398 /• NickRs on Fluoriferous Sulphuric Acid. 



till the liquid had a slight movement, without actually boiling; 

 this operation was continued for a greater or less time, usually 

 thirty hours sufficing to obtain the sulphuric free from fluorine. 



To test its purity the following method may be adopted. Pat 

 about thirty grams of the acid in a platinum crucible large 

 enough to contain twice this quantity; add ten grams of dis- 

 tilled water, and immediately cover it with a plate of rock crys- 

 tal which had been prepared with wax and traced oyer with 

 regular geometrical figures, — such figures being more easily dis- 

 tinguished than others from any accidental striae or markings. 

 The rock-crystal should be kept cool with water, which for this 

 purpose should be often renewed. The vapor of the water pro- 

 duced by the heat at the first contact of the water and acid, 

 naturally condenses at first upon the cooled rock crystal. It is 

 indispensable that this condition should be fulfilled; for the 

 slight dew covering the bed of wax intercepts the fluohydnc 

 acid, which, as is well known, is yqyj soluble in water, and acts 

 only moderately on glass when it is free from humidity. 



When the acid is so far diluted as' not to heat up by the addi- 

 tion of a new quantity of water, it is necessary to have recourse 

 to the lamp or sand-bath. 



The temperature should be such that the hand cannot bear 

 direct contact Avith the crucible. After two hours, the operation 

 is completed. Then withdraw the crystal plate and remove 

 the wax, wiping with care and cooling it. If there is no tracing 

 apparent to the naked eye, and none while the film of vapor 

 from the breath covers it, it gives evidence of purity from 

 fluorine. 



When, as in the old process, we operate with a plate of glass, 

 and act upon carbonate of lime, such as calcined shells, shell of 

 lobsters, &c., it takes but little time to obtain the corrosion; an 

 effect which has, in fact, been already attributed to the presence 

 of fluorine. This is what was done by M. Jenzsch, with the 

 spars and aragonites,^ and also by Mr. Wilson. These effects 

 are easily explained; for a new affinity enters into play between 

 the carbonates, an affinity which modifies that between the sul- 

 phuric and fluohydric acids, and the latter acid, as a consequence, 



cannot fail to be disengaged. 



We may then, in the preceding experiment, use one of these 

 agents, and the better to insure success, it should be moistened 

 with a little distilled water. The experiment may then be fin- 



ished in half an hour, especially if aided by heat. This process, 



based on the partial neutralization of the sulphuric acid under 

 examination, is more sensitive and, especially, more expeditious 

 than the preceding, 



r 



* Poggendorflf's Annalen^ 1865. 



