FIXATION OF BLOOD-FILMS 203 



tubercle bacillus answers well. It is mounted upon a tripod 

 stand, and the heat applied at one end. After a time the tem- 

 perature of various portions of the plate is tested by the 

 application of a few drops of water ; the point at which the drop 

 assumes the " spheroidal state " {i.e., takes the form of a sphere, 

 and does not wet the plate) is about the right point to use. The 

 slides or films are placed at this point for the appropriate time. 



II. Fixation by Perchloride of Mercury. — Flood the film with a 

 saturated watery solution of perchloride of mercury, allow to act 

 for a minute, and wash for a minute under the tap, or by washing 

 it in a vessel of water. 



This method of fixation is about the best that can be used for 

 general purposes. 



III. Fixation by Alcohol and Ether. — This is very simple ; the 

 films are placed in a mixture of equal parts of alcohol and ether 

 for at least half an hour. 



This method of fixation is good, but slow. 



IV. Fixation by Formalin. — There are several methods by which 

 the fixative action of formalin can be used for blood-work. Of 

 these, the use of a mixture of i part of formalin with 9 parts 

 of absolute alcohol answers perfectly. The films are immersed in 

 this for half a minute, and then washed thoroughly under the tap. 



This method of fixation is both good and rapid. 



Staining Blood-films for the Investigation of their Cells. 



There are a great many methods of staining blood-films, and 

 all depend upon the division of stains into two varieties, the acid 

 and the basic. All the stains which are used in this branch of 

 histology are salts, and in some of these salts the acid radicle 

 does the staining, in others the basic. 



Acid stains are those in which the colouring property resides in 

 the acid of the salt. A familiar example is picrate of potash, a 

 yellow stain in which the picric acid is the active ingredient. 

 The acid stains in chief use are eosin, acid fuchsin, and orange 

 G. Substances which stain with an acid stain after suitable 

 exposure to a mixture of an acid and a basic stain are called 

 oxyphile, or, from the frequent use made of eosin as an acid stain, 

 eosinophile. 



Basic stains are those in which the colouring property resides 

 in the basic radicle of the salt ; they include all the stains which 

 are in use for staining bacteria, and they all colour the nuclei of 



