“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.