CARNOT' s MODIFICATION 271 



ble. The capsule, after cooling, is covered with a funnel, and from 

 30 to 40 cubic centimeters of water are poured upon the ignited 

 precipitate, followed by three cubic centimeters of glacial acetic 

 acid. After the evolution of carbon dioxid is finished the funnel 

 is removed and washed, the residual liquor is evaporated to 

 near dryness, water added, and the evaporation repeated two 

 or three times and finished by evaporating to dryness. The res- 

 idue is taken up with 30 or 40 cubic centimeters of water con- 

 taining one per cent, of acetic acid. After heating for a mo- 

 ment the fluorid of calcium which remains insoluble is collected 

 and washed with water slightly acidulated with acetic. The 

 washing is finished with pure water. The absence of sulfates in 

 the last wash water should be ascertained by a careful test. It 

 is rather difficult to filter the calcium fluorid, and some precau- 

 tions to avoid a turbid filtrate will be found necessary. The pre- 

 cipitate is dried, ignited and the fluorid of calcium, obtained in a 

 perfectly pure state, weighed. The purity of the residue can be 

 determined by dissolving with concentrated sulfuric acid, after* 

 wards diluting a little and precipitating by alcohol. The sulfate 

 of lime obtained in this way should correspond, molecularly, to 

 the original calcium fluorid. According to Lasne, this method, 

 if carefully followed, gives results which are rigorously exact. 



231. Carnot's Modification. Carnot has proposed to shorten 

 the method of Lasne in the following manner: 



In place of receiving the fluorid of silicium in caustic soda, 

 the gas is conducted into a solution of fluorid of potash, the 

 delivery tube being plunged into mercury to avoid obstruction. 

 There is thus formed a fluosilicate, which is precipitated by 

 alcohol, collected upon a tared filter and weighed. Lasne con- 

 sidered this method to be, in fact, more rapid, but dangerous. It 

 is always inconvenient, according to him, to use in determination 

 a body of the same nature as that which is to be determined, a 

 process which renders all final verification impossible. 



Goutal has criticised the method of Lasne. preferring the shorter 

 method of Carnot mentioned above. 29 His objections to the 

 method of Lasne are based upon the following: 



w Annales de Chimie analytique, 1897, 2 : 401. 



