N I T

N I T

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have a way of lining the iron pots with anenamel glazing, which mult unite 'all the advan-tages of iron and glafs.

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Nitric acid with its varieties and combinationshas been the fubjedt of more laborious, pro-found, and various chemical refearch than per-haps any other fubftance that has ever occupiedthe chemift: and it is well entitled to thisattention on account of the readinefs withwhich it is decompofed, and the great afliftancewhich as a chemical reagent it affords in the ana-lyfis of almoft every other fubftance in nature.

From the admirable refearches of Scheele ,Cavendifh, Prieftley, Lavoifier, Berthollet , Davyand many others, we have now acquired a toler-ably accurate idea of the nature and combinationsof this acid, though fome important points ftillremain uncertain.

The appearance and colour of this acid varyconfiderably: in one ftate it is perfectly limpidand colourlefs, and when poured out fends forthwhitifh pungent fumes: it is then properly calledNitric acid. In another ftate it is yellow orangeor fometimes of a dark red, emitting copiousred fumes, when it is termed in modern nomen-clature Nitrous acid.

Lavoifier was of opinion that the differencebetween thefe two acids confifted in the degreeof oxygenation, the former being the moft, thelatter the leaft oxygenated. This diftinftionhowever, though in one fenfe it expreffes thefact, is not perfeftly accurate, fince it will befeen by numerous experiments to be prefentlyrelated, that the nitric acid becomes convertedinto tile nitrous fimply by abforption of nitrousgas without any obvious difference in the degreeof oxygenation of the nitric acid itfelf, andconfequently when either acid is faturated withan alkali the refulting fait is the fame in both,and not a more oxygenated fait with the nitric,and a lefs oxygenated with the nitrous, as wouldhappen were the acid itfelf different in each.Therefore though the adtual quantity' of oxy-gen is greater in a given weight of nitric acidthan in the fame weight of nitrous acid, andthough they are both compofed of azot andoxygen, there appears to be only one ftate ofoxygenation of this acid, and the nitrous gaswhich makes the difference between the nitrousand nitric acids muft be here confidered asdoing little more than diluting the acid, andthus diminifhing the afftual quantity of it in ajgiven weight.

Pure colourlefs nitric acid is invariable in itscompofition as far as has hitherto been found,and no other varieties of it are produced indiftillation than what arifes from the quantityof water with which it is diluted, and withwhich it wili mix in every proportion.

Nitrous acid on the other hand exifts in anindefinite number of varieties according to thequantity of nitrous gas which it contains, whichlaft again is regulated both by the quantity ofwater prefent, and by the temperature. Thereis however a point of faturation of nitric acidwith nitrous gas, at which it affumes a blue-green colour. With lefs quantities of nitrousgas the colour is green, olive, red, and yellow.The degree of dilution with water howevermuch influences the colour of the acid.

There is another form of acid which alforequires particular notice, and it is that whichDr. Prieftley in his moft admirable refearcheson this acid called Nitrous •vapour. It is nitrousacid in the form of red vapour, and confifls ofnitrous gas holding nitric acid in folution. Itis produced in feveral methods ; for example,if to nitrous gas be added oxygen gas, a redcloud is formed, which, if the gaffes be con-fined over water, is readily abforbed, but if nowater be prefent it remains as a permanent gas,and this is nitrous vapour. It is alfo expelledfrom the fuming nitrous acid by Heat, and it isproduced during the folutien of bifmuth, andfome other of the metals in nitric acid, wherethe action is very violent, the nitrous gas al-ways generated in thefe folutions, here diffolv-ing and carrying up a portion of the acid.

Nitrous gas therefore when in large quantityhas a ftrong tendency to volatilize the acid,and this is more powerful in proportion to itsconcentration, or inverfely as the water whichit contains. The proportions of nitrous gas andnitric acid in nitrous vapour vary confiderably.

There are four diltinct fubftances con-cerned in the fubjeft of nitric acid, namely:Nitrous gas, (whole properties have been partlydefcribed under the article Eudiometry , andwill be further noticed in their place,) Nitrousvapour , or a gafeous combination of nitrous gasand nitric acid, Nitrous acid, compofed ofnitric acid, water, and nitrous gas in varyingproportions, and Nitric acid, or the pure fatu-rated form of azot and oxygen, rendered liquidby union with water. Another combination ofthefe elements, Nitrous oxyd or gafeous oxyd ofazot, (dephlogijhcated nitrous air ofPrieftleyq willbe feparately defcribed, and its properties neednot here be confidered.