428 PROTOPLASM 



instrument. Elements can thus be quantitatively determined. 

 Of some elements, as little as 0.0004 gram can be detected. 



The quantities of heavy metals present in plants and animals 

 are exceedingly small compared with those of the alkali metals, 

 but they are still sufficient to make one wonder how the organism 

 can tolerate metals as highly toxic as are copper, zinc, and tin. 

 Copper is a striking example of a poisonous element. Its 

 toxicity is so great that in some early experiments by Nageli 

 the concentration was reduced to an absurdity. Nageli found 

 that a trace of copper in water (produced by leaving a few copper 

 coins in water for a time) quickly killed the filamentous alga 

 Spirogyra. He then diluted the copper water until, by estima- 

 tion, but one molecule of copper remained in a liter of water, 

 and still the alga died. He rinsed the glass thoroughly and put 

 in fresh distilled water, and the alga still died. The effect was 

 so startling that Nageli gave to it the special name "oligo- 

 dynamic." The experiment was a rather crude one, and 

 undoubtedly more copper ions were present than Nageli thought 

 (they probably clung to the walls of the vessel), but it at least 

 showed that copper is highly toxic in very low concentrations. 

 Other heavy metals, such as silver, are equally poisonous. A 

 silver coin laid upon an agar culture of bacteria will leave a space 

 free from bacteria around it, owing to minute quantities of the 

 metal which have gone into solution. The toxic action of heavy 

 metals is made use of in antiseptics. 



Metals probably exhibit their toxicity by a "poisonous" 

 action on the catalytic powers of other metals. Bredig found 

 that 0.000003 gram of colloidal platinum per liter of water 

 produced a marked acceleration in the decomposition of hydrogen 

 peroxide. The shghtest trace of a catalytic poison — 0.000001 

 gram of hydrocyanic acid per liter — will impede this action of 

 platinum. Often, however, the reverse situation is true; a trace 

 of a second element is necessary before the benefits of the first 

 are realized, as in the needed presence of copper if iron is to 

 prevent anemia. Poisonous metals are tolerated by plants and 

 animals because the quantity is very minute — in this case, the 

 element may not only cease to be a poison but become a necessity, 

 for it is generally recognized in physiology that substances which 

 in higher concentration are poisonous may be beneficial in very 

 low concentrations — or the element may be in another physical 



