The nutritive value of different kinds of food has also been represented theoretically, by supposing it to be very nearly in proportion to the quantity of nitrogen, or of gluten, which vegetables contain. Though this cannot be considered as a correct principle, yet as the ordinary kind of food on which stock is fed contains in general an ample supply of carbon for respiration, with a comparatively small proportion of nitrogen, these theoretical determinations are by no means without their value, and they approach in many cases very closely to the practical values above given, as deduced from actual trial. Thus, assuming that 10 lbs. of hay yield a certain amount of nourishment, then of the other vegetable substances it will be necessary, according to theory, to give the following quantities, in order to produce the same effect: Hay, 10 Turnips, 60 Clover hay, 8 Carrots, 35 Vetch hay,[26] 4 Cabbage, 30 to 40 Wheat straw, 52 Pease and Beans, 2 to 3 Barley straw, 52 Wheat, 5 Oat straw, 55 Barley, 6 Pea straw, 6 Oats, 5 Potatoes, 28 Indian corn, 6 Old potatoes, 40 Oil-cake, 2 [26] Both cut in flower. If the feeder be careful to supply his stock with a mixture or occasional change of food, he may very safely regulate the quantity of any one he ought to substitute for a given weight of any of the others, by the numbers in the above tables—since the theoretical and practical results do not in general very greatly differ. As has been already stated, it is not strictly correct that this or that kind of vegetable is more fitted to sustain animal life, simply because of the larger proportion of nitrogen it contains; but it is wisely provided, that along with this nitrogen in all plants, a certain proportion of starch or sugar and of saline and earthy matter are always associated—so that the quantity of nitrogen may be considered as a rough practical index of the proportion of some of the important saline and earthy ingredients also.