Memoirs of Dr. Joseph Priestley

by Joseph Priestley

Chap. 7th. The elastic force of the Air depends on its nitro-aereal particles. In what way exhausted air is reimpregnated with them. Of the elements of Heat and Cold. This chapter contains experiments to shew that the elasticity of the air is owing to the nitro-aereal particles contained in it: which may be destroyed by the burning of a candle or other combustible substances, and also by the breathing of animals. When the atmospheric air contained in a glass jar inverted over water, will no longer support flame or animal life, the water rises in the jar, owing to the diminished elasticity of the air, not being able to counteract the pressure of the surrounding atmosphere on the water p. 100. He finds p. 101 that the diminution by burning a taper in a given quantity of the air, is about one thirtieth of the whole, and by the breathing of mice and other animals about one fourteenth. Thence he concludes p. 106 that by means of respiration the elastic part of the air enters into the blood, and that the sole use of the lungs is not as some suppose, to break down the blood in its passage into very minute particles. That combustion and respiration have similar effects on atmospherical air, he concludes, p. 108, from the fact, that a candle and a small animal inclosed together in a glass jar over water, the one will not burn, nor the other remain alive above half the time that they would if alone. Mayow however, did not consider his nitro-igneous and elastic particles to be either pure air, or even a component part of the common air, as air, notwithstanding the ambiguity of the passages in p. 114 and 118; but as particles of a different nature, attached to and fixed in the atmospheric particles; and detached (excussas) by the means above mentioned, p. 118 and 121. His explanation of elasticity generally in this chap. and of the difficulty arising from the obvious resistance to the Atmosphere, and the expansibility of the air in which a taper has been extinguished, or an animal died, seem too obscure and unintelligible to merit transcribing. It is evident however upon the whole from p. 123 compared with p. 100 and 135 that he conceived the diminution of such air to arise from diminished elasticity, but he supposes it to be denser than common air 123. In a subsequent part of this chapter p. 128 et seq. he states his theory of the manner in which deteriorated air recovers its loss, viz. that the nitro-aereal particles being lighter than the atmospherical, float abundantly in the higher regions; and that the part of the atmosphere deprived of them below, being forced upward by the pressure of the atmosphere above, obtains a renewal of these particles by mixture with the strata where they abound.

The element of fire, he supposes to reside in the body of the Sun, which is no other than a mass of nitro-aereal particles driven in perpetual gyration with immense velocity. Cold, which he considers as some thing positive (p. 130) he thinks consists in these particles assuming a pointed form, and moving not in gyration but strait forward. Much of his reasoning indeed throughout the book, savours greatly of the mechanical and corpuscular philosophy prevalent in his day.

Chap. 8th. On the nitro-aereal spirit as inspired by animals. Formerly he thought that in respiration the nitro-aereal particles were rubbed or shaken off (atterere, excutere 146) from the common air by the action of the lungs, at present he thinks the air itself enters the mass of the blood, is there deprived of these particles, and of part of its elasticity. To prove this he produces an experiment of the diminution of air by the vapours from iron dissolved in nitrous acid: but the beautiful deductions of Dr. Priestley from a similar experiment, never occurred to him; on the contrary he expressly states that it is an Aura, but not Air p. 145 and though afterward in chap. 9 p. 163, 164 he inclines to doubt, yet again in p. 168 he denies it that character.

In p. 146 he proceeds to state the uses of these nitro-aereal particles, which (147) he considers as the principle of life and motion both in animals and vegetables. By the mutual action of the nitro-aereal, with the sulphureo-saline particles contained in the blood, a fermentation is excited necessary to animal life, and to the warm fluid circulation of the blood (ad sanguinis æstum.) To these particles imbibed from the air, he attributes the difference in colour between the venous and arterial blood; and he shews this, from the numerous air bubbles arising in an exhausted receiver from warm arterial blood: but his experiment to illustrate the difference, from the colour produced by the nitrous acid with vol. alk. seems very little to the purpose p. 150.

To the fermentation arising from this mixture of nitro-aereal particles with the blood, he ascribes animal heat, and accounts satisfactorily for the increased heat of the body during strong exercise, from the more frequent inspirations occasioned by the exertion (p. 152, 306:) but his replies to the objections of Dr. Willis, drawn from the phenomena of fermenting mixtures, are very inconclusive.

Chap. 9th. Whether air can be generated anew. He repeats the experiment of dissolving iron in dilute nitrous acid, and finds that though some of the vapour be absorbed, a portion still remains uncondensible even by severe cold. On substituting dilute vitr. for nitr. acid he finds an aura which is hardly absorbed or condensed at all. Hence he doubts whether these auræ be not entitled to the appellation of air, especially as by subsequent experiment he shews that they are equally expansible with common air. In making this last experiment he exhibits the method of transferring air from one vessel to another (Tab. 5. Fig. 5.) much in the manner afterwards described by Mr. Cavendish in 1766.[45] From the inability of these auræ to support animal life (Tab. 5. Fig. 6.) he concludes finally that they are not air, though not very dissimilar p. 171. The succeeding five chapters do not seem to contain any facts or conjectures that can add to Mayow’s reputation. His Hypotheses are completely superceded by the more accurate knowledge of the present day. In his tract on quick lime p. 225 he seems to have forestalled the acidum pingue of Dr. Meyer published exactly a century afterward. It may be noted that in his treatise on the Bath waters p. 259, he describes fishes as collecting vital air from the water, and respiring like land animals. (Aereum aliquod vitale ab aquà, veluti aliàs ab aurà secretum et in cruoris massam trajiciatur.) The air bladder he considers rather as a reservoir of air to be inspired, than a receptacle for excreted air; though the latter opinion is made probable by Dr. Priestley.[46]

[45] Boyle had invented an apparatus for transferring air from one receiver of an air-pump to another, but not under water.

[46] See Nich. Journ. v. 3 p. 119 on the probability of fishes separating oxygen from the water they inhabit.

The first part of his Treatises on Respiration is chiefly anatomical. In p. 300 et seq. he states more fully his opinion, that vital air, is of a nitro-saline nature: that it is the principle of life, both in Animals and Vegetables: that combined with the sulphureo-saline particles in the blood, it is the stimulus to the muscular fibre, and of course to the heart as a muscle, p. 305; but that the fermentation occasioned by the introduction of these particles into the blood, is not confined to the left ventricle of the heart, but commences, in the passage of the blood through the lungs, and continues in the Arteries. This evidently approaches the theory, advanced by Dr. Goodwyn in his tract on the Connection of life with respiration about sixteen years ago, viz. that the pure air combined with the blood is the stimulus to the left ventricle of the heart, and produces the alternate contraction, and dilation on which the circulation depends. Dr. Lower, in his treatise de motu sanguinis, and Fracassati, and Dr. Frederick Slare attributed the change of the colour of venous blood into a florid red, to the combination of the air with it. Lower I believe preceded Mayow, who quotes him, p. 148; the date of Fracassati’s and Dr. Slare’s’ observations I have not been able to ascertain, but they must have been near the time of Mayow. Lowth. Ab. v. iii. p. 237.

In his third treatise on respiration, he explains the Animal œconomy of the fœtus in utero, by suggesting that the fœtus is supplied by the placenta, not with venous, but with arterial blood brought by the umbilical Arteries; so that the required stimulus of the nitro-aereal particles being thus conveyed, supercedes the necessity of the lungs for the purpose. This he ingeniously illustrates by the known experiment, that a dog into whom arterial blood is infused, though respiring with great difficulty before, hardly respires at all. A similar theory he applies to the life of the chick in ovo. This treatise seems to have suggested Dr. Beddoes’s illustration of his theory of consumption from the state of pregnancy.