84 A MANUAL OF BACTERIOLOGY 



written on the label. Every morning this shelf 

 should be examined, and the hardening solution 

 changed in those requiring it, the date being each 

 time written on the label, so that it may be seen 

 at a glance how long the tissue has been in the 

 fluid, and whether the hardening agent ought to be 

 renewed. Muller's fluid and bichromate of potash 

 preparations may be placed by themselves, and 

 need only be looked at occasionally/ 



The best hardening agents are absolute alcohol, 

 methylated spirit, Muller's fluid, chromic acid solu- 

 tion, potassium bichromate solution, and osmic acid. 

 (1.) Pieces of an organ, etc., should be cut from 

 J in. to 1 in. cubes, and placed in one of the 

 hardening solutions. If absolute alcohol or methy- 

 lated spirit is used the tissues should remain in the 

 spirit from two to three days. Many delicate 

 tissues, however, cannot be placed in strong spirit 

 without shrinking ; to obviate this such tissues are 

 first placed in dilute spirit (one part of water to 

 two parts of spirit). In this mixture the tissues 

 remain about twenty-four hours and are then trans- 

 ferred to the strong spirit for one or two days. 

 After this they are ready for imbedding and cutting. 

 (2.) Muller's fluid is an excellent hardening agent. 

 To prepare it, dissolve two parts of potassium 

 bichromate, one part of sodium sulphate, and 100 

 parts of distilled water. The preparations to be 

 hardened should remain in the fluid from two to 

 three weeks. When the fluid becomes cloudy it re- 

 quires changing ; but it retains its hardening pro- 

 perties for a long time. The preparations, after 



