Subsequent experiments with nitric oxide showed that air from red precipitate or from “_mercurius calcinatus_” (red oxide of mercury in each case, although prepared in different ways) was “between four and five times as good as common air.” He proceeds:[6]--“Being now satisfied with respect to the _nature_ of this new species of air, viz. that being capable of taking more phlogiston from nitrous air, it therefore originally contains less of this principle, my next inquiry was, by what means it comes to be so pure, or, philosophically speaking, to be so much _dephlogisticated_.” He therefore went on to heat the various oxides of lead, but without any special results worth chronicling. On moistening red-lead with nitric acid, however, and distilling the mixture, he obtained, in successive operations, air which was “five times as good” as common air. This process formed lead nitrate, which on distillation yielded nitric peroxide and oxygen; the gas was, of course, collected over water, which absorbed the peroxide, allowing pure oxygen to pass. He found that red-lead was not the only “earth” which produced this effect; but that “flowers of zinc” (zinc oxide), chalk, slaked lime, and other substances also gave a gas, when distilled with nitric acid, which was “better” than common air. In some cases he broke up nitric acid by heat into water, nitric peroxide, and oxygen; in others he heated nitrates. His conclusion is: “_Atmospherical air_, or the thing we breathe, _consists of the nitrous acid and earth_, with so much phlogiston as is necessary to its elasticity; and likewise so much more as is required to bring it from its state of perfect purity to the mean condition in which we find it.”[7] When such experiments were made by heating nitrates in a gun-barrel, “phlogisticated air” was obtained. This was nitrogen, for the iron had reduced the oxides of the latter, and combining with their oxygen, had formed nitrogen; moreover, it had absorbed to a greater or less extent the oxygen simultaneously produced. Having concluded that respirable air was a compound of nitrous acid, phlogiston, and earth, Priestley endeavoured to ascertain what was the nature of this earth. He concludes “that the _metallic earths_, if free from phlogiston, are the most proper, and next to them the _calcareous earths_.” “Dephlogisticated air may be procured from any kind of earth with which the spirit of nitre will unite.” A few quantitative experiments would surely have refuted this erroneous conclusion. Those which he attempted to make were very crude. A bladder (of which he does not give the capacity) was filled with Phlogisticated air, and weighed 7 dwts. 15 grs. Nitrous air " " 7 " 16 " Common air " " 7 " 17 " Dephlogisticated air " " 7 " 19 "