In Dr. Edward Smith’s treatise on ‘Food,’ the foot-pound equivalent of each kind of food is specifically stated in such a manner as to lead the student to conclude that this represents its actual working efficiency _as food_. Other modern writers represent it in like manner. Here, then, comes the bearing of these theories on my subject. A practical dietary or _menu_ is demanded, say, for navvies or for athletes in full work; another for sedentary people doing little work of any kind. According to the new theory, the best possible food for the first class is fat, butter being superior to lean beef in the proportion of 14,421 to 2,829 (Smith), and beef fat having nearly eight times the value of lean beef. Ten grains of rice give 7,454 foot-pounds of working-power, while the same quantity of lean beef gives only 2,829; according to which 1 lb. of rice should supply as much support to hard workers as 2½ lbs. of beefsteak. None of the modern theorists dare to be consistent when dealing with such direct practical applications. I might quote a multitude of other palpable inconsistencies of the theory, which is so slippery that it cannot be firmly grasped. Thus, Dr. Pavy (page 403), immediately after describing bacon fat as ‘the most efficient kind of force-producing material,’ and stating that ‘the _non-nitrogenous_ alimentary principles appear to possess a higher dietetic value than the _nitrogenous_,’ tells us that ‘the performance of work may be looked upon as necessitating a _proportionate supply_ of _nitrogenous_ alimentary matter,’ and his reason for this admission being that such nitrogenous material is required for the nutrition of the muscles themselves. A pretty tissue of inconsistencies is thus supplied! Non-nitrogenous food is the best force-producer—it corresponds to the fuel of the steam-engine; the nitrogenous is necessary only to repair the machine. Nevertheless, when force production is specially demanded, the food required is not the force-producer, but the special builder of muscles, the which muscles, according to theory, are _not_ used up and renewed in doing the work. It must be remembered that the whole of this modern theoretical fabric is built upon the experiments which are supposed to show that there is no more elimination of nitrogenous matter during hard work than during rest. Yet we are told that ‘the performance of work may be looked upon as necessitating a proportionate supply of nitrogenous alimentary matter,’ and that such material ‘is split up into two distinct portions, one containing the nitrogen, which is eliminated as useless.’ This thesis is proved by experiments showing (as asserted) that such elimination is not so proportioned.