SPECIAL METHODS. 169 



few seconds, in a mixture of one part sea-water and two 

 parts fresh water to which was added enough potassium 

 ferrocyanide to give the solution the specific gravity of sea- 

 water. Then the algae were rapidly passed through a vessel 

 of pure sea-water and placed for one half to two seconds in 

 a mixture of two parts sea-water, one part fresh water, and a 

 few drops of ferric chloride* The depth of the coloring is 

 increased markedly by repeating the proceeding several 

 times. 



For producing Turnbuirs blue, which Noll thinks less 

 suitable, he used the corresponding solutions of potassium 

 ferricyanide and ferrous lactate. 



, 2970. But it should be remarked concerning these stain- 

 ings that they are gradually destroyed, probably by the 

 -excretion of alkali. But they can be renewed at any time 

 by placing the algae in a solution of potassium ferro- (or 

 ferri-) cyanide acidified with pure hydrochloric acid. 



Finally it may be observed that, according to Noll's 

 researches, the vitality of the algae is not destroyed by 

 these manipulations and the precipitate is in this way very 

 uniformly deposited in the membranes, provided they pre- 

 sent no chemical differences, so that they show the same 

 depth of color in all the layers. 



2p7d. Zacharias (IV, 488) has lately used Congo red in the 

 same manner as Berlin blue. He worked with root-hairs of 

 Lepidium, which he placed for 1530 minutes in a solution 

 of Congo red in water from the public supply arid then 

 allowed to grow further in moist air. But, since a decom- 

 position of Congo red takes place in light, the seedlings 

 must be cultivated in the dark. 



2976. Congo red was earlier used by Klebs (III, 502) in 

 the investigation of the growth of the membranes of various 

 algae. This author found that Congo red has the remark- 

 able property of leaving membranes already formed color- 

 less or almost so, while it gives a red color to forming 



* This solution must be freshly prepared each time it is used, as it de- 

 composes in a short time. 



