326 Dr. Priestley’s Experiments and Obfervations on 
nitre, or about 23! grains of nitre cryftallized in mean tempe- 
rature *. 
The difficulty of finding even an approximation of the quan- 
tity of acid, or of air expended in forming the brown calx, 
mud be very great ; and this circumftance (hews the diiad van- 
tage of tiling metallic veflels, although in other refpedts more 
commodious for your large experiments than thole of glafs. 
I think it is probable, that the quantity of acid expended in 
this manner is as much or more than the quantity retained in 
the folution ; but I am lorry I am not able to give you any 
grounds for a true determination. However, the experiment 
which I made with the calx was as follows. I diflolved it in 
vitriolic acid, and having faturated with alkali the fuperfluous 
acid, I precipitated the folution by a fixed alkali of known 
ffrength, and I found that the quantity of alkali reqnilite for 
this precipitation was as much as would laturate ten grains of 
the oil of vitriol before mentioned. This quantity of vitriolic 
acid is fo. much lefs than I Ihould expedf, that 1 regret I have 
not an opportunity of repeating the experiment. 
The next point to be determined is the Jpecies of acid that is 
contained in the green liquor. The acid produced in your 
former experiments of burning inflammable and dephlogifli- 
cated airs was determined to be the nitrous ; and every trial 
which I have made of this green liquor has convinced me, 
that the fame acid is contained in it alfo. Upon adding to this 
liquor fome vitriolic acid and diftilled water, I obtained by 
* I fay the mean temperature , becaufe the proportion of vrater retained by 
falts varies very confiderably, according to the temperature in which they are 
cryftallized ; the warmer the weather, the lefs water being retained. The confi- 
deration of this fa£t ought to enter into til® eftimates of the proportions of the 
««mponent parts of falts. 
7 
diflillation 
