kofer in the "Zeitschrift für Biologie" (1866, p. 466). In the case before us, however, the investigators contented themselves with weighing the food eaten and estimating its contents of nitrogen. We have seen that Professor Flint elsewhere insists upon the importance of the food eaten in its effects on the excretion of nitrogen, but, both in his own experiments and Dr. Pavy's, there were, according to their own estimates, great variations in the amount of nitrogen ingested from day to day, as, for example, 65·68 grains and 161·72 grains, or, on another occasion, 522·42 grains and 871·92 grains on two successive days in Dr. Pavy's experiments, and 144·70 grains and 383·04 grains in Professor Flint's.
Such great and sudden variations as these could not but impair the accuracy of the experiments, and cause corresponding fluctuations in the amount of nitrogen excreted, as has been sufficiently shown by the investigations of Voit and others already alluded to, and the results bear testimony that such was the case.
Furthermore, not only did the quantity of nitrogen ingested from day to day vary, but even these varying amounts were not accurately determined by analysis, but simply, with a few unimportant exceptions, estimated from the average composition of similar articles as given by Payen. Neither Dr. Pavy nor Professor Flint appears to have even taken the trouble to estimate the water of the various articles of food, but to have simply weighed them in the fresh state—a fact which alone deprives the results of all claim to strict accuracy, since the water content of such articles as fresh meat or bread, for example, is quite variable, and the proportion of nitrogen in the fresh substance of course varies correspondingly. While such a method may give an approximation to the truth, it is impossible that, when applied to such a varied diet as that taken in these experiments, it should give results of scientific exactness.
The estimations of the ureal nitrogen appear to have been made after approved methods, and are to be assumed to be correct; but, even if we assume the accuracy of the estimates of nitrogen in the food as well, the results of Dr. Pavy do not show what he claims for them. They do, indeed, show that there was an increase in the average daily excretion of nitrogen during work over that during rest of 194·12 grains, and, at the same time, an increase of 201·63 grains in the average amount of nitrogen daily ingested. The only conclusion which can be drawn from these figures is, that during work more nitrogen was excreted because more was taken in the food. That muscular exertion caused any increase in the excretion of nitrogen we have no evidence.
With Professor Flint's experiments the case is somewhat different. There the amount of proteine taken in the food was considerably less during work than during rest, while the excretion of nitrogen remained about the same, so that the relative excretion was increased.
