l86 BACTERIOLOGICAL DIAGNOSIS. 



for themselves. It happens to every medical man to 

 find it necessary to send tumours, &c. to a laboratory to 

 obtain a pathological diagnosis ; and in very many 

 cases the materials are treated in a way which abso- 

 lutely prevents good sections from being obtained. 

 Many fixing fluids are in use, and any of them may 

 be selected, but it is absolutely necessary that the 

 material to be investigated .should be cut into small 

 pieces and put into a large bulk of the fluid at once. 

 This is especially necessary in the case of material 

 removed at a post-mortem examination, where the 

 tissues and organs have already undergone alteration. 



As regards the size of the slices which are to be 

 placed in the hardening fluid, it is sufficient to say that 

 they should never exceed J inch in thickness, and if 

 perchloride of mercury is used should be even thinner. 

 The other dimensions of the block are of less importance. 



The bulk of the fluid in which the block is placed 

 should be at least twenty times that of the block, and it 

 is not advisable to place two blocks in the same vessel. 



The fluids which we shall recommend for this purpose 

 are : 



i. Perchloride of mercury in normal saline solution. 

 This is prepared by dissolving common salt in water 

 in the proportion of seven grammes to a litre (about 3^ 

 grains to the ounce), and saturating this solution whilst 

 hot with perchloride of mercury. The solution must be 

 allowed to cool completely ;'as it does so crystals of the 

 mercury salt will separate out. 



This fluid fixes completely in twenty-four hours, or 

 less, and gives most excellent results. Its powers of 

 penetration are not very great, so that slices of tissue 

 which are to be fixed in it should be thin. 



The after-treatment of the blocks fixed in this fluid 



