INTKODUOTION. » 



zinc still undissolved, is to be evaporated to a syrupy consist- 

 ence, saturated with potassic iodide, an<l lastl}' enough pure 

 iodine added to render the whole a deep red or brown. Cell- 

 walls that have incrusting matters, for instance, cork-cells and 

 most wood-ceils, are turned yellow b^- this reagent. It is known 

 as Schulze's reagent. Behrens advises the preparation of modi- 

 fications of this important reagent, all depending on the relative 

 amount of iodine and the degree of dilution. A little practice 

 in their use will suggest the cases to wliich each is specially 

 applicable. Solutions of iodine color protoplasm, and other 

 albuminoid bodies, yellow to deep brown. 



35. Owing to the tendency of iodine solutions to form hvdri- 

 odic acid, it is recommended by man3- authors that they be kept 

 out of the light ; but this precaution is not necessary unless the 

 investigation calls for pure iodine alone ; in such a case it is 

 better to use only freshly prepared solutions. 



The following reagents are arranged in alphabetical order. 



36. Acetic acid. (Jlacial acetic acid diluted by two or four 

 parts of water, or the ordinary concentrated acid of the shops, is 

 used (1) to neutralize the alkali in Hanstein's method (see 24) ; 

 (2) to discriminate between oxalates and carbonates, the latter 

 dissolving with effervescence in it, the former remaining un- 

 changed in it, but dissolving quietly in hydrochloric acid ; (3) in 

 the study of the nucleus. 



37. Alcohol. Common strong alcohol, or the so-called " 95 

 per cent," is widely employed for the preservation of micro- 

 scopic material. In it soft tissues become hardened. This is a 

 great advantage in the case of specimens which arc too 3'ielding 

 to be eleanlj- cut when fresh. If it is desirable to again soften 

 tissues which have been hardened bj' the action of alcohol, it is 

 merely necessary to soak them for a short time in water, when 

 they will assume nearly the consistence the}- had when fresh. 

 This reagent produces certain marked changes in the contents of 

 vegetable cells : the protoplasmic matters become more or less 

 shrunken, many oils and fats are dissolved, and certain sub- 

 stances in solution in the cell-sap are separated out (see 183). 



38. The air which occurs in intercellular spaces and in all 

 drj' specimens is generally removed with ease by the action of 

 alcohol, especially if a little heat is applied. 



39. Alcohol is of use also in the preparation of some of the 

 staining agents. 



40. Absolute alcohol contains only the merest trace of water. 

 Hence it must be used instead of ordinary alcohol whenever the 



