74 INJURY, RECOVERY, AND DEATH 



NaCl 0.52 M and CaCl 2 0.278 M have the same conduct- 

 tivity as sea water. If we wish to compare the toxicity 

 of NaCl 0.278 M with that of CaCl 2 0.278 M we may dilute 

 the sea water until it has the conductivity of NaCl 0.278 

 M. Tissue placed in this may be used as a control. At 

 the outset we make the resistance of the control equal 

 to that of the tissue in NaCl 0.278 M, or we divide the 

 resistance of the control by a figure which reduces it to 

 the same value (and divide all subsequent readings 

 of the control by the same figure). We then express all 

 readings of the tissue in NaCl 0.278 M as per cent, of the 

 reading of the control which is taken at the same time. 

 All readings of the tissue in CaCl 2 0.278 M are likewise 

 expressed as percentage of the readings of a control in 

 sea water having the same conductivity as CaCl 2 0.278 M. 

 Stronger solutions may be treated in the same way, using 

 sea water which has been concentrated by evaporation. 



Attention may be called to a further difficulty in deter- 

 mining toxicity. If tissue of Laminaria be transferred 

 from sea water to pure solutions of toxic salts their 

 relative toxicity sometimes appears to be different from 

 that which is observed when the same substances are 

 added directly to the sea water. Similar considerations 

 may be found to apply to animals and plants which live 

 on land or in fresh water, in which cases Ringer's solu- 

 tion or the water of soils and rivers may play the same 

 role as the sea water in experiments with marine forms. 

 These differences depend largely on the antagonistic 

 action of salts, which will be discussed in Chapter IV. 



It may be added that in some cases variations in the 

 supply of oxygen may cause changes in relative toxicity; 

 and in view of the fact that the temperature coefficient 

 is not the same in all cases of toxic action it seems 



