Determination of the Relative Affinities of Acids. 557 



iris produced a relatively enormous " time-lag " in following 

 changes of luminous intensity, which we know does not exist. 



There seems, in fact, to be little doubt that the difference 

 is due to the retina itself becoming sensitive to long waves 

 after rest, which were incapable of affecting it w T hen it was in 

 some way fatigued by exposure to the ordinary bright light 

 of day. 



The next and obvious step is to find the respective wave- 

 lengths corresponding to the different temperatures. This 

 point, however, and others, cannot be determined without 

 some additions to the present apparatus, and will form the 

 subject of a future paper. 



LY. Neva Method of Determining the Relative Affinities of 

 certain Acids. By M. Caret Lea *. 



THIS method of measuring affinities is based on the 

 principle that the affinity of any acid is proportionate to 

 the amount of base which it can retain in the presence of a 

 strong acid selected as a standard of comparison for all acids. 

 The standard acid being in all cases kept exactly at the same 

 dilution. 



An example will make this clearer. Sulphuric acid is 

 here taken as the standard, and its presence or absence in the 

 free state is ascertained by means of the herapathite test 

 (described in this Magazine for July 1893). For simplicity, 

 we will suppose that the quantity taken is always a gram- 

 molecule at a fixed rate of dilution. It is evident that two 

 gram-molecules of sodium hydroxide would exactly saturate 

 it. If, now, we take a given acid, we may find that a 

 quantity of its sodium salt corresponding to three gram- 

 molecules of sodium hydroxide will exactly extinguish the 

 reaction of a gram-molecule of free sulphuric acid. With still 

 another acid we may find that a quantity of its sodium salt 

 corresponding to four gram-molecules of sodium hydroxide 

 is needed to extinguish the sulphuric reaction. Then the 

 affinity of the second acid is exactl} T twice as great as that of 

 the first. At the point where the free sulphuric-acid reaction 

 was extinguished, the second acid under examination retained 

 twice as much sodium as the first, and this quite independently 

 of any question of comparative basicity. 



Throughout the series of determinations here to be described 

 the sulphuric acid was used invariably at the same degree 



* Communicated by the Author. 

 Phil. Mag. S. 5. Vol. 37. No. 229. June 1894. 2 Q 



