Naturally, the individual proteoses show marked differences in their rate of conversion into secondary or final products. Take as an illustration some results[140] obtained with caseoses formed in the digestion of the casein of milk. Thus, heterocaseose, a primary product, yielded only fifteen per cent. of peptone after ninety-four hours at 40° C. with a strong pepsin-acid solution. Protocaseose, however, containing some deuterocaseose, under like conditions, yielded thirty-two per cent. of peptone in one hundred and nineteen hours, while pure deuterocaseose gave sixty-six per cent. of peptone in one hundred and thirty-seven hours. Evidently, then, the first-formed soluble products of gastric digestion, _i. e._, the primary proteoses, are only slowly converted into peptone, since they must first pass through the intermediate stage of deuteroproteose, which is plainly not a rapid process. The deutero-body, on the other hand, once formed is more rapidly converted into peptone, but even this is in no sense a rapid process. Hence, in the artificial digestion of proteids with pepsin-hydrochloric acid, solubility of the proteids may be quite rapid, and even complete in a very short time, but the resultant products will be mainly proteoses and not peptones. The latter are truly formed and in considerable amount, but proteoses, either as primary or secondary bodies, are invariably present and usually in excess of the peptones. [140] Chittenden and Hartwell, loc. cit., p. 22. In this connection the question naturally arises how far we are to trust these results in their bearing on the natural process of digestion as it occurs in the living stomach. Obviously, the conditions are quite different in the two cases. In artificial digestions, we have especially the influence of an ever-increasing percentage of soluble products on the activity of the ferment, a condition of things generally considered as more or less inhibitory to enzyme action. We have attempted to measure the real value of this influence by experiments[141] conducted in parchment dialyzing tubes, in which the conditions are made favorable for the removal of at least some of the products of digestion as fast as they are formed. In these experiments, the dialyzer tubes containing the proteid and pepsin-acid were immersed in a large volume of 0.2 per cent. hydrochloric acid (about three litres), which was gradually changed from time to time, the whole mixture being kept at 40° C. during the entire period of the experiment. The extent of peptonization was then ascertained by analysis of both the contents of the dialyzer tubes and of the surrounding acid, the results being compared with those obtained from control experiments carried on in flasks. Without considering the results in detail, it may be mentioned that the slow and incomplete peptonization so characteristic of artificial gastric digestion is not materially modified by this closer approach to the natural process.