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We fhall firft mention fome circumftancesthat occur during the diftillation of nitric acidthat affedt the nature of the produdt. If theacid that comes over be examined at differentperiods of the diftillation, the firft portion willhave the greateft fpscific gravity and alfo theleaft colour. Dr. Prieftley b has an experimentto this purpofe. On receiving the acid at fourtimes, the firft produce was very pale, andheavier than the fucceeding portions, effer-vefced violently with water, emitting red fumes.

The fecond part of the produce was ftill paler,and alfo effervefced with water. The thirdpart was brown, and mixed with water quietly:the fourth was of. a deep orange colour. Proufthas given the following experiments on thisfubjedl. A pure nitric acid c was prepared bynitre and fulphuric acid which gave no precipi-tate with nitrat of barytes, and therefore con-tained no fulphuric acid. This was boiled toexpell fome of the fuperfiuous nitrous gas, afterwhich it remained yellow, and of the fpecificgravity of 1.52. The fame acid being againdiftilled, the firft product was 1.51 fp. g. andlefs yellow: the fecond produa which was ftillpaler was alfo 1.5 1, but the remainder in theretort which was quite colourlefs, was only1.47. It may be observed that the ftronger theacid is the lefts heat it requires to be volatilized.

Mr. Perceval d found on diftilling two pounds ofnitre with one of oil of vitriol and dividing theprodua into three equal parts, that the fhftportion was 1.494 fp. gr. the fecond 1.485, andthe third 1.442.

From thefe experiments it is fhewn to beinrpoffible to concentrate the nitric acid bydiftillation, fo that when the moft concentratedacid is required, very dry nitre and ftrongfulphuric acid mult he ufed, and the firft produa:fet apart. Mr. Kirwan e found that the Itrongeftacid he could procure was 1.5543 fp. gr. at 6o°

It was yellowilh red, and exceffivdy fuming andvolatile. I his laft circumftance is owing to thefmail quantity of water contained in the mate-rials before diftillation, as Dr. Prieftley hadalfo rem eked, for the quantity of nitrous gaswhich nitric acid is capable of diffolving ismverfely in proportion to the water it contains,lence it is that even a concentrated pale acid(w rich however ftill holds much nitrous gas ino ut lon ) will give out copious red fumes of*Th S V fi' P ° ur on dilution with water. .

e effects of heat on nitric acid are curious

• Dr - Prieftle 7 was the firftthat examined this fubjedt with accuracy; many

* Experiments on Air, vel.i. . j ourn , de Phyf , Tom _ 4?j p . ;9

of his experiments deferve particular mention.In moft of them the acid was incloled in glafstubes hermetically fealed by melting the end ofthe glafs by which all adtion of external fub-ftances was prevented except heat and lightwhich could permeate the glafs. Some ftrongnitric acid of a pale yellow was put into a glafstube four feet long and one third of an inch indiameter, the fpace occupied by the acid beingabout two inches of the tube, and the open endthrough which it was introduced was clofedhermetically. The tube thus prepared washeld in the hand at the empty end, and the otherprefented to a common fire. The iirft effectwas the acid affuming an orange colour through-out, after which an orange-coloured vapourappeared above the furface of the acid andgradually afeended higher in the tube, at thefame time that the acid grew paler and at laftwas left again colourlefs. The red vapour rofehigher and higher in the tube as the heat wascontinued, leaving a confiderable fpace betweenit and the acid, fumetimes as much as ten ortwelve inches of the tube, quite tranfparent.On continuing the heat however the quantityof vapour increafed and the colour deepened.It was then removed from the fire, when thered vapour prefently defeended to the furfaceof the colourlefs acid, and entering into itcommunicated the orange colour. The acidhowever did not feem to be more coloured thanat firft by this procefs, but on repeating it withthe fame tuhe for many times, and boiling itfor a confiderable time the vapour changed fromorange to green, and the acid remained perma-nent^ of an orange-red. The effedt of heatupon the coloured acid is quite the reverfe ofthat on the nitrous vapour, for when the vapouris heated its colour always was found to deepento a dark orange-red, whereas the colouredacid becomes pale and limpid. If the nicfl:coloured acid be boiled in open vefli ls io as toallow the efcape of the nitrous vapour, it re-mains pale after it is cold, but in clofed tubes(as above related) the vapour generated by thehest and confined over the furface of the acid,is (in part at leaft) again ahlorbed on cooling,and imparts its own orange colour.

That the effeft of heat is actually to expellthe vapour from the acid, is proved from thecircumftance that when a part of the tube wasfoftened by heat, the preffure of the vapourwithin burft through the glafs at that part.The degree in which the acid is comprefled bythe nitrous vapour, or (which is the lame) theRrval Irifh Tranf. veliv. « Ibid. p. ij,.