S6 Gn the best Method of decpmpos'r^ 



and calcine slrongly in order to evaporate the sulphaie of 

 animouia and conjpletely deconij^ost' the chroiiiate oF am- 

 jnonia : lastly, weigh the oxide of chrome. The latter, 

 when boiled with nitric acid, onght to give no sign of sul- 

 phuric acid with the nitrate of baiyics. 



If the proportions of the chromate of barytes which wc^ 

 have obtained are very exact, as there hi every reason to 

 believe they arc, the same analysis may serve for those of 

 other chromates which are soluble in water. 



Process for ollainhg the Chromic Acid pure. 



Among the various methods which may be employed for 

 preparing ihis acid, the most preferable, in our opinion, 

 consists in decomposing the chromate of barytes by the sul- 

 phuric acid : all other processes were attended with more or 

 jess difiTiculties. 



We must therefore dissolve the chromate of barytes in 

 weak nitrous acid ; afterwards carcj^ully precipitate it by 

 means of sulphuric acid, so that all the salt may be decom- 

 posed without the sulphuric acid being in excess. If by 

 iihance we exceed this point, we must separate the superabun- 

 dance of the acid by barytes water. We shall then find whe- 

 ther we have seized the point at which the precipitate formed 

 by chromic acid in barytes water is redissolved entirely iu 

 nitric acid, and at which the sulphuric acid does not disturb 

 ihis chromic acid. 



We then filter the liquor and carefully evaporate it, par- 

 ticularly towards the latter part of the operation, that vie 

 may not decompose the chromic acid : this evaporation 

 must be repeated several times to dryness, in order to expel 

 all the nitric acid. 



When the chromic acid is highly concentrated, irregular 

 {mammeloniiees) nmsses are formed, in which we see small 

 red crystals grouped together, but they are not permanent 

 in the air, as they. attract humidity from it. 



The chromic acid thus purified is .of a deep red colour, 

 has a very acid, but austere and metallic taste; ii is solu- 

 ble in alcohol, which speedily deconsposes it, for the so- 

 lution becomes green. 



luusy Method of determining the Quaniiiy of Chromic Acid 

 combined or mixed loith several Saline Substances. 



We must put into the solution containing chromic acid 

 a slight excess of nitric acid, if the salt be not of itself so- 

 luble nor acid : we must then pour in a little hydro-sulphu- 

 ret uf amuiouia, and auicer«ite for iouic lime iu a el'>sG, flasks 



after 



