XXxii INTRODUCTION. 



visible, can often be made evident by treating the preparation 

 with certain colouring matters. The principle of this means of 

 research consists in the circumstance that various constituents 

 of the tissues become quickly stained with colouring matters, 

 or combine with them, whilst others do not. The tissues should 

 be dipped in the solutions of the colouring agents, allowed to 

 remain in them for some time, and then washed. Cceteris 

 paribus, the concentration of the solution stands in inverse 

 relation to the length of time required in order that certain 

 effects should be produced. It is therefore advantageous to 

 use very dilute solutions, and to prolong the time of their 

 action. The more gradual this is, the more scope is afforded 

 for exact researches. 



A division of the colouring reagents can be made first, into 

 those, the solutions of which, when examined by transmitted 

 light, show the same absorption colours they impart to the 

 tissues ; secondly, into those which impart to the tissue one of 

 their own proper absorption colours ; and lastly, into those whose 

 solutions absorb no definite colour, or are, as we are accustomed 

 to say, colourless. 



In the two last-mentioned cases, after saturation with the 

 fluid, some chemical process must take place. An example of 

 the first kind is seen in carmine, the alkaline solutions of which 

 impart their own colour to the tissues ; an example of the 

 second kind is met with in chloride of gold, the solutions of 

 which are pale yellow, whilst the tissues that are saturated 

 with it assume a violet tint ; and an example of the third 

 kind is found in nitrate of silver, the solutions of which are 

 colourless, but yet stain the tissues of a dark brown hue. The 

 secondary chemical change may either occur without further 

 addition, or some means must be employed to induce it. When 

 the tissues are macerated in dilute solutions of perosmic acid, 

 they assume, sooner or later, according to their chemical 

 nature, a black colour, without any addition ; but those which 

 have been in solutions of nitrate of silver require exposure to 

 light before the chemical change, which consists in the precipi- 

 tation of silver, will occur. Gerlach introduced the method of 

 examination by staining the preparation into practice. His 

 first experiments were made with carmine. At the present 



