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degree of denfity of the vapour, much affe£ts■the colouring of the acid, fo that other thingsbeing the fame, the acid will affume a highercolour when heated in a fhort tube than in along one where the vapour can expand.

r l he contadf of air was not found at all•neceflary to the colouring of nitric acid, foron confining feparate portions of acid in tubeswith different fpecies of air and in vacuo, andapplying an equal degree of heat to all, theyall affirmed nearly the fame fhade of colour ina given time.

The fmalleft quantity of combuftible (orphlogiftic) matter will almoft immediately ren-der pale nitric acid high coloured when addedto it, and the fame change takes place in a fhorttime even when the matter is only contiguousto it, as for example when the acid is kept in aphial flopped with a cork.

Fuming nitrons acid is made colourlefs byfimple expofure to the air, but this feems to beeffefted folely by the great dilution which itgradually undergoes from the water which itabforbs from the atmofpliere. Thus Dr.Prieftley found on expofing a glafs tumblercontaining fome of the acid to this open air,that in a day or two it became colourlefs, andfor feveral months it gained weight notwith-ftanding the lofs which it muff have undergoneby evaporation, till it had acquired double itsoriginal weight, and of courfe was proportion-ally dilute.

Light alfo has a fimilar effect with heat inColouring pale nitrous acid, which, as thefame excellent obferver has remarked, takesplace in a few days, but it appears to a£t onlyon the vapour of the acid, for even thepalefl acid when concentrated is conllantlylending forth a vapour which is at firfl colour-lefs, and this vapour when coloured by thelight feems to rea£t on the acid and to efferit afimilar change. Light however is not neceffaryto affift the aclion of heat lince the fame changewas found by heating the acid in opake veffels asin tranfparent ones. To prove that the colorifica£lion of light was exerted only on the vapourof the acid, Dr. P. made a quantity of colour-lefs acid by boiling the common yellow acidhaflily, and letting it cool in the dark, and putdifferent portions of it in phials, fome of themquite full, others only half full and with diffe-rent fpecies of air incumbent upon them, andexpofed them all to the fun’s rays for a fewdays. After this, time the acid in all the phialsthat were only half full was confjderably colour-ed* but in the full bottles it remained as

colourlefs as water. The fame effect watproduced on the vapour when a vacuum waimade over the acid as foon as introduced. Alfoon throwing the ftrong light and heat of a lenson the upper part of a phial half full of acid,the vapour became fpeedily coloured, and thisbeing imbibed by the acid gradually tinged it,the colour fpreading from above downwards.

Whenever pale nitric acid is made to yield aquantity of red nitrous vapour there is at thefame time a production of oxygen gas, andalfo as appears from Dr. Prieftley’s experiment,a quantity of azotic gas.

He found on filling a tube with the pale acid,boiling it within the tube and doling it her-metically when boiling, (in which way almollevery particle of atmofpheric air is excluded bythe boiling vapour) that after the acid has beencoloured by heat and red vapour formed, aquantity of air ruffed out of the tube whenopened, which was found to conlill of a largeproportion of oxygen and fome azotic gas.Thus it appears that oxygen gas and nitrousvapour can exilt in the fame place and in aHate of confiderable condenfation, without mu-tual decompofition.

The action of heat on nitric acid thereforeappears to be the following: fuppoling the acidto contain no feparable portion of nitrous gasin folution, the heat appears firfl: to decompofepart of the acid into nitrous gas and oxygen,and thefe being expelled as foon as formed,carry up in folution a certain portion of unde-compounded acid, and the three together fovrnnitrous vapour. The oxygen however feems notto be an elfential conllituent of the vapour andperhaps is only mechanically mingled with it.If the acid is clofely confined in a veffel whichit does not fill, the vapour remains un-changed in the empty fpace above the acid,and when the heat is withdrawn, a portion ofit (more or lefs according to the denfity -andelallicity) is again abforbed by the acid andrenders it coloured. A part at lealt of theoxygen gas remains in the gafeous Hate un-altered, but it is by no means improbable thatas foon as the heat is removed, another part ofthe oxygen may again combine with the nitrousgas which it had quitted and return into thebody of acid.

Where pale nitric acid is enclofed in vefleltwhich it entirely fills it is probable that nocontinuance of heat could render it coloured,for as the colouring is owing to its diffoivingnitrous gas, and as this gas cannot be formedwithout the previous decompofition of a portion