208 THE MICROSCOPE AND ITS REVELATIONS. 



son's blue made neutral, the blue will in no long time entirely drive out 

 the red; but by carefully watching the process, it will be seen that the 

 different tissues will change color in different times, the softer cells giving 

 up their red and taking-in the blue more quickly than the harder; so that 

 by stopping the process at the right point (which must be determined by 

 taking-out a section, dipping it in alcohol, and examining it under the 

 microscope), the two kinds of cells are beautifully differentiated by their 

 coloring. 1 The best effects are usually produced by Carmine and Indigo- 

 carmine, Logwood and Picro-carmine, Carmine or Logwood and Aniline- 

 blue or Aniline-green. But very much has yet to be learned on this sub- 

 ject; and the further investigation of it will be likely to produce results, 

 that will amply repay the time and labor bestowed. 



204. Chemical Testing. It is often requisite, alike in Biological and 

 in Mineralogical investigations, to apply Chemical Tests in minute quan- 

 tity to objects under Microscopic examination. Various contrivances have 

 been devised for this purpose; but the Author would recommend, from 

 his own experience, the small glass Syringe already described (Fig. 106), 

 with a fine-pointed nozzle, as the most convenient instrument. One of 

 its advantages is the very precise regulation of the quantity of the test to 

 be deposited, which can be obtained by the dextrous use of it; whilst 

 another consists in the power of withdrawing any excess. Care must be 

 taken in using it, to avoid the contact of the test-liquid with the packing 

 of the piston. Whatever method is employed, great care should be taken 

 to avoid carrying away from the slide to which the test-liquid is applied, 

 any loose particles which may lie upon it, and which may be thus trans- 

 ferred to some other object, to the great perplexity of the Microscopist. 

 For testing Inorganic substances, the ordinary Chemical Reagents are of 

 course to be employed; but certain special Tests are required in Biologi- 

 cal investigation, the following being those most frequently required : 



a. Solution of Iodine in water (1 gr. of iodine, 3 grs. of iodide of potassium, 1 

 oz. of distilled water) turns Starch blue and Cellulose brown; it also gives an in- 

 tense brown to Albuminous substances. 



b. Dilute Sulphuric Acid (one of acid to two or three parts of water), gives to 

 Cellulose that has been previously dyed with iodine a blue or purple hue; also, 

 when mixed with a solution of sugar, it gives a rose-red hue, more or less deep, 

 with Nitrogenous substances and with bile (Pettenkofer's test). 



c. What is known as Schulze's Test is a solution of Chloride 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 serves . like the 

 preceding, to detect the presence of Cellulose ; and has the advantage over sul- 

 phuric acid of being less destructive to the tissues. Each will sometimes succeed 

 where the other fails; consequently, in doubtful cases, both should be employed. 



d. Concentrated Nitric Acid gives to Albuminous substances an intense yel- 

 low. 



e. Acid Nitrate of Mercury (Millon's Test) colors Albuminous substances 

 red. 



/. Acetic Acid, which should be kept both concentrated and diluted with from 

 3 to 5 parts of water, is very useful to the Animal Histologist from its power of 

 dissolving, or at least of reducing to such a state of transparence that they can no 

 longer be distinguished, certain kinds of membranous and ribrous tissues, so that 

 other parts (especially nuclei) are brought more strongly into view. 



g. Solution of Caustic Potass or Soda (the latter being generally preferable) 

 has a remarkable solvent effect upon many Organic substances, both Animal and 

 Vegetable; and is extremely useful in rendering some structures transparent, 



1 See " Journ. of Eoy. Microsc. Soc.," Vol. iii. (1880), p. 694. 



