The proper adjustment of the relative quantities of the great groups of nutritive elements in the food is a matter the importance of which cannot be over-rated, for it is in fact the foundation of successful and economical feeding; and this will be readily understood if we consider what would be the result of giving to an animal a supply of food containing a large quantity of nitrogenous and a deficiency of fat-forming compounds. In such circumstances, the animal must either languish for want of the latter, or it is forced to supply the defect by an increased consumption of food, in doing which it must take into the system a larger quantity of nitrogenous compounds than would otherwise have been requisite, and in this way the other elements, which are present in abundance, are wasted, and the theoretical and practical value of a food so constituted may be very different, and it is only when the proportions of the different groups are properly attended to that the most economical result can be obtained. It can scarcely be said that the experiments yet made by feeders enable us to fix the most suitable proportion in which those substances can be employed, although experience has led them to the use of mixtures which are in most cases theoretically correct; thus they combine oil-cakes or turnips with straw, which is poor nitrogenous, and rich in fat-forming elements; and in general it will be found that where different kinds of food are mixed, the deficiencies of the one are counterbalanced by the other, and though this has hitherto been done empirically, it cannot be doubted that as our knowledge advances it will more and more be determined by reference to the composition of the food.