12 BOTANICAL MICROTECHNIQUE. 



Finally, under this head, the use of some other strongly 

 refractive media will be described. 



i. The Ordinary Method of Transfer from Water to Canada Balsam. 

 (a) Dehydration. 



14. Dehydration by alcohol commonly does not present 

 the least difficulty. In case of microtome sections it is suffi- 

 cient to cover them with alcohol and then let the alcohol 

 flow off ; free-hand sections are best placed in small dishes 

 or cups with alcohol, and left for a longer or shorter time, 

 according to their thickness. 



But there often results from direct transfer from water to 

 alcohol the shrinking of the cells or their collapse from the 

 too rapid withdrawal of their water. Several methods have 

 been employed to prevent this collapse of the cells, whose 

 essential feature lies in the very gradual replacement of the 

 water by the alcohol. 



This can be effected by placing the preparations in turn 

 in different mixtures of water and alcohol, each of which 

 exceeds the previous one in its proportion of alcohol. For 

 instance, one may prevent collapse by placing the prepara- 

 tions first in 10% alcohol and then in order in 30$, 50%, 70$, 

 90$, and finally in absolute alcohol. The time between 

 the transfers must depend upon the thickness of the tissues. 

 With delicate objects, as, for example, unicellular algae, 

 intervals of a few minutes each are sufficient. 



In the case of filamentous algae the transfer can be much 

 simplified by binding them together with a thread. 



15- Gradual dehydration can be accomplished by a method 

 devised by J. af Klercker, which consists in allowing abso- 

 lute alcohol to flow slowly into io# alcohol through a fine 

 capillary tube. 



16. The dehydrating vessel* recommended by Fr. E. 

 Schulze (I) brings about the gradual replacement of water 



* This may be obtained of Warmbrunn, Quilitz & Co., Berlin, C., Rosen- 

 thaJerstr., 40, at the price of Mk. 2.75 (67 cents). 



