Methods of Analysis 231 



washed in copper'-free water and dried. Careful observation in- 

 dicated that adhering copper salts deposited from water solution 

 were completely removed by this treatment. ■ It is probable that 

 the acid penetrates plant tissues somewhat in the time employed 

 and removes some copper. The results are, therefore, probably 

 severely conservative. 



2. Roots grown in soil cultures containing copper cannot be 

 safely cleansed with HCl, which does not readily dissolve silicates 

 and sulphides of copper, and which cannot be allowed to remain 

 in contact with plant' roots for more than a few seconds. 



Carbon dioxide in water was finally selected as a mild, slow 

 but finally effective solvent for the purpose. Samples of roots 

 were first very thoroughly washed in copper-free well-water, then 

 placed in five-liter jars with ground glass covers, a stream of 

 washed CO, passed, the jars shaken and treatment with COo 

 repeated until the water was saturated, then allowed to stand 

 with occasional shaking for twenty-four hours. The solution 

 was then siphoned or filtered off and the treatment repeated 

 until, on evaporating the bulky filtrates, no more copper was 

 found. To prevent putrefaction during long-continued washings, 

 a pinch of thymol w^as added to each washing. From nine to 

 thirty-one washings were found necessary to cleanse plant roots 

 thoroughly, the process being laborious and time-consuming. 

 When the sample yielded no more copper to wash waters it was 

 dried, burned and copper determined according to the method 

 for small amounts in plant ashes. 



Following is a record of washings for examples of roots 

 cleaned by this process: 



(1) Corn roots grown in a pot culture of soil containing 0.01 

 per cent of copper as basic carbonate. 



Quantitative by 

 ^2^* *^^* electrolysis 



First wash distinct 



Fifth wash distinct 



Ninth wash doubtful 1 liter of filtrate no Cu 



(2) Corn roots grown in a pot culture of soil containing 0.05 

 per cent copper as Cu.,S. 



Quantitative by 

 electrolysis 



