54 Estimations occurring in Agricultural Analysis [83, 84 



and run in the requisite amount of sugar, all but "5 c.c. Boil, 

 and see whether the liquid is blue ; if in doubt, test with acid 

 K 4 Fe(CN) 6 . Then add sugar, drop by drop, boiling after each 

 addition until no copper is left in solution. Read off the 

 result again. 



A third determination is necessary for accuracy. This time 

 run in the whole amount of sugar solution except i c.c. The 

 operation will thus be done very rapidly, and hence very accu- 

 rately. Take this last result as correct. 



When the Fehling's solution has been thus standardised, 

 calculate what weight of glucose was contained in the quantity 

 of solution used, and label the Fehling's solution bottles. 

 Five c c. solution I. and 5 c.c. solution II. = x glucose, where 

 x is the weight calculated. 



83. It has already been pointed out that different results 

 may be obtained by using solutions of different strength. This 

 necessitates that the solution we work with shall always be of 

 nearly the same strength as the solution used in standardising. 

 Should a preliminary determination show it to be much 

 stronger, it must be diluted to approximately the right 

 strength. 



84. Gravimetric Method. — The volumetric method 

 described above is rapid and, in practised hands, good, but is 

 always a stumbling block to students. Messrs Clowes and 

 Coleman, in their ' Text-book of Quantitative Analysis,' describe 

 a gravimetric method which the author has found very satis- 

 factory, and which students will be able to carry out without 

 much difficulty. 



Filter paper absorbs a certain amount of copper sulphate 

 which obstinately refuses to be washed out. It is therefore 

 necessary to prepare an asbestos filter, and it is on the prepara- 

 tion of this filter that the accuracy of the determination depends. 

 A 'calcium chloride' tube of the shape shown on the 



