MATILDA M. BROOKS 333 



Fig. 1 shows the manner in which the rate of production of CO2 

 changes under the influence of MgCl2, in concentrations of 0.01, 0.05, 

 0.03, 0.5, and 1.0 m. During the first 10 minutes the bacteria are 

 under normal conditions and the curve (broken line) is horizontal. 

 After this, at the point marked on the abscissa, the salt is added. 

 For example, the addition of sufficient MgClo to make the concentra- 

 tion 0.03 M produces an increase in the rate which remains constant 

 during the period of experimentation. When the concentration is 

 0.05 and 0.01 m the rate is normal, while in higher concentrations 

 there is a decrease in rate. These curves are selected from a number 

 of similar typical curves and each represents one experiment. 



Fig. 2 shows the eflfect of various concentrations of MgCl2 upon the 

 rate of respiration expressed as per cent of the normal. The rate 

 indicated is that produced after the bacteria had been in contact with 

 the salt for 1 hour. The figure shows that there is an increase in the 

 rate of production of CO 2 at 0.03 m and a decrease in rate at concen- 

 trations higher than 0.05 m. 



The effect of osmotic pressure in these experiments is probably- 

 negligible, since it has been shown in a previous paper^ that respiration 

 is normal in a properly balanced solution obtained by mixing 1.0 m 

 KCl and 1.0 m CaCh in the proper proportions. 



Fig. 3 shows the antagonism between the salts. Thus Curve A 

 shows that when 9.3 parts of 0.8 m NaCl, and 0.7 parts of 0.8 m MgCli, 

 were added to the bacteria, the rate of respiration remained normal. 

 The other proportions did not give normal rates. 



Curve B illustrates the effect of combinations of CaCU and MgCl2 

 upon the rate of production of CO 2. There is no normal rate with 

 any combination of these salts at 0.8 m concentration. There is a 

 maximum in the curve at 8 parts of 0.8 m CaCh, combined with 2 

 parts of 0.8 m MgCU, where the antagonism is slight. In experi- 

 menting upon lower concentrations of Ca and Mg in the same propor- 

 tions, no normal rate was obtained until the concentrations were so 

 dilute that they would of necessity, according to their separate curves, 

 produce normal rates. It may be added that the combination of the 

 two monovalent salts, NaCl and KCl, does not give a normal rate. 



In order to find out whether the pH value of the liquid containing 

 the bacteria was changed when the salts were added, thereby influ- 



