22 Preparation of Substances 



crystals in water in a test tube give no test 

 for chlorin ions when a soluble silver salt is 

 added. 



The microscope can be used to advantage here. 

 If square right angle blocks (sodium chlorid) are 

 formd adhering to the long nitrate bars the crystals 

 will have to be redissolvd. If no blocks of sodium 

 chlorid are seen it may be taken for granted that 

 purification may be brot about by continued, drop by 

 drop, washing wdth cold water. The crystals when 

 pure may be dried by pressing between filter paper or 

 by allowing to stand over one exercise. 



NOTES 



The Witt filter consists of a perforated 

 porcelain disk in a funnel fitted into a 

 heavy glass suction flask, having connection 

 with an aspirator. It is a convenient 

 and rapid means of filtering when prop- 

 erly used. The student should be supplied 

 with paper filters of a diameter about one 

 centimeter greater than that of the porcelain 

 plate. Should the plates become chipt they 

 can still be used if a small piece of filter 

 paper is torn off and laid over the damaged 

 place. Unless the chipt place is so closed 

 pressure will make a hole in the filter paper 

 allowing the precipitate to pass thru into the 

 filtrate. 



The flask should be supphed with rubber 

 tubing of ordinary thickness, not pressure 



