80 BACTERIOLOGY 



crystals of ice which form, the organs are usually not cut 

 until they have undergone a hardening process. 



In Histology an entire series of reagents is employed for 

 hardening, the use of which is, however, impracticable in 

 Bacteriology, because they deprive bacteria of their property 

 of taking up aniline colours easily. The most convenient way 

 is to harden the pieces, which should each be a cubic centi- 

 meter in size, singly in absolute alcohol, which must be 

 changed several times. The alcohol may be obtained as 

 free from water as possible in the following way : Crystals 

 of copper sulphate (blue vitriol) are heated in an iron 

 capsule with frequent stirring until they have completely 

 parted with their water of crystallisation and subsided to a 

 white powder, which, after cooling, is introduced into a bottle 

 and the alcohol is poured over it, when it greedily extracts 

 the water therefrom, becoming again blue. As the piece of 

 tissue contains water, it sinks to the bottom if thrown into 

 absolute alcohol, and the hardening process goes on more 

 slowly in the lower than in the upper half of the vessel, 

 the alcohol above being less rich in water. Hence it is 

 advisable to keep the organ in the upper part of the alcohol, 

 either by means of a layer of cotton wool, or by suspending 

 it with a thread fastened outside. 



A half per cent, chromic acid solution, with or without the 

 addition of platinum chloride and acetic acid, has also been 

 recommended, as in it the bacteria are well preserved. After 

 eight days the pieces are rinsed in water until that which 

 flows away shows no yellow coloration, and the hardening 

 is then completed in alcohol. 



Instead of chromic acid, a concentrated aqueous solution 

 of picric acid renders good service. The pieces are left in 

 this for two days, washed for twenty-four hours in water, 

 and transferred first to dilute, and from that to absolute, 

 alcohol. 



