adipose tissue with consequent 
higher quantity of protein tissue of the lightweight and thin pigs 
appears to result in a measurable masking of the real firmness or soft- 
ness of the fat. The low relative thickness of the adipose tissue acts 
in a similar manner. In many instances there are wide variations in 
the fatness of animals of the same weight within the 125-pound limit. 
On a corn ration the fat pigs of 100 pounds are usually more com- 
parable, from the grading standpoint, to animals weighing consider- 
ably more than 125 pounds. This, in part, is explained by the greater 
firmness of the body fat as shown by the refractive index and also by 
the character of the adipose tissue. 
It appears, from an examination of the data on a large number of 
lightweight " check " pigs, that, from the market standpoint, the car- 
cass grading is a safe criterion of the condition of the carcass. How- 
ever, greater emphasis must be placed on the actual composition of 
the fat in studying the changes produced by feeds and other factors 
on the body fat. There have been numerous instances in which 
variations in the feed before the experimental period, which were par- 
ticularly reflected in the refractive index and not in the gradings, 
have had a marked influence on the final results of the experimental 
feeding. 
In addition to the determining of grade limits, further comparison 
has been made of the parallelism between the refractive index and 
the iodine number as they measure increasing softness or hardness. 
Group averages of iodine numbers have been made which are com- 
parable, so far as a smaller number of determinations permit, with 
the refractive-index averages used in preparing the various graphs of 
this bulletin and of Department Bulletin 1407(4). From com- 
parison of the data on the same samples, corresponding values for 
the two constants were determined and a complete scale of the iodine 
number which paralleled the refractive index was constructed. The 
results from the rations listed in Group A of Table 1 were used for 
the most part in constructing this parallel scale, shown in Figure 4. 
The refractive index is given at the left and iodine number at the 
right as abscissas. The weights of the hogs are charted as ordinates 
to show progressive changes in the two values. 
The results shown in Figure 4 confirm the statement that the iodine 
and refractive-index values are usually in close agreement. The 
rations listed in Group A of Table 1 have furnished values showing a 
