Another enzyme which may be of importance in puering is erepsin, the enzyme of the intestinal juice, which is responsible for the completion of the digestive process. The pancreatic enzymes act upon the peptones produced by the pepsin of the stomach, splitting them up into simpler compounds, while the erepsin acts further upon these products. It dissociates albumoses and peptones into amino acids, taking as it were the last traces of nutriment from the food passing through the intestine. It acts best in alkaline solutions. This ferment is very widely distributed in the animal kingdom, and occurs in other organs and tissues besides the intestines. The quantity of erepsin in the fresh fæces must be considerable, since a dog secretes from 400 to 500 c.c. of intestinal juice per day. It remains to be shown whether this retains its properties after excretion, and for how long. A most important point in connexion with the activity of enzymes is the reaction of the medium, i.e. its acidity or alkalinity, or, more strictly speaking, its hydrogen ion concentration. A very slight increase or decrease of the acidity or alkalinity of the liquor will diminish the rate of action of the enzyme by a large amount, and in some cases cause the action to cease. In all cases enzymes have an optimum acidity or alkalinity; in other words, for every enzyme there is a particular hydrogen ion concentration at which its activity is at a maximum. The work of Soerensen, to which I have already referred, gives a very complete account of this aspect of enzyme action and also of the methods he employed to investigate it. His work should certainly be carefully studied by anyone wishing to take up this part of the subject. The same remarks as to optimum conditions apply to temperature, although the effect of this is better known. Most tryptic enzymes act best at the body temperature, viz. 98° to 100° F. (37° to 40° C.), and hence puering should be conducted at this temperature. In the case of the hen or pigeon-dung bate the enzymes have not yet been studied, so far as I am aware, so that it is not possible to give an account of them. At the same time it seems highly probable that, as these bates are employed at comparatively low temperatures, the enzyme action is kept back, and therefore the chief action would be a chemical one.