198 TEST-LIQUIDS. — STAINING PROCESS. 



sulphuric acid of being less destructive to the tissues. Each will 

 sometimes succeed where the other fails ; consequently, in doubtful 

 cases, both should be employed. 



4. Concentrated Nitric Acid gives to Albuminous substances an 

 intense yellow : when diluted with about four or five parts of 

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

 Animal and Vegetable tissues, when these are boiled or macerated 

 in it. 



5. Acetic Acid (which should be kept both concentrated and 

 also 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 transparence that they 

 can no longer be distinguished, certain membranes, fibres, &c, 

 whilst others are brought strongly into view. 



6. A cid Nitrate of Mercury (Millon's test) colours Albuminous 

 substances red. 



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

 rally preferable) has a remarkable solvent effect upon many Organic 

 substances, both Animal and Vegetable, and is extremely useful in 

 rendering some of their structures transparent, so that others are 

 brought into view ; whilst it has a special action upon Horny 

 tissues, which enables their component cells to be more readily 

 distinguished. 



8. Alcohol dissolves Resinous substances and many Vegetable 

 Colouring matters, and renders most Vegetable preparations more 

 transparent ; on the other hand, by its coagulating action on Albu- 

 minous substances, it renders many Animal tissues (as Nerve -fibres) 

 more opaque, and thus brings them into greater distinctness. 



9. Ether dissolves not only Resins, but Oils and Fats. 



10. Chromic Acid hardens Animal tissues, especially Nerve- 

 fibres. 



145. Staining Process. — Much attention has been given of late 

 years to the effects of another kind of testing, in which advantage 

 is taken of the various degrees of attraction for cei'tain Organic 

 Colouring matters, which are possessed by different Tissues ; so that 

 whilst some are stained very quickly when immersed in colouring 

 solutions, others require a much longer contact with them ; and 

 thus the former may be distinguished in the midst of the latter, 

 with a certainty and clearness attainable by no other method. 

 Although there are particular instances in which Magenta may be 

 employed with advantage, the colouring substance most generally 

 serviceable is Carmine; and the following is given by Dr. Beale, 

 who had large experience of this process, and has obtained im- 

 portant results by its use, as the best mode of applying it. Ten 

 grains of Carmine in small fragments are to be placed in a test- 

 tube, and half a drachm of strong Liquor Ammonias added ; by 

 agitation and the heat of a spirit-lamp the carmine is soon dis- 

 solved, and the liquid, after boiling for a few seconds, is to be 



