70 UHAPTEB V. 



Absolute alcohol is held to rank as a fixing 1 agent because it 

 is said to kill and harden with such rapidity that structures 

 have not time to get de-formed in the process by the energetic 

 dehydration that unavoidably takes place. Dilute alcohol is 

 held to rank as a fixing agent in virtue of being of such a 

 strength as to possess a sufficiently energetic coagulating 

 action and yet contain enough water to have but a feeble 

 and innocuous dehydrating action. The intermediate grades 

 do not realise these conditions, and therefore should not be 

 employed alone for fixing. But they may be very useful in 

 combination with other fixing agents (such as corrosive sub- 

 limate or nitric acid) by greatly enhancing their penetrating 

 power ; 70 per cent, is a good grade for this purpose. 



A recent writer, however, TELLYESNICZKY (Arch. f. Mik. 

 A not., Hi, 2, 1898, p. 219), disagrees with the doctrine of the 

 supposed importance of the grades of alcohol used for fixing, 

 finding them all equally bad. He finds no difference at all 

 between the action of absolute alcohol and that of 90 per 

 cent, or that of 70 per cent. They all cause a remarkable 

 amount of shrinkage, and probably a notable amount of solu- 

 tion of cell-constituents. 



A point that is not suffieientlv attended to is the chemical purity of the 

 alcohol, which has its importance in respect of the preservation of delicate 

 histological detail and the application o delicate stains. Pure alcohol 

 ought to leave only the very slightest solid residue on evaporation; 

 rubbed between the hands it ought not to leave a strong odour behind after 

 evaporation ; and ought to show neither an acid nor an alkaline reaction 

 (this latter much more commonly occurs than is imagined). It should be 

 tested with very sensitive litmus paper, which it is well to leave in it for 

 some hours. See hereon MAYER, in Mitt. Zool. Stat. Neapel, x, 1891, p. 180, 

 where is given a test for alkalinity: a solution of 1 gramme each of hse- 

 matein and chloride of aluminium in 100 c.c. of alcohol, added in the 

 proportion of 1 : 100 to the alcohol to be tested, ought not to be precipitated 

 in twenty-four hours. 



For fixing, alcohol is a very third-class reagent, only to be 

 used where better ones cannot be convenientXy employed. 

 For hardening it is a very important one. When used alone, 

 it is indeed inferior as a hardening agent to most of the 

 reagents discussed above ; but when judiciously employed to 

 complete the action of a good fixing agent, it renders most 

 valuable services. 90 to 95 per cent, is the most generally 

 useful strength. Weaker alcohol, down to 70 per cent., is 



