TEST-LIQUIDS — MOUNTING OF OBJECTS. 213 



3. Solution of cUoride of zinc, iodine, and iodide of potassium, 

 made in the following way : — Zinc is dissolved in hydrochloric 

 acid, and the solution is permitted to evaporate, in contact with 

 metallic zinc, until it attains the thickness of a syrup ; this syrup 

 is then saturated with iodide of potassium, and iodine is last 

 added. This solution (which is known as Schultz's test) serves, ' 

 like the preceding, to detect the presence of cellulose, and has 

 the advantage over sulphuric acid of being less destructive to the 

 tissues. Each will sometimes succeed where the other fails ; con- 

 sequently, in doubtful cases, both should be employed. 



4. Concentrated Nitric Acid gives to albuminous substances 

 an intense yellow ; when diluted with about two or three parts 

 of water, it is very useful in separating the elementary parts 

 of many Animal and Vegetable tissues, when these are boiled 

 in it. 



5. Acetic Acid (diluted with from three to five parts of water) 

 is a most useful test-liquid to the Animal Histologist, from its 

 power of dissolving, or at least of reducing to a state of such 

 ti'ansparency that they can no longer be distinguished, certain 

 membranes, fibres, &c. ; whilst others are brought strongly into 

 view. 



6. Acid Nitrate of Mercury (Millon's test) colors albuminous 

 substances red. 



7. Solution of caustic Potash or Soda (the latter being gene- 

 rally preferable) has a remarkable solvent effect upon many or- 

 ganic substances, both Animal and Vegetable. 



8. Alcohol dissolves resinous substances and many vegetable 

 coloring matters, and renders most vegetable preparations more 

 transparent ; on the other hand, by its coagulating action, it 

 renders many animal tissues (as nerve-fibres) more opaque, and 

 thus brings them into greater distinctness. 



9. Mher dissolves not only resins, but oils and fats. 



Section 2. Mounting of Objects. * 



115. The Microscopist not merely desires to prepare objects ' 

 for examination, but, where possible, to preserve them in such a j 

 manner that they may be inspected at any future time. This" 

 may be so eflfectually accomplished in regard to many substances, 

 that they undergo no kind of change, however long they may be 

 retained ; and even delicate structures whose composition renders 

 them peculiarly liable to decay, may often be kept, by complete 

 seclusion from the air, and by immersion in a preservative fluid, 

 in a state so nearly resembling that in which they were at first 

 prepared, that they will continue, during an indefinite length of 

 time, to exhibit their original claaracters with scarcely any de- 

 terioration. The method of " mounting" objects to be thus pre- 

 served, will differ, of course, both according to their respective 

 natures, and also to the mode in which they are to be viewed, 



