262 -Dr Colquhoun 07i the Asmy 



the quantity of flux which is taken will of course require to 

 be increased, as 100 parts of the carbonate are equivalent to 

 only 5Q parts of pure lime. 



In preparing silica, it is only necessary to reduce colour- 

 less transparent quartz, or calcined flint, to an impalpable 

 powder. The pulverization of the flint is materially facilita- 

 ted by the previous calcination. 



Pure white coloured pipe-clay, which is composed of little 

 else than alumina and silica, the silica being almost always 

 the preponderating ingredient, will be found a very useful 

 material in any case where the presence of a small quantity of 

 alumina is likely to promote vitrification in the furnace. It 

 will be observed that all clays, however similar in appearance, 

 if obtained from different localities, are liable to variation in 

 the proportion of their constituents, and it is therefore proper, 

 in the prosecution of a series of assays, to preserve a stock of 

 the same clay in the laboratory, so as to make sure of always 

 employing a reagent of the same description in each experi- 

 ment. Without this precaution it is not always safe to com- 

 pare the results of different assays with each other. 



Fluor spar may be obtained by reducing to powder a tran- 

 sparent fragment of fluor or Derbyshire spar. This spar 

 should be carefully selected, so as to be free from pyrites, a 

 matter which requires the greater attention, as specimens often 

 occur, through which minute crystals of pyrites are dissemi- 

 nated to a considerable amount. 



Borax, before being used, should always be well calcined, 

 to get rid of its water. Let it be first exposed to the moderate 

 temperature of a sand-bath, until it ceases to swell up, and then 

 let it be acted on by a dull-red heat. While yet hot, it should 

 be pounded, sifted, and transferred into a stoppered bottle. 

 Unless this be done before the borax has been allowed to be- 

 come cold, or if the stopper do not accurately fit the neck of 

 the bottle, the salt will speedily regain its water by absorbing 

 the atmospheric humidity. 



These seem to be all the substances which can be required 

 to compose any flux in the assay of ironstones. We shall next 

 consider in what manner the ore itself may be fitted for the 

 crucible. 



