108 METHODS IN PLANT HISTOLOGY 



7. Two per cent aqueous solution of iron-alum, until the stain is satis- 

 factory. This can be determined only by examining frequently under 

 the microscope. 



8. Wash in water, 2 hours. If this washing is not thorough, the continued 

 action of the iron-alum will cause the preparations to fade. 



9. Transfer to 10 per cent glycerin, and allow the glycerin to concentrate 

 until it has the consistency of pure glycerin. It is not necessary to use 

 an exsiccator. Merely put the glycerin into shallow dishes, and leave 

 it exposed to the air, but protected from dust. If the material is in Petri 

 dishes or other dishes with a large surface, 3 or 4 days will be sufficient. 

 This process may be hastened by warming, if the temperature does not 

 go above 35° C. If the reduction from 10 per cent glycerin to pure 

 glycerin is accompUshed in less than 48 hours, the change in the con- 

 centration is so rapid that material is likely to suffer. 



10. Wash out the glycerin with 95 per cent alcohol. It will be necessary 

 to change the alcohol several times. From 10 to 20 minutes will be 

 sufficient if the alcohol is changed frequently. This alcohol cannot be 

 used again for the same purpose, but it will be useful in cleaning one's 

 hands and in cleaning dishes which have contained Venetian turpen- 

 tine. 



11. Complete the dehydration in 100 per cent alcohol: 10 minutes should 



be sufficient. 



12. Most failures are now ready to occur. 



From the absolute alcohol the material is transferred to a 10 per 

 cent solution of Venetian turpentine in absolute alcohol. The turpen- 

 tine thickens as the alcohol evaporates, and when it reaches the con- 

 sistency of pure glycerin the material is ready for mounting. The 10 

 per cent Venetian turpentine is very sensitive to moisture, and most 

 failures are due to this characteristic ; consequently the concentration 

 cannot be allowed to take place with the turpentine exposed to the air 

 of the room. Use an exsiccator. This will not only absorb the moisture 

 from the air, but will soon remove the alcohol from the turpentine 

 mixture. Make an exsiccator as follows: Place a saucer full of soda 

 lune (sodium hydroxide with Hme) on a plate of glass, and cover with 

 a bell jar. This is a simple and effective exsiccator. Instead, you may 

 simply scatter soda lime in the bottom of any low museum jar with 

 tight-fitting cover. The tin cans, with tight covers, in which you get 

 your pound of "Improved Vacuum Coffee" make good exsiccators for 

 small amounts of material. You may improvise other forms ; the essen- 



