202] Method of Analysis 127 



many other of the special methods used in agricultural analysis, 

 or indeed in any other branch of technical analysis, it is very 

 necessary that the instructions as to quantities, &c, should be 

 exactly followed out. 



202. Weigh out 2-5 grams of the finely powdered mineral 

 on a watch glass. Transfer to a beaker, washing the glass with 

 a little hot water. Add a little dilute HC1, and digest on top 

 of the water oven until all effervescence ceases. Now add 20 

 to 30 c.c. of pure strong HC1, and evaporate thoroughly to dry- 

 ness on the water bath. This will get rid of any fluorine which 

 may be present. When dry dissolve in about 10 c.c. of dilute 

 HC1 (one part of strong acid to four of water), allowing it to 

 digest until nothing is left undissolved excepting the siliceous 

 matter. This operation may be materially assisted by break- 

 ing up any lumps with a stirring rod. Pour the liquid into a 

 250-c.c. flask, washing the residue into the flask with the 

 smallest possible quantity of water— 25 c.c. should be sufficient. 

 Measure out 10 c.c. of pure strong H 2 S0 4 with a pipette, and 

 run it into the flask. A thick precipitate of CaSO t will be 

 formed. Shake the flask gently in a rotatory manner until the 

 liquids are thoroughly mixed, and allow to cool. Fill up to the 

 250-c.c. mark with 95 per cent, alcohol. On allowing this 

 to stand, the liquid will be found to contract. Shake it up 

 well ; allow it to cool, and make up to the mark again with 

 alcohol and shake. Place a funnel with a dry filter paper in 

 the mouth of a 200-c.c. flask, and filter off the liquid until 

 200 c.c. have been collected. Evaporate these 200 c.c. (which 

 contain the Fe and Al in 2 grams of the mineral) in a large 

 platinum dish. This may be started over a water bath, but as 

 soon as most of the alcohol has evaporated it must be placed on 

 a pipeclay triangle over a rose burner. Heat until the sulphuric 

 acid begins to fume strongly. This will thoroughly char any 

 organic matter which might otherwise interfere with subse- 

 quent operations. Allow the dish to cool, then wash it out 



