138 THE DIGESTIVE FLUIDS. 



to 0.3 per cent., it is capable of dissolving albumins, with the forma- 

 tion of albumoses and so-called peptones (see below). This can 

 readily be demonstrated as follows : an artificial gastric juice is 

 prepared by dissolving a pinch of one of the commercial prepara- 

 tions of pepsin in dilute hydrochloric acid (0.1-0.2 per cent.) to 

 which a flake of boiled beef-fibrin is then added, The mixture is 

 kept at a temperature of about 40 C., when it will be noted that 

 after a short time the fibrin begins to swell and is subsquently dis- 

 solved. In the solution which thus results albumoses and peptones 

 can be demonstrated. Other acids, such as sulphuric acid, nitric 

 acid, phosphoric acid, lactic acid, and even acetic acid, are also 

 capable of rendering pepsin physiologically active, but much larger 

 amounts are necessary to bring about the same result. In the case 

 of phosphoric acid, for example, an acidity of 10-12 pro mille is 

 necessary. Carbonic acid and hydrocyanic acid, on the other hand, 

 are without effect. 



Unlike chymosin, ordinary pepsin does not bring about the coag- 

 ulation of casein, but Pekelharing has shown that in acid solution 

 the pure substance also coagulates milk. It further causes the 

 precipitation of so-called plastein (see below) in concentrated 

 solutions of albumoses. 



In neutral and alkaline solutions pepsin is inactive ; it is rapidly 

 destroyed by sodium carbonate, even in very small amount. Its 

 resistance to higher temperatures is to a great extent dependent upon 

 the reaction of its solutions. In neutral solution it is destroyed at 

 55 C. ; in the presence of 0.2 per cent, of hydrochloric acid this 

 result is reached only at 65 C., and in the presence of peptones 

 and certain salts a temperature of 70 C. is necessary to bring about 

 the same end. In the dry state, on the other hand, the ferment 

 may be heated to 100 C., and even higher, without being de- 

 stroyed. At temperatures lower than 40 C. pepsin is still active, 

 but less energetically so, and at C. its action ceases altogether. 



Alcohol precipitates pepsin from its solutions without effecting its 

 subsequent activity, unless the exposure has been prolonged. Some 

 of the salts of the heavy metals, such as the acetates of lead and 

 platinum chloride, as also tannic acid, magnesium carbonate, and 

 ammonium sulphate, likewise cause the precipitation of impure 

 forms, at least, but are without effect upon the ferment itself. 

 Uranyl acetate is an excellent reagent ror the precipitation of 

 ferments (and albumins), even with a neutral or feebly acid reaction. 

 Like the albumins, pepsin does not diffuse through animal mem- 

 branes, but is precipitated. 



To a certain extent the rapidity of digestion is dependent upon 

 the amount of pepsin that is available ; but, as in the case of all 

 ferments, very small quantities are sufficient to effect an amount of 

 chemical change that is apparently out of all proportion to the 

 amount present. Thus, Petit claims that a pepsin preparation, 

 which he prepared himself, was capable of dissolving 500,000 times 



