when accompanied by Silica. 353 



evolved during the action of the oil of vitriol, from the dust 

 which is gathered during a protracted process of powdering ; 

 but the presence of this acid in small quantity is of no im- 

 portance, and the powdering of the rock is the most trouble- 

 some part of the investigation. 



It is otherwise with weathered granite and trap, which 

 contain chlorides and carbonates, and give off hydrochloric 

 and carbonic acids when treated with sulphuric acid. These 

 gaseous acids materially interfere with the processes de- 

 scribed by the frothing which they occasion, and by their 

 tendency to sweep away the hydrofluoric acid which may 

 accompany them. In my earlier trials, accordingly, I treated 

 the powdered pieces of rock with hydrochloric acid, and 

 washed them with water, then dried them, and heated them 

 with oil of vitriol. The preliminary treatment, however, 

 risked, and, I have no doubt, occasioned, the loss of the fluo- 

 rides present in the mineral, which were soluble in water or 

 in hydrochloric acid, and latterly I abandoned this process. 

 I refer to it here only because it explains certain of the less 

 perfect etchings which are exhibited. 



In later trials, a simpler and more satisfactory process has 

 been put in practice. The powdered rock has been added to 

 oil of vitriol in the cold, in small quantities at a time, so as 

 to prevent any great rise in temperature. So long as the 

 heat evolved is not considerable, there is no risk of fluorine 

 escaping, either as hydrofluoric acid or as fluoride of silicon, 

 whilst any chlorides or carbonates present are decomposed, 

 and the hydrochloric or carbonic acids evolved are carried 

 away before their escape can interfere with the evolution of 

 fluorine. When the oil of vitriol is afterwards raised to its 

 boiling point, the fluoride of silicon is liberated, and little 

 difficulty attends its collection and identification. 



The ashes of plants are somewhat less easily examined. 

 They almost invariably contain charcoal, which occasions the 

 evolution of sulphurous acid with hot oil of vitriol. Sul- 

 phurous acid, however, does not very materially interfere 

 with the detection of fluorine, as it can be expelled by heat- 

 ing the distillate before adding ammonia, which is the pro- 

 cess I have hitherto generally followed. It may also be 



