32 THE AMERICAN MONTHLY [February, 



35 centigrammes of salicylic acid are boiled together for ten minutes. The 

 fluid thus obtained is of a I'edder hue, and its stain a more vivid red than that 

 of the preceding fluid. Vegetable and animal tissues stain in about twenty- 

 four hours. 



' 4. Satisfactory results were obtained by boiling 25 centigrammes of car- 

 mine with 50 cc. saturated solution of boric acid for ten minutes and filtering. 

 The fluid thus obtained much resembles in its action and appearance picro- 

 carmine.' 



We now come to the metallic substances, osmic acid, nitrate of silver, and 

 chloride of gold ; these are used in the form of weak aqueous solutions, and 

 are very useful in certain kinds of animal histological work. 



Osmic acid can be obtained in two forms — pure, in crystals, and as a 1% 

 solution. If obtained in the latter form, it should be diluted with an equal 

 bulk of water ; or, if the crystals are purchased, a ^% solution should be made. 

 It is very volatile, and as it is easily decomposed by actinic light, or by being 

 brought into contact with organic substances, it should be kept in a stoppered 

 bottle in a dark place. Osmic acid combines the properties of a hardening, 

 stainino;, and fixings agfent : as a staining ag^ent it acts bv bleaching fat cells, 

 for which it has a great affinity. In the study of medullated nerve-fibres, a 

 1% solution is the most useful strength. It should be allowed to act upon the 

 nerve for fifteen minutes, after which time it will be found to have completely 

 blackened the phosphorized fats of the myeline. 



Nitrate of silver is used for staining the intercellular substance of epithelium, 

 cartilage, and the cornea; also for demonstrating ' Ranvier's Crosses' in 

 medullated nerve-fibres. Tissues to be stained with silver nitrate solution 

 must be perfectly fresh, and should invariably be first rinsed in distilled water 

 to remove soluble chlorides. They are then placed in a ^% solution until they 

 assume a milky appearance, after which they are removed into ordinary water, 

 washed, and mounted in glycerin or Canada balsam. The mounted prepara- 

 tions are exposed to a strong light until thev tvn-n brown, and always afterwards, 

 when not in use, they should be kept in a dark place. In cases where the solu- 

 tion has been used for showing the existence of epithelium, an examination un- 

 der the microscope will show that the silver has been precipitated in the inter- 

 cellular substance ; but in medullated nerve-fibres, the silver lines will be 

 found transversely on the cement uniting the nerve-segments, and longitudi- 

 nally on the axis-cylinder for a short distance on each side of the transverse 

 bar. 



Chloride of gold is a most useful reagent for staining cartilage, tendon, de- 

 veloping capillaries, nerve-plexuses, and nerve-ganglia. As in staining with 

 silver-nitrate solution, the tissue must be perfectly fresh, and must be washed 

 in distilled water ; it is then transferred to a 1% solution of the salt for half 

 an hour, washed, and placed in water acidulated with acetic acid, 3 drops of 

 acid being added to every ounce of water. In twenty-four hours the gold will 

 be completely reduced, and the tissue, which will be of a deep violet-color, 

 is then washed and mounted. 



If a tissue is thick or dense and covered with epithelium, the method of 

 staining with gold chloride solution is not altogether satisfactory ; in such cases 

 the following process yields much better results. Wash the fresh tissue in 

 distilled water, place it in filtered lemon juice for five minutes, wash it again 

 in water, and transfer it for half an hour to a 1% solution of gold chloride ; 

 the tissue is again washed, and is then transferred to a mixture of three parts 

 of water, and one part of formic acid. Treatment vt^ith this for twenty-four 

 hours reduces the gold and removes the epithelium, so rendering the under- 

 lying parts more transparent and suitable for examination. The tail of a half- 

 grown tadpole treated in this way makes a very fine preparation for the mi- 



