Intelligence and Miscellaneous Articles. 315 



attempt has ever been made to deprive concentrated sulphuric acid 

 of spec. grav. 1*848 of its water, without previously causing it to 

 enter into combination and form a salt. The same may also happen 

 when monohydrated sulphuric acid is employed in the preparation 

 of fluoboric and fluosilicic acids, which are considered as substances 

 having great affinity for water. 



The reaction which M. Barreswil employs he considers as ex- 

 tremely simple. He mixes anhydrous phosphoric acid, with the 

 sulphuric acid of commerce, and leaves them in contact, and the 

 mixture is afterwards heated : the combination of the two acids pro- 

 duces an increase of temperature, and some acid vapours soon ap- 

 pear ; but this is prevented by proceeding cautiously, and keeping 

 the acids in a freezing mixture : by distillation anhydrous sulphuric 

 acid is disengaged, and vitreous hydrated phosphoric acid remains ; 

 the distillation is effected in the same way as the Saxon acid. 



A circumstance which struck the author in this operation, is the 

 fact of the innocuousness of the mixture of monohydrated sulphuric 

 acid and anhydrous phosphoric acid, with respect to organic matters, 

 si;ch as paper and cotton, which are instantaneously destroyed by 

 the Saxon acid. The author considers this circumstance as a proof 

 that the sulphuric acid in the mixture is not anhydrous, but becomes 

 so when heat is applied. 



Even if the reaction above described possesses interest in a theo- 

 retical point of view, M. Barreswil admits that as a manufacturing 

 process it is unimportant, and will hardly be regarded as a ready 

 method of obtaining anhydrous sulphuric acid. The high price of 

 phosphorus, and the difficulty of preparing anhydrous phosphoric 

 acid, ai-e obstacles to the employment of the process. — Comptes 

 Jiendus, Juillet 5, 1847. 



OBSERVATIONS ON SILICA. BY M. DOVERI. 



It results from the experiments detailed by the author — 



1. 'I'hat the alkaline silicates, when decomposed by acids, and 

 particularly hydrochloric acid, deposit the greater part of the silica 

 which they contain if the acid in excess be added drop by drop ; 

 whereas the same quantity of acid added at once does not occasion 

 the precipitation of the smallest portion of silica. 



2. That silica, once precipitated, does not redissolve in acids, 

 whatever may have been its origin, whether precipitated from an 

 alkaline silicate by an acid, or from fluoride of silicium by water. 



3. That weak acids, as the carbonic, sulphiu'ous, boracic and the 

 vegetable acids, decompose the alkaline silicates at common tempe- 

 ratures, and precipitate the silica either as a jelly or in gelatinous 

 flocculi. 



4. That very finely-divided silica, whether anhydrous or hydrated, 

 is capable of decomposing the aqueous solutions of the alkaline car- 

 bonates, and dissolving in the solution at a boiling heat. 



5. That silica precipitated at common temperatures from a solu- 



