It is clear, then, from what I have stated, that neither the amount of flesh-formers, nor of fat-formers, contained in a given quantity of a substance is a measure of its nutritive value; nevertheless it would be incorrect to infer from this that the numerous analyses of feeding substances which have been made are valueless. On the contrary, I am disposed to believe that the composition of these substances, when correctly stated by the chemist, enables the physiologist to determine pretty accurately their relative alimentary value. Theory is certainly against the assumption that food is valuable in proportion to its content of nitrogen; nor has practice less strongly disproved its truth. An illustration drawn from the nutrition of plants will make this matter more apparent. Every intelligent agriculturist knows that guano contains nitrogen and phosphoric acid; both substances are indispensable to the development of plants, and therefore it would be incorrect to estimate the manurial value of the guano in proportion to the quantity of nitrogen it was capable of yielding. If the value of manures were determined only by their per-centage of nitrogen--a mode by which certain chemists still estimate the nutritive value of food--then woollen rags would be worth more than bones, and bones would be more valuable than superphosphate of lime. The truth is, that the analysis of feeding stuffs and manures is sometimes of little value if the condition in which the constituents of these substances exist be undetermined. For example, the analysis of one manure may show it to contain 40 per cent. of phosphate of lime, and three per cent. of ammonia, whilst, according to analysis, another fertiliser may include 20 per cent. of phosphate of lime, and two per cent. of ammonia. Viewed by this light solely, the first manure would be considered the more valuable of the two, whereas it might, in reality, be very much inferior. If the phosphate of lime in the manure, containing 40 per cent. of that body, were derived from coprolites or apatite, and its ammonia from horns, the former would be worth little or nothing, and the latter, by reason of its exceedingly slow evolution from the horns, would possess a very low value. If, on the contrary, the phosphate of lime, in the manure comparatively poor in phosphate, were a constituent of bones, and its ammonia ready formed (say as sulphate of ammonia), then, its value, both commercial and manurial, would be far greater than the other.