XXVI INTRODUCTION. 



blood, and amniotic fluid in which a little metallic iodine* has 

 been dissolved the so-called iodized serum; finally, very diluted 

 solution of neutral salts may be particularly recommended. If 

 the formed elements have been already modified in their 

 chemical characters by the addition of other reagents, if, for 

 example, they have been soaked in a dilute solution of bichro- 

 mate of potash, or of chromic acid, water alone may be added. 

 Reagents which induce coagulation of the formed elements, and 

 a consequent hardening of the tissues, cause them also to become 

 cloudy. In order to examine such changed elements with any 

 advantage by means of transmitted light, it is customary to 

 apply highly refractive fluids, which, when they penetrate into 

 their interior, render them transparent. The employment of 

 these means have led to very remarkable advances in micro- 

 scopic art. 



The highly refractive medium must be soluble in the fluid in 

 which the tissues had previously been macerated. Glycerine 

 is a highly refractive liquid of this nature, and it is soluble in 

 water. Tissues can therefore be removed from watery solutions 

 and immersed in glycerine, or what comes to the same thing, 

 glycerine may be directly employed as a fluid for mounting 

 microscopical preparations. Oil of turpentine is still more highly 

 refractive, but it is insoluble in water. A tissue cannot therefore 

 be removed from a watery solution into oil of turpentine. But 

 alcohol is soluble both in oil of turpentine and in water. If, 

 therefore, it be desired to impregnate a tissue with oil of tur- 

 pentine, it is first removed from its watery solution into absolute 

 alcohol, and from this into the turpentine. In cases where the 

 tissue forms a membrane, it is only requisite to spread it out 

 when fresh ; to add a drop of some indifferent fluid, and then 

 to cover it with a plate of thin glass. This plan, however, is 

 only feasible when the membrane is not too thick. 



As a general rule, fresh tissues are more or less transparent, 

 but after death they become cloudy. When, therefore, dead 

 membranes are spread upon the slide, and are required to be 



* The ainniotic fluid should be pure and almost destitute of smell. A 

 trace of putrefaction renders it less available. The addition of iodine 

 colours the fluid of a feeble yellow tint. 



