20 



THE PROPERTIES OF SOLUTIONS. 



[CH. I. 



which can be mixed in various proportions to give a series 

 of solutions of a known P H > which have been accurately 

 determined by the electrical method. A given amount of a 

 suitable indicator is added to a measured volume of the 

 fluid, and also to equal volumes of the standard test 

 solutions, contained in tubes or vessels as uniform as 

 possible. The solutions that give exactly the same tints 

 have the same hydrogen ion concentration, provided that 

 this concentration is in the range of the indicator employed. 

 The results are not as accurate as the electrical method, 

 owing to the difficulty of exactly matching the tints and 



SOURCE OF LIGHT. 



Water. 



Coloured 

 fluid. 



B 



Standard 



Indicator. 



Ground 

 glass 



EYE. 



Fig. 3. Plan of arrangement of tubes in Cole and Onslow's Comparator. 



also of the effect of proteins, salts, and other substances 

 on the colour developed. If the fluid be coloured it is 

 obvious that this simple method can only give very approxi- 

 mate results. Walpole overcame the difficulty by viewing 

 the (standard solution + indicator) through a layer of the 

 coloured fluid of the same depth as that of the (coloured 

 fluid + indicator). A special instrument was devised for 

 this purpose. Hurwitz, Meyer and Ostenberg used 

 Walpole 's principle, but employed test tubes held in a box 



