EXPERIMENTS ON FIXATION 317 



A flocculent coagulum forms instantly throughout the fluid and 

 remains there. 



Note that the final concentration of ferric sulphate in each tube 

 is 10/ 



^* 2 /O' 



The coagulation of egg-albumin by mercuric chloride 



The following experiments illustrate the effect of acidity on 

 coagulation, and of sodium chloride and potassium iodide on the 

 coagulates. 



Add to egg-white twice its volume of distilled water. Centrifuge 

 to clear. 



Put 3 ml of the albumin solution in each of two test-tubes 

 marked A and B. 



Make these solutions : — 



A. Mercuric chloride, sat. aq. . . . 6-5 ml 

 Distilled water . . . . . 0-5 ml 



B. Mercuric chloride, sat. aq. . •6-5 ml 

 Acetic acid, glacial ..... 0-5 ml 



Add solution A to test-tube A; close; turn upside down once. A 

 thick white clot forms at once throughout the fluid ; the next day 

 it has fallen somewhat. 



Add solution B to test-tube B; close; turn upside down once. 

 Fine white flocculi form at once throughout the fluid; the next 

 day the appearance is unaltered. 



The experiment shows that acetic acid reduces the coagulating 

 power of mercuric chloride. 



To each tube add 2 ml of a saturated solution of sodium 

 chloride ; close the tube and turn upside down once. The coagulum 

 in A dissolves ; that in B remains unaltered. 



The experiment may be repeated with the substitution of potas- 

 sium iodide for sodium chloride. The result is essentially the same, 

 but the coagulate in A turns bright orange momentarily before 

 dissolving, while that in B turns bright orange momentarily and 

 then reverts to its former state : it does not dissolve. 



(Whether sodium chloride or potassium iodide be used, the 

 fluid in A will very gradually become cloudy on prolonged 

 standing.) 



The coagulation of nucleoprotein by various fixatives 

 Make a 4% aqueous solution of nucleoprotein, prepared from the 

 thymuses of calves by the method given on p. 314. 



