BACTERIOLOGICAL DIAGNOSIS. 



of tissue soaked in paraffin which is just melted for at 

 least twelve, and more often twenty-four hours. To do 

 this properly involves the use of some sort of an incu- 

 bator. This might possibly be rigged up out of a tin 

 biscuit-box in the manner already described, though 

 considerably more heat would be necessary, as the 

 paraffin melts at about 50 C. But the writer has often 

 embedded the blocks by placing them in bottles con- 

 taining the paraffin at such a distance from the fire that 

 the paraffin is never completely melted, but always 

 shows a thin solid layer on the surface. To do this it 

 is only necessary to look at the bottle occasionally and 

 move it a little further from the fire if the paraffin is 

 completely melted and vice versa. The process may be 

 stopped at night without any harm resulting, and if the 

 soaking only continues for a few hours at a time it is of 

 no consequence so long as the total period is made up. 



CASTING THE BLOCKS. 



Special metal moulds are used in the laboratory 

 (fig. 31). A pill box will do quite well. A small amount 

 of melted paraffin is poured into the box and the piece 

 of tissue is taken from the bottle containing the melted 

 paraffin with a pair of forceps (previously warmed so as 

 to prevent the paraffin from setting upon the points) and 

 placed in the paraffin in the pill box. It is necessary to 

 see that the surface from which sections are to be cut 

 should be placed downwards. The box is then filled 

 up with melted paraffin, and placed in a cool place or 

 surrounded with water. The moment a firm film has 

 formed over the surface the whole is plunged in cold 

 water to hasten the setting of the paraffin ; the more 



