“but that atmospherical air, or the thing that 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.”
Priestley’s “complete discovery of the constitution of the air we breathe” was thus wholly erroneous: he was very far indeed from having a clear conception of its real nature.
Priestley’s description of the main properties of oxygen is however accurate, and lecturers in chemistry are indebted to him for some striking experimental illustrations of them.
“I easily conjectured,” he says, “that inflammable air would explode with more violence and a louder report by the help of dephlogisticated than of common air; but the effect far exceeded my expectations, and it has never failed to surprise every person before whom I have made the experiment.... The dipping of a lighted candle into a jar filled with dephlogisticated air is alone a very beautiful experiment. The strength and vivacity of the flame is striking, and the heat produced by the flame in these circumstances is also remarkably great.... Nothing would be easier than to augment the force of fire to a prodigious degree by blowing it with dephlogisticated air instead of common air.... Possibly platina might be melted by means of it.
“From the greater strength and vivacity of the flame of a candle, in this pure air, it may be conjectured that it might be peculiarly salutary to the lungs in certain morbid cases.... But perhaps we may also infer from these experiments that though pure dephlogisticated air might be very useful as a medicine, it might not be so proper for us in the usual healthy state of the body: for, as a candle burns out much faster in dephlogisticated than in common air, so we might, as may be said, live out too fast, and the animal powers be too soon exhausted in this pure kind of air. A moralist, at least, may say that the air which Nature has provided for us is as good as we deserve.... Who can tell but that, in time, this pure air may become a fashionable article in luxury. Hitherto only two mice and myself have had the privilege of breathing it.”
An experiment which Priestley says “I had the pleasure to see at Paris, in the laboratory of Mr Lavoisier, my excellent fellow-labourer in these inquiries, and to whom, in a variety of respects, the philosophical part of the world has very great obligations,” led him into a train of inquiry upon the action of nitric acid upon a wide range of organic substances, from which however no general results followed, in spite of much experimenting. He had at one time the idea that a fundamental difference existed in the behaviour of animal and vegetable matter with respect to nitric acid, but the observations were contradictory, and although it is readily possible to interpret the phenomena in the light of our present knowledge, they led Priestley to no definite conclusions.
Of more importance is the work on the “Fluor Acid Air”—a substance discovered by “Mr Scheele, a Swede; from which circumstance the acid is often distinguished by the name of the Swedish acid.” Priestley sought to make the air by heating Derbyshire spar (fluor spar) with oil of vitriol in glass vessels,
“as in the process of making spirit of nitre from saltpetre; and the most remarkable facts that have been observed concerning it are, that the vessels in which the distillation is made are apt to be corroded; so that holes will be made quite through them; and that when there is water in the recipient, the surface of it will be covered with a crust of a friable stony matter.”
What Priestley actually produced by this method of experimenting was more or less pure silicon fluoride, which he proceeded to collect, in his usual fashion, over quicksilver.
“I had no sooner produced this new kind of air but I was eager to see the effect it would have on water, and to produce the stony crust formed by their union, as described by Mr Scheele; and I was not disappointed in my expectations. The moment the water came into contact with this air the surface of it became white and opaque by a stony film.... Few philosophical experiments exhibit a more pleasing appearance than this, which can only be made by first producing the air confined by quicksilver, and then admitting a large body of water to it. Most persons to whom I have shown the experiment have been exceedingly struck with it.... The union of this acid air and water may also be exhibited in another manner, which to some persons makes a still more striking experiment, viz., by admitting the air, as fast as it is generated, to a large body of water resting on quicksilver.... It is, then, very pleasing to observe that the moment any bubble of air, after passing through the quicksilver, reaches the water, it is instantly, as it were, converted into a stone; but continuing hollow for a short space of time, generally rises to the top of the water.... I have met with few persons who are soon weary of looking at it; and some could sit by it almost a whole hour, and be agreeably amused all the time.”
Priestley’s attempts to explain the real nature of the fluor acid air were, as may be expected, not very happy.