192 THE MICROSCOPE 



delicate vegetable structures. The acid seems to be taken up by 

 each granule of the protoplasm, and there to be decomposed, 

 giving to the granule the characteristic grey colour, thus at the same 

 time both hardening and staining. (Parker — -Joiirnal, R.M.S., 

 Vol. 7, /. 381.) We use the following mixture : — 9 parts of a 

 \ per cent, solution of chromic acid, with i part of a i per cent, 

 solution of osmic acid, which answers for many purposes better 

 than osmic acid alone, the brittleness produced by osmic acid 

 being completely avoided. After being subjected to this agent, 

 the specimens should be treated with 30 per cent, alcohol, 

 gradually increased in strength to absolute. 



Molybdate of Ammonia is recommended as affording a cool 

 blue-grey or neutral tint general stain, which affords a pleasant 

 ground colour to parts strongly coloured by bright selective stains. 



Chemical Testing.— The following short list will be found 

 useful : — 



(a) Solution of iodine in water. Take iodine, i grain ; iodide 

 of potassium, 3 grains ; distilled water, i ounce. This solution 

 turns starch blue, cellulose brown ; also gives an intense brown 

 colour to albuminous substances. 



{b) Dilute sulphuric acid (i part of acid to 2 or 3 parts of 

 water) gives cellulose that has been already dyed with iodine a 

 blue or purple hue,; also when mixed with sugar, it gives to 

 nitrogenous substances a rose-red hue, more or less deep. — 

 (Pettenkofer's test). 



[c) Schultze's test is a solution of choride of zinc, iodine, and 

 iodide of potassium ; it detects cellulose. 



{d) Concentrated nitric acid gives albuminous substances an 

 intense brown. 



{e) Acid nitrate of Mercury (Millow's test) colours albuminous 

 substances red. 



(/) Acetic acid, both concentrated and diluted with from 

 3 to 5 parts of water, is useful to the animal histologist. 



{g) Solution of caustic potash or soda (the latter generally 

 preferable) has a remarkable solvent effect upon many organic 

 substances, both animal and vegetable. 



