112 HOW TO WORK 



peculiar points of structure or arrangement not visible before, or it 

 produces a chemical change upon the substance, from which we 

 gain important information. Solutions of magenta and solutions of 

 tannin have been much used in investigations upon the blood cor- 

 puscles. The action of tannin upon the red blood corpuscle is very 

 peculiar ; it has been specially studied by Dr. Roberts, of Manchester 

 (" On peculiar appearances exhibited by blood corpuscles under the 

 influence of solutions of Magenta and Tannin " Proceedings of the 

 Royal Society, vol. XII, p. 481, No. 55, April, 1863). The solution 

 is made by dissolving 3 grains of tannin in an ounce of distilled 

 water. One drop of blood may be mixed with four or five drops 

 of the tannin solution and a portion of the mixture examined under 

 the microscope. 



2O5. Solutions of Nitrate of silver. Of late years nitrate of silver 

 has been used for staining tissues. Recklinghausen and His have 

 employed this plan with great success. A weak solution may be 

 imbibed by delicate tubes, and part being precipitated in the tube, 

 perhaps as a chloride or in combination with some albuminous ma- 

 terial, subsequently becomes decomposed by the action of light, and 

 a very dark line results, and thus the position of a previously per- 

 fectly invisible channel is clearly demonstrated. The outlines of 

 epithelial cells and the intervals between them may be demonstrated 

 by this process. Transparent connective tissue and the outer part 

 <>f cells can thus be coloured, the nuclei remaining perfectly colourless 

 and transparent. The nuclei by longer immersion will also be 

 coloured.* The appearances may be made to vary very much by 

 modifying the mode of procedure and the time which the preparation 

 is allowed to remain in the solution. After soaking in the nitrate of 

 silver solution for some time the specimen must be placed in dis- 

 tilled water, or in a weak solution of common salt, in order to wash 

 away the nitrate which adheres to the surface or occupies the intervals 

 between the cells. When this has been effected the specimen is 

 exposed to daylight or sunlight until the requisite degree of blacken- 

 ing has been obtained. The strength of the solution employed may 

 be varied according to circumstances. Recklinghausen uses a very 

 dilute solution, consisting of i part of nitrate of silver to 400 800 

 of distilled water. 



The structure of the cornea has been recently investigated by His, 

 after the tissue had been prepared according to this plan. The so- 

 called 'intercellular substance' (formed material) only may be coloured, 

 or, after the whole structure has been thoroughly impregnated with 



* The nuclei (germinal or living matter) as long as they are alive resist the 

 action of the fluid, but when they die it is imbibed by them. 



