OF VITAL PHENOMENA 145 



loses most of the Hg ions if the concentration of the NaCl is 

 high, but gains the Na 2 HgCl 4 , which more than compensates 

 this loss, so that the total Hg in solution is greater than before. 



Although the solubility of HgCl 2 is increased by the addition 

 of NaCl, and the mercury may be used in greater concentration, 

 this does not mean that the toxicity is increased. The question 

 as to whether the ions or molecules are the more toxic must 

 first be answered. This seems to vary with the duration of the 

 exposure to the poison, as shown by the following experiments: 



Paul and Kronig (1896) determined the disinfecting power 

 of differently dissociated salts of mercury on Anthrax spores. 

 Those salts which are poorly dissociated are less toxic than those 

 which are more completely dissociated. In other words, the 

 toxicity depends chiefly on the mercuric ions. The following 

 table shows the number of colonies that developed after ex- 

 posure to the toxic solutions: 



The same rule had previously been found by Kahlenberg and 

 True (1891) to apply to the toxicity of copper salts, and seems 

 to be approximately true of all heavy metals. It is only ap- 

 proximately true because the molecules are toxic, although not 

 so toxic as the ions, during brief periods of exposure, and the 

 rule does not hold for prolonged exposure. 



If salts of heavy metals are allowed to act on organisms for 

 a long time, their toxicity is approximately proportional to the 

 total quantity of the heavy metal they contain. Stevens (1898) 

 and Clark (1899) grew Penicillium and Aspergillus in such solu- 

 tions, finding that the inhibition of growth is approximately pro- 

 portional to the quantity of the heavy metal, without regard to 

 the combination in which it exists. It is probable that the heavy 

 metal ions combine with the protoplasm, thus leading to further 

 dissociation of the salt until all is dissociated and combined. 



