NITRIC ACID. 447 



nitre will be produced, from which the acid may be extracted. Min- 

 gled in proper proportions, the gases nearly disappear, in conse- 

 quence of their combination. 



(b.) The usual process for nitric acid.* A large retort, with an 

 adopter, tubulated receiverf and sand heat ; the lute, is clay 5 sand and 

 flour, equal measures, mixed and kneaded together ; proportions of 

 the salt and acid, nitre 6, sulphuric acid 4 to 6 ; the nitre in fragments : 

 if the receiver is not tubulated there should be an opening through it 

 for gas to escape; heat slowly raised, receiver kept cool; there 

 should be no water in it, if we would obtain a strong acid ; the pro- 

 cess lasts one hour, or two or more, according to the quantity ; there is 

 some red gas at the beginning, and much towards the end ; the retort 

 is clear in the middle of the experiment, and when the residuum 

 puffs up, we stop the process. To extract the caput mortuum, J warm 

 water is poured in very cautiously, and, at first, in very small portions, 

 for there is great heat and ebullition ; proceed carefully till the super- 

 sulphate of potassa is thus dissolved ; if left in, it almost infallibly 

 breaks the retort by crystallizing ; the excess of acid may be driven 

 away by heat, or neutralized by chalk, and then crystals of sulphate 

 of potassa will be obtained. 



(c.) Nitric acid is freed from muriatic, and in a good degree from 

 the sulphuric, by nitrate of silver ; the sulphuric acid is better re- 

 moved by distilling it again from J of the original quantity of very pure 

 nitre. 



(d.) Nitrate of baryta also separates the sulphuric acid ; it must 

 be re-distilled off from the precipitate, leaving J or T \ in the retort. 



(e.) Pure nitre is prepared by dissolving and crystallizing nitre 

 several times, the last time with the addition of nitrate of silver, to 

 precipitate the muriatic acid. Such nitre will, if the salt does not soil 

 the neck of the retort, and the heat is cautiously raised, and is not 

 raised too high, give pure nitric acid in the first process. 



(/.) The colored acid may be made clear by long and cautious 

 heating, to expel the nitric oxide gas, and a receiver may be used to 

 save any acid that rises. The acid is more easily cleared by a little 

 black oxide of manganese, placed in the retort which imparts oxygen 

 and converts the nitrous into nitric acid. Probably any other nitrate 

 would answer to afford nitric acid, but the nitrate of potash is the best.|| 



* Nitre may be decomposed by sulphuric acid in small quantities, with a naked glass 

 retort, over a lamp or live coals, and the adopter is not indispensable, as the receiver 

 is very little heated in the process. 



t Furnished with a bent tube if you wish to collect the gases evolved. 



t An old fanciful name given to the solid residuum from distillation. 



Or nitrate of lead ; but nitrate of silver is preferable. 



|) Dr. Thomson, (First Principles, Vol. I, p. 107,) says that pure nitre is anhy- 

 drous, and that if a little water be mechanically lodged between the plates of the 

 crystals, it is easily dissipated by a moderate heat or by fusion. When such nitre in 

 the proportion of 12| parts is decomposed by 6 \ parts of the strongest sulphuric 



