Pharmacodynamics of Salts and Drugs 89 



When a plate of metal is placed in water, a certain amount of the 

 metal passes into the water in the form of positively charged par- 

 ticles, so that the solution becomes positively charged, the metal 

 negatively charged. The tendency of different metals to throw off 

 these positive particles varies, and this tendency may be measured 

 in so many volts if the metal is placed in a solution of one of its salts 

 of known strength. If this is done and the difference in voltage 

 between the metal and solution is measured, one obtains a series of 

 values for the different metals, and these values are known as the 

 solution tension series of the metals. The measurements are gener- 

 ally referred to the metals when immersed in normal ionic solutions 

 of their salts (Table i). 



By a reference to Table i showing this series it will be found that 

 potassium and sodium stand at one end of the series, these metals 

 having in normal ionic solutions a negative voltage of 2.92 and 2 . 54 

 respectively, as compared with the solution; while at the other end 

 are the noble metals, gold, silver, platinum, and mercur}', which 

 immersed in normal ionic solutions become electropositive to the 

 extent of more than one volt. 



The solution tension measures therefore the difference in po- 

 tential between the solution which contains a known quantity of 

 the ions of the metal and the metal itself; and it expresses the 

 difference between the tendency of the ion to deposit itself on the 

 metal plate, and the tendency of an atom of the plate to become an 

 ion. It will be seen that the values of the solution tension depend 

 entirely on the concentration of the ions in the solution, and the pres- 

 ence of a plate or particle of the metal. For our purposes, therefore, 



