2^. Dr Colquhoun 07i the Aasaij 



these ores. The amount of lime thus used will vary accord- 

 ing to circumstances between 5 and 20 or even 30 per cent, of 

 the ore, being increased or diminished according to the amount 

 and the nature of the earthy matter which the ore contains. 



In assaying ores which are overcharged with silica, and des- 

 titute of clay, it will often be found advantageous to employ, 

 in conjunction with the lime, a portion of pure clay, varying 

 in amount from 5 to 10 per cent, of the ironstone, according 

 to the abundance of the silica. 



In the same manner, in order to promote fusibility in any 

 argillaceous ore which is more than usually destitute of silica, 

 it will be proper to add a certain amount of that substance, 

 varying from 8 to 12 per cent, of the ore, in order to form a 

 flux along with the lime. 



But before leaving this topic it is material to observe, that 

 of all the ingredients which are associated with iron in the ore, 

 clay is the most frequent and abundant, and that which re- 

 quires the greatest delicacy in adjusting to it the due propor- 

 tion of its counteracting flux. If the lime be exhibited in ex- 

 cess, the resulting scoria is difficultly fusible and viscous ; and 

 of course, obstructs the coalescence of the particles of reduced 

 metal into a single mass ; but if too small a portion of lime be 

 employed, the consequence will be, either that vitrifaction 

 will take place defectively in the furnace, or that the portion 

 .of the siliceous ingredient of the clay which is left unsaturat- 

 ed, will dissolve and unite with a portion of the oxide of iron, 

 so as to shield it from reduction. It is therefore an object of 

 primary importance to the assayist to determine the exact pro- 

 portion of argillaceous matter contained in any ore, before he 

 commences its assay ; and as this may be easily done, even by 

 those who have very little experience in chemical manipulations, 

 it ought in all cases to be made the subject of a separate and 

 preliminary experiment. This analysis may be conducted in 

 the following manner. 



Take 100 grains of the pulverized ore ; put it into a wide- 

 mouthed flask, and pour over it an ounce measure of muriatic 

 acid, which has been previously diluted with two ounce mea- 

 sures of water. An effervescence will instantly take place oc- 



