714 A MANUAL OF PHYSIOLOGY . 



portion ten times its bulk of a 5 per cent, solution of magnesium 

 sulphate. Let stand, with frequent stirring, for twenty-four hours. 

 Then strain through several folds of linen, press out the residue, and 

 filter through paper, (i) With the filtrate of the watery extract make 

 the following observations : 



(a) Reaction. To litmus-paper acid. 



(b) Determine the temperatures, at which coagulation of the 

 various proteins in the extract takes place, according to the method 

 described on p. 8.* Put some of the watery extract in the test- 

 tube, and heat the bath, stirring the water in the beakers occasionally 

 with a feather. Note at what temperature a coagulum first forms. 

 It will be about 47 C. Filter this off, and again heat ; another 

 coagulum will form at 56 to 58. Filter, and heat the filtrate ; a 

 third slight coagulum may be formed at 60 to 65 C., but this 

 represents merely a residue of the myosinogen which was left in 



FIG. 265. ARRANGEMENT FOR MEASURING THE VELOCITY OF THE NERVE- 

 IMPULSE. 



A, travelling plate of spring myograph ; M, muscle lying on a myograph plate; 

 N, nerve, lying on two pairs of electrodes, E and E' ; C, Pohl's commutator 

 without cross wires ; K, knock-over key of spring myograph (only the binding- 

 screws shown) ; K', simple key in primary circuit ; B, battery ; P, primary coil ; 

 S, secondary coil. 



solution at the previous heating. A fourth precipitate (of serum- 

 albumin) will come down at 70 to 73. Saturate some of the watery 

 extract with magnesium sulphate ; a large precipitate will be formed, 

 showing the presence of a considerable amount of globulin. Filter 

 off the precipitate and heat the filtrate ; coagulation will again occur 

 at very much the same temperatures as before, although the total 

 amount of precipitate will be less. Note in particular that there 



* It should be remembered that the temperature of heat-coagulation of 

 any substance is by no means an absolute constant. It depends on the 

 reaction, the proportion and kind of neutral salts present, perhaps on the 

 strength of the protein solution and the manner of heating. A solution 

 of egg-albumin, e.g., can be coagulated at a temperature much below 70 

 when it is heated for a week. Small differences in the temperature of 

 heat-coagulation, unless supported by well-marked chemical reactions, 

 are not enough to characterize protein substances as chemical individuals. 



