If we consider experiment f as accurate, with regard to the relative quantities of carbonic acid and nitrogene produced, they are to each other nearly as 20 to 8; that is, allowing 2 for the nitrous gas, and consequently, reasoning in the same manner as Lavoisier, concerning the composition of nitric acid, it should be composed of 1 nitrogene to 3,38 oxygene. But though the quantity of oxygene in this estimation is far short of that given in his, yet still it is too much. From whatever source the errors arise, whether from the evolution of phlogisticated nitrous acid, or the decomposition of water, or the production of nitrous gas, they all tend to increase the proportion of the carbonic acid to the nitrogene.
I am unacquainted with any experiment from which accurate opinions concerning the different relative proportions of oxygene and nitrogene in the nitric and nitrous acids could be deduced. Lavoisier’s calculation is founded on his fundamental experiment, and on the combination of nitrous gas and oxygene.
Dr. Priestley’s experiment mentioned in section 12, on the absorption of nitrous gas by nitrous acid, from which Kirwan[51] deduces the composition of the differently colored nitrous acids, was made over water, by which, as is evident from a minute examination of the facts[52], the greater portion of the nitrous gas employed was absorbed.
XIV. The opinions heretofore adopted respecting the quantities of real or true acid in solutions of nitrous acid of different specific gravities, have been founded on experiments made on the nitro-neutral salts, the most accurate of which are those of Kirwan, Bergman, and Wenzel. The great difference in the results of these celebrated men, proves the difficulty of the investigation, and the existence of sources of error.[53] Kirwan deduces the composition of the solutions of nitrous acid in water, from an experiment on the formation of nitrated soda. In this experiment, 36,05 grains of soda were saturated by 145 grains of nitrous acid, of specific gravity 1,2754. By a test experiment, he found the quantity of salt formed to be 85,142 grains.[54] Hence he concludes that 100 parts of nitrous acid, of specific gravity 1,5543, contain 73,54 of the strongest, or most concentrated acid.
Supposing his estimation perfectly true, 100 parts of the aëriform acid of 55° would be composed of 74,54 of his real acid, and 25,46 water. In examining, however, one of his later experiments,[55] we shall find reasons for concluding, that the acid in nitrated soda cannot contain much less water than the aëriform acid. A solution of carbonated soda, containing 125 grains of real alkali, was saturated by 306,2 grains of nitrous acid, of specific gravity 1,416. The evaporation was carried on in a temperature not exceeding 120°, and the residuum exposed to a heat of 400° for six hours, at the end of which time it weighed 308 grains. Now according to my estimation, 306 grains of nitric acid, of 1,416, should contain 215 true acid; and we can hardly suppose, but that during the evaporation and consequent long exposure to heat, some of the nitrated soda was lost with the water.
Bergman estimates the quantity of water in this salt at 25, and the acid at 43 per cent; but his real acid was not so concentrated as Kirwan’s, consequently the nitric acid in nitrated soda should contain more water than my true acid.
Wenzel, from an experiment on the composition of nitrated soda, concludes that it contains 37,48 of alkali, and 62,52 of nitrous acid; and 1000 of this acid, from Kirwan’s calculation, contain 812,6 of his real acid; consequently, 100 parts of my aëriform acid should contain 93,28 of Wenzel’s acid, and 6,72 of water.
I saturated with potash 54 grains of solution of nitric acid, of specific gravity 1,301. Evaporated at about 212°, it produced 66 grains of nitre. This nitre exposed to a higher temperature, and kept in fusion for some time, was reduced to 60 grains.
Now from the table, 54 of 1,301, should contain 26,5 of true acid. But according to Kirwan’s estimation, 100 parts of dry nitre contain 44[56] of his real acid, with 4 water; consequently 60 should contain 26,4.
Again, 90 grains of acid, of specific gravity 1,504, saturated with potash, and treated in the same manner, gave 173 grains of dry nitre. Consequently, 100 parts of it should contain 47,3 grains of true acid.