ON SILICA IN AN IGNEOUS EOCK, 471 



An Accurate and Rapid Method of estimating the Silica in an 

 Igneous Rock. By J. H. Plater. 



[A communication ordered by the General Committee to be printed in extenso 



among the Eeports.] 



Prepare a flux by intimately mixing together two parts of bicarbonate 

 of soda, two parts of bicarbonate of potash, and one part of nitrate of 

 potash, all finely ground. 



The mixture must be heated until dry and perfectly pulverulent. 



In a platinum crucible, six centimetres high, mix thoroughly half a 

 gramme of the very finely powdered rock with one gramme of the flux, 

 place a cover on the crucible, and heat the contents to fusion. 



"With a Fletcher's gas crucible furnace, the chimney being used, com- 

 plete fusion can be effected in five minutes, with a small blowpipe gas 

 furnace in three minutes. Working with rocks containing not less than 

 45 per cent, of silica I have never found a crucible attacked. I have not 

 used the process with less siliceous rocks. 



After fusion remove the cover from the crucible and replace it by a small 

 glass funnel cut off at the apex of the cone ; then, without waiting for the 

 fused mass to cool, pour in it through the funnel 3 cc. of strong nitric acid. 

 Energetic action ensues, but there is no projection of the solid or fluid 

 contents of the crucible. Place it at once on a water bath and keep it for 

 ten minutes at a temperature of 100° C; at the end of that time if all hard, 

 and dark particles have not disappeared add 2 cc. of nitric acid, stir with 

 a platinum rod, and continue to heat. As soon as the disintegration of 

 the glass is completed wash with as little water as possible the crucible 

 cover, funnel, and rod, then evaporate the jelly and the washings to- 

 gether. After having driven off the greater part of the water, when 

 there is no longer any fear of projection, remove the crucible to a sand 

 bath and heat just below 250° C. until all nitrous fumes shall have dis- 

 appeared. 



The operation has up to this point been carried on in one vessel; now 

 turn the mass into a small glass beaker, washing the crucible, first with 

 5 cc. of water, then with 3 cc. of hydrochloric acid ; heat the beaker to 

 100° C, that the bases may be quickly dissolved by the acid, until the 

 silica left be white and free from red specks ; add now more water and 

 pour the contents of the beaker on to a 590 Schleicher and Schulls filter, 

 placed over a filter-pump ; drain as dry as possible, and calcine at once in 

 a small crucible. "Weigh the crucible and its calcined contents, con- 

 sisting of silica mixed, if these bodies be present in the rock, with 

 minute quantities of iron oxide and alumina, and a much larger quantity 

 of titanic acid. After weighing moisten very slightly with four or five 

 drops of strong sulphuric acid, stir the moistened silica with one gramme 

 of perfectly pure ammonium fluoride, put a cover on the crucible, and heat 

 it, first of all gently, then more strongly, finally to a full red heat. If the 

 few milligrammes left in the crucible are not perfectly soft repeat the 

 process ; but this it should not be necessary to do. 



The weight of the crucible and the calcined residuum having been 

 ascertained, it must be deducted from the previously recorded weight ; the 

 difference will give the weight of the silica present in half a gramme of 

 the rock. 



