GENERAL TECHNIC 9 



is the fact that the mercuric chlorid sometimes produces dark, 

 irregular precipitates in the tissues. This may be remedied, however, 

 by the use of iodine and iodid of potassium in the hardening process 

 (see under Hardening, p. lo). 



(ii) Pi'mf aa(/ is an excellent fixative for cytoplasm. It may be 

 used in: (a) Saturated aqueous solution; (b) saturated solution of 

 picric acid in i-per-cent. aqueous solution of acetic acid; (c) saturated 

 solution of picric acid in 2-per-cent. aqueous solution of sulphuric 

 acid. 



II. Hardening 



Most fixatives are also hardening agents if their action is pro- 

 longed. This is, however, often detrimental. It is, therefore, 

 customary, after fixation is complete, to carry the specimens, with or 

 without washing, through successively stronger grades of alcohol 

 for the purpose of hardening the tissues. For general histological 

 purposes the specimens may be transferred directly to 70-per-cent. 

 or 80-per-cent. alcohol, which should be changed once or twice. 

 In the case of delicate tissues the first grade of alcohol should be 

 40-per-cent. or 50-per-cent., the second 70-per-cent., and the third 

 80-per-cent. The specimens should remain in each grade from 

 twelve to twenty-four hours. 



Washing the tissues after fixation is not a matter of indifference. 

 In some cases water should be used, while in other cases water is 

 liable to undo the action of the fixative, in which cases alcohol must 

 be used for washing. 



After fixation in alcohol no washing, of course, is necessary. 

 Specimens fixed in strong alcohol are embedded immediately (see 

 Embedding, p. 11), or preserved (see Preserving, p. 10). After 

 fixation in dilute alcohol the specimens are passed through the graded 

 alcohols up to 80-per-cent. 



After fixation in formalin, specimens are placed directly in 96- 

 per-cent. alcohol without washing in water. Specimens fixed in any 

 solution containing picric acid should not be washed in water, but 

 passed directly through the alcohols; and it is usually necessary to 

 change each grade in order to wash out the picric acid. 



Specimens fixed in osmic acid or any solution containing osmic 

 acid should be washed in running water before being passed through 

 the graded alcohols. After solutions containing potassium bichro- 

 mate the specimens should be washed in water sufficiently to remove 



