236 THE MICROSCOPE. 



is not too strong, b. Use a large bulk of fluid in 

 portion to the material, c. Change the fluid frequently. 

 d. If freezing be employed, take care that the speci- 

 men is thoroughly frozen. 



3. a. Always use a sharp razor, b. Take it with one 

 diagonal sweep through the material, c. Make the 

 sections as thin as possible ; and d. Remove each one as 

 soon as cut, for if sections accumulate on the knife or 

 razor they are sure to get torn. 



4. a. Do not be in a hurry to stain, but b. Re- 

 member that a weak colouring solution permeates the 

 section better, and produces the best results; and 

 c. That the thinner the section the better it will take 

 the stains. 



5. a. Always use glass slips and covers free from 

 scratches and bubbles, and chemically clean, b. Never 

 use any but extra thin circular covers, so that the speci- 

 mens may be used with high powers, c. Always use 

 cold preservatives, except in the case of glycerine jelly, 

 and never use warmth to hasten the drying of balsam 

 or dammar, but ran a ring of cement round the cover. 



6. Label specimens correctly, keep them in a flat 

 tray and in the dark. 



Dr. Cook pointed out that the results obtained by 

 logwood were often unsatisfactory, and not fairly stable, 

 because it must be understood that its colouring material 

 consists of two substances, hsematoxylin and hoema- 

 tein, differing from each other by two equivalents of 

 hydrogen. The first named, containing the larger 

 amount of hydrogen, is soluble in alum solution, while 

 the latter, the heematein, is only slightly so, and is of 

 no use for the colouring of animal tissues. Haematoxy- 

 lin forms compounds with various metallic oxides ; and 

 a solution of hoematoxylin, alum, and a metallic oxide, 

 has a clear purple colour, becoming red on the addition 

 of an acid. If an alkaline earth, hydrated earthy 

 phosphate, be suspended in it, it will absorb the 

 colour, and the solution w r ill become purple. If the 

 solution be treated with a very small percentage of a 

 chromate, the purple will be gradually replaced by a 

 yellowish-brown colour; or if a tissue, stained with 



