The above facts furnish us, I think, a good illustration of how dependent these proteolytic enzymes are upon the proper conditions of temperature, to say nothing of other conditions, for the full exercise of their peculiar power. Toward acids, alkalies, metallic salts, and many other compounds they are even more sensitive than toward heat, and much might be said regarding the effects, inhibitory or otherwise, produced by a large number of common drugs or medicinal agents on these two ferments. Any lengthy discussion of this matter, however, would be foreign to our subject, and I will only call your attention in passing to one or two points which have a special bearing upon the general nature of the enzymes. Take, for example, the influence of such substances as urethan, paraldehyde, and thallin sulphate on the proteolytic action of pepsin-hydrochloric acid[37] and we find that small quantities, 0.1 to 0.3 per cent. tend to increase the rate of proteolysis, while larger amounts, say one per cent., decidedly check proteolysis. Similarly, among inorganic compounds, arsenious oxide, arsenic oxide, boracic acid, and potassium bromide[38] in small amounts increase the proteolytic power of pepsin in hydrochloric acid solution, while larger quantities check the action of the ferment in proportion to the amounts added. Again, with the enzyme trypsin, similar results with such salts as potassium cyanide, sodium tetraborate, potassium bromide and iodide[39] may be quoted as showing not only the sensitiveness of the ferment toward foreign substances, but likewise its peculiar behavior, viz., stimulation in the presence of small amounts and inhibition in the presence of larger quantities. [37] Chittenden and Stewart: Studies in Physiol. Chem., Yale University. Vol. 3, p. 64. [38] Chittenden and Allen, _Ibid._, vol. 1, p. 76. [39] Chittenden and Cummins, _Ibid._, vol. 1, p. 112.