242 THE COAGULATION OF MILK. 



even down to the commencement of the present (nineteenth) cen- 

 tury. The prevalent opinion, based on the curdling of sour milk, 

 was that the precipitation effected by rennet was indirect, an acid 

 being first formed which then caused the precipitation of the 

 curd. The elucidation of the true state of the case was reserved 

 for BERZELIUS (I.) in 1840, and his discovery was soon afterwards 

 supplemented by the labours of FR. SELMI (I.), C. G. LEHMANN (I.), 

 HEINTZ (I.), and VOELCKER (I.), who showed that the action of 

 rennet is quite independent of the formation of acid. 



The identity of this enzyme with the pepsin discovered by 

 SCHWANN ( 18) was disproved by 0. HAMMARSTEN (I.) in 1872. 

 This observer was the first to successfully separate these two gastric 

 secretions, an operation which DESCHAMPS (I.) had failed to effect 

 thirty-two years earlier. Hammarsten, however, did not make use 

 of the name chymosin, proposed by his predecessor, but adopted 

 the ancient appellation, lab. "We are also indebted to the Swedish 

 chemist for deep researches into the activity of this substance. It 

 acts on casein alone, not on lactose or lactalbumin. 



A divergence of opinion still prevails as to the nature and 

 course of this reaction, and we will therefore merely refer to the 

 investigations made on these points by the undernamed workers : 

 A. DANILEWSKY and P. RADENHAUSEN (I.), W. EUGLING (I.), F. 

 SCHAFFER (II.), E. DUCLAUX (VIII.), M. ARTHUS and C. PAGES (I.), 

 A. S. LEA and W. S. DICKINSON (I.), S. RINGER (I.), P. WALTHER(!.), 

 and A. FICK (I.). SCHREINER (I.), in 1877, showed that milk when 

 boiled is no longer coagulable by rennet, but the reason for this 

 behaviour was not ascertained until eleven years later, when FR. 

 SOLDNER (I.) found that the lab reaction can only proceed in the 

 presence of soluble salts of lime, which latter are precipitated by 

 boiling. For the same reason coagulation does not occur in milk 

 that has been rendered alkaline ; and the most favourable condition 

 is one of very slight acidity. The constitution of this enzyme is 

 still unknown. Its decomposing power is unusually high, one part 

 by weight being (according to Soldner) sufficient to throw down at 

 least one hundred million parts of casein. As A. MAYER (III.) 

 has shown, the optimum temperature for the reaction is 37 C., the 

 coagulation taking three times as long at 25 C. Above 45 C. the 

 enzyme is paralysed, and is destroyed at 70 C. Coagulation does 

 not ensue immediately upon the addition of the lab, but only after 

 a lapse of from several minutes to some hours, according to the 

 temperature. 



The occurrence of this enzyme in Nature is by no means rare. 

 It was found by Roberts in the stomach of birds, and in that of 

 fishes by Benger, and is also present in the cell-sap of various 

 plants, e.g. of butterwort (Pinguicula vulgaris and P. alpind), 

 withania (Punceria coagulant), fig-tree (Ficus carica), artichoke 

 (Cynara scolimus), and others. It is also excreted by several 

 species of Schizomycetes, particulars of which will be given in the 



