230 



Mr. R. F. D'Arcy on the 



contain a maximum percentage of hydrate, and this will he 

 the solution having maximum viscosity. 



Experiments on Chrome- Alum Solutions. , 



It is well known that a solution of chrome alum when 

 heated to 70° undergoes a change which is easily traced by 

 the colour changing from violet to green, and that on cooling 

 the solution remains for a long time green, but gradually 

 returns to its original state. Experiments were made on the 

 viscosity of a solution before and after being heated; the 

 results are given in the following Table, and shown graphically 

 in fig. 4 : — 



Table XVIII.— Viscosity 



of ^q normal solution of Chrome 

 Alum. 



Mean 

 Temp. 



Time. 

 Seconds (7). 



Pressure. 

 Cm. of water at 15° (jp). 



pt 

 1000' 



Viscosity. 



Water at 



15° = 1. 



Colour of sol. 



o 



16 



1132 



28-42 



320 



1-245 



violet 



162 



1115 



28-42 



31-7 



1-224 



violet 



29-8 



817 



28-42 



23-2 



•896 



violet 



49-3 



567 



28-42 



16-05 



•620 



violet 



.58-8 



483 



28-42 



13-67 



•528 



colour changing 



69-4 



413 



28-42 



11-70 



•452 



green 



49-35 



528 



28-42 



150 



•579 



green 

 [had been heated 



29-85 



753 



28-42 



21-4 



•826 



to 100°.] 



1605 



1031 



28-42 



29-3 



1-132 



green 



Experiments were also made with a solution which was 

 saturated in the cold with similar results. 



This change of colour has been explained in two ways : (1) 

 that it is due to dehydration ; (2) more recently and per- 

 haps more satisfactorily as due to the decomposition of the 

 normal chromium salt with formation of basic and acid salts. 

 The diminished viscosity may perhaps be taken to indicate a 

 decomposition of complex molecules into simpler ones, but 

 does not seem capable of discriminating between these two 

 theories. Probably osmotic pressure being chiefly dependent 

 on the number of molecules in solution would yield a crucial 

 method of experiment from the physical point of view. 



The alteration of viscosity is so marked in this case that it 

 would be sufficient to show that a change had been produced 

 by heating ; hence it seems that in cases where there is no 

 alteration of colour determinations of viscosity may be used 



