Part 14
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 His Hypotheses are completely superceded by the more accurate knowledge of the present day.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.
In a subsequent Essay on animal spirits, he conceives them to be, if not the same with the nitro-aereal part of the atmosphere, yet to consist of this, so far as they are necessary to the production of muscular motion, which he attributes entirely as before to nitro-aereal particles, p. 24 and 40, of chap. 4, on the animal spirits.
I do not observe any thing else in Mayow’s book worth noting on the present occasion; or sufficiently connected with pneumatic Chemistry.
From the analysis thus given of[47] what Mayow has advanced, it appears, that he clearly comprehended the atmosphere to consist of a mixture of two parts, the one the efficient cause of life and of combustion, the other not of itself necessary to either.
[47] At the time this was written neither Dr. Bostock’s treatise on respiration or the books therein quoted p. 200 had arrived here. Nor have I had an opportunity of consulting the references there made to Prof. Robinson, Dr. Thompson, Dr. Yeates, or Fourcroy’s account of Mayow.
That the vital part of the air, was also a constituent part of nitre, the effects of both being in essential particulars the same.[48]
That the vital part of the atmosphere entering the blood through the vessels in the lungs, is conveyed to the left ventricle of the heart, and becomes the stimulus to the contractions of that muscle, and is equally essential to the whole system of muscular contraction.
[48] Mr. Ray wrote “A dissertation (in 1696) about respiration,” in which he supposes the air to pass from the bronchia and lungs into the substance of the blood, and there (pabuli instar) it foments and maintains the vital flame which he supposes to be in the sulphureous parts of the blood, as the air foments the common flame of a candle, and that the nitre has nothing to do with it. See Durham’s collection of Ray’s letters.
That the vital part of the atmosphere thus combined with the blood becomes also the source of animal heat.
That this vital part is equally necessary to the fœtus in utero as to the adult, and that the use of the lungs in the former case is superceded by the functions of the umbilical artery and placenta; by means of which, blood already impregnated with the vital air, is conveyed to the fœtus.
That the respiration of fishes, is dependant on the particles of air mixed with watery element they inhabited.
That heat, flame, and combustion, depend on two universal principles, and the gentleness or violence of their mutual conflict: the one being a principle of inflammability universally diffused in combustible bodies, and the other the vital or igneous part of the atmosphere.
These propositions evidently touch upon the most brilliant of the pneumatic discoveries of the authors already quoted; and not a little extraordinary it is, that they should have remained so long unknown, unnoticed, and not understood.
The sulphur of Mayow is decidedly the Phlogiston of Stahl; the fire air of the former is the fire air of Scheele, the dephlogisticated air of Priestley, and the Oxygen of Lavoisier.
The combination of oxygen with the blood by means of respiration, first discovered as was thought by Lavoisier, is clearly stated by Mayow; who has also forestalled the elaborate theories of Crawford on animal heat, of Goodwyn, on muscular stimulus, and of Beddoes on the succedaneum for respiration in the fœtus.
Boyle, though he must certainly have known of Mayow, neither quotes him, nor uses, or improves on his experiments; though as I have already remarked, he seems to have had notions of the atmosphere much like those adopted by Mayow. Whether this neglect arose from the pride of birth, or the pride of knowledge, or the pride of age, (for Boyle was almost twice the age of Mayow) or from jealousy of Mayow’s abilities, cannot now be ascertained. From that time until Hales published his statics in 1726, pneumatic experiments were neglected, and the mathematical philosophy which Newton’s discoveries rendered fashionable, absorbed for many years the attention of men of Science, particularly in England. The way in which Lemery, Hales and Brownrigg speak of Mayow, evidently shews that his theories were not understood, nor his merits appreciated.
That Mayow was unknown to Black and Cavendish until of late years, is highly probable at least, if not absolutely certain. Neither these philosophers, nor Dr. Priestley, could have passed over Mayow’s book, without being struck with his ideas, and publicly referring to them in their chemical works.
That Dr. Priestley was unacquainted with Mayow is certain, from the limited extent of his reading at the early period of his experiments (from 1770 to 1776 or 1777,) in books of chemistry and theoretic physiology: from Mayow, not being quoted by any of the writers whose works Dr. Priestley would be likely to consult except Hales and Brownrigg, and not by them in a manner to induce any farther curiosity: from their being unnoticed by Black, Cavendish, Sir John Pringle, and Lavoisier, in particular: from the custom that Dr. Priestley had of acknowledging the sources of his ideas in all cases where they originated from the discoveries of others, as in his references to Hales, Brownrigg, Cavendish, &c; and from his making no mention of Mayow in his express account of the labours of his predecessors on the subject of animal respiration. That both he and Sir John Pringle before the Royal Society in 1772 and 1776 should expressly treat the _history_ of discoveries in which Mayow bore so distinguished a part, and omit noticing him altogether, had they known of his works, is incredible. It is evident that he was then an obscure writer, and not in repute, or he would have occurred to them; or some of their philosophical friends would have suggested the propriety of referring to his publications.
Neither is it likely that Scheele would have been acquainted with Mayow’s writings, though it is singular that he escaped the notice of Lavoisier who I believe was employed under government in the collection of essays on the theory and manufacture of saltpetre and in the superintendance of the saltpetre works, especially as Mayow was mentioned though disrespectfully by Lemery, in his paper on nitre before referred to. But there certainly is no evidence that Lavoisier obtained his ideas of oxygen and its combination with the blood from Mayow, or his theory of metallic calcination from Jean Rey, though his obligations to Dr. Priestley have not been always acknowledged with the candour and liberality that men of science would expect from Lavoisier.
Mayow had more than ordinary discernment in comparing known facts, and drawing conclusions from them, but he does not appear to have had the talent of imagining decisive experiments, of varying them, of observing and noting all the natural phenomena attendant upon them, or sufficient industry in pursuing them. It is one thing to make a plausible conjecture, and another to verify it. Those alone are entitled to the honour of discoveries who not merely start the theory, but take the pains of pursuing it by experiments and resting it on the basis of well conceived and accurately ascertained facts, sufficiently numerous and varied to obviate the most prominent objections. Mayow has reasoned with great acuteness and conjectured with singular felicity, but he added little to the mass of philosophical KNOWLEDGE in his day. He composed and decomposed nitre and ascertained the existence of vital air in this substance as well as in the atmosphere, but he did not collect, exhibit, and examine it. He knew how to make artificial air from nitrous acid and iron, but all the extraordinary properties of this gas, remained unobserved by him as well as by others until collected and imprisoned by Dr. Priestley, and exposed to the question under his scrutinizing eye. Indeed as an experimentalist Dr. Priestley stands unrivalled. The multiplicity of his experiments, their ingenuity, their bearings upon the point in question, their general importance, and their fidelity, were never equalled upon the whole, before or since. Nor is it any detraction from their merit with those who are accustomed to experiment, that they hold out no pretensions to that suspicious accuracy, which has too often depended more upon arithmetical calculations than upon actual weight and measure. The many kinds of aeriform fluids discovered by Dr. Priestley, the many methods of procuring them, the skilfull investigation of their properties, the foundation he laid for the labours of others, the simplicity, the novelty, the neatness, and the cheapness of his apparatus, and his unequalled industry, have deservedly placed him at the head of pneumatic Chemistry. Nor should it be forgotten that while he thus outstripped his predecessors and contemporaries in the field of experiment, it formed not as with them the business of his life, but (among other branches of literature and philosophy successfully cultivated) the occupation of his leisure hours, the relaxation from what he deemed more important, more laborious, and more obligatory pursuits.
Before his time (excluding Mayow) Boyle had discovered that air might be generated, fatal to animal life. It was known that common air would only serve a certain time for the purposes of combustion and respiration. The mephitic exhalations from natural Grottoes had been remarked. Inflammable air both natural and artificial had been exhibited before the royal society. Hales had ascertained the presence of air in a great number of substances where it was not commonly suspected though he had not the skill to examine the properties of the air produced. Black had ascertained the presence of fixed air in limestone, and Brownrigg, Lane, and Venel had illustrated the theory of mineral waters. But it was the paper of Cavendish in 1766 on fixed and inflammable air produced from various substances by means of acids, fermentation and putrefaction, that first introduced a stile of experimenting in pneumatic chemistry, more neat, more precise, and scientific than had hitherto been known.
The attention of Dr. Priestley, however to these subjects was not originally excited by the works of his predecessors, but by the _accident_ of his proximity to a brew-house at Leeds, where of course fixed air (a subject that had attracted much attention about that time) would be produced in a large way. It was thus that one experiment led to another, until the fruits of his amusements were the discoveries on which his philosophical reputation is principally founded. It is no more than justice to his character to mention in this place, that of all men living he was the freest from literary deception and the vanity of authorship. He never claims the merit of profound investigation or great foresight, for discoveries that might easily have been so stated as if they had been the pure result of those qualifications, but which were in reality the offspring of accident and circumstance. He excites others to patient labour in the field of experiment, from observing that success does not depend so much on great abilities or extensive knowledge, as on patient attention, and perseverance; and that much of his own reputation was owing to the discovery of facts that arose in the course of his pursuits, the result of no previous theory, unlooked for and unexpected. In v. 3 p. 282 of his experiments on air he says “Few persons I believe have met with so much unexpected good success as myself in the course of my philosophical pursuits. My narrative will shew that the first hints at least of almost every thing that I have discovered of much importance have occurred to me in this manner. In looking for one thing I have general found another, and sometimes a thing of much more value than that which I was in quest of. But none of these unexpected discoveries appear to me to have been so extraordinary as that I am about to relate (viz. the spontaneous emission of dephlogisticated air from water containing a green vegetating matter) and it may serve to admonish all persons who are engaged in similar pursuits, not to overlook any circumstance relating to an experiment, but to keep their eyes open to every new appearance and to give due attention to it however inconsiderable it may seem.”[49] To this candour of disposition, and the readiness with which he acknowledged his mistakes and his oversights, even those who opposed his opinions bear honourable testimony. “The celebrated Priestley himself (says M. Berthollet in his reply to Kirwan on Phlogiston p. 124 of the Eng. translation) often sets us the example, by rectifying the results of some of his numerous experiments.”
[49] See also the 1st, vol. of his early edition of experiments on air p. 29.
Numerous indeed those experiments were as well as important: far too numerous to be particularised here; though it may not be improper to call to the recollection of the reader some of the more interesting facts which we owe to Dr. Priestley, and the times of their discovery and communication.
The first of his _publications_ on pneumatic chemistry was in 1772, announcing the method of impregnating water with fixed air, and on the preparation and medicinal uses of artificial mineral waters; a discovery that domesticated much of the knowledge that had heretofore been disclosed only in the works of learned societies; and that beautifully exemplified how much of the health and the pleasure of common life, might depend on the ingenious researches of men of science. Though this was the first publication of Dr. Priestley on the chemistry of the airs, he had certainly commenced his experiments in this branch of Science, soon after his arrival at Leeds, and as early at least, as 1768. In the year 1771 he had already procured good air from saltpetre; he had ascertained the use of agitation, and of vegitation as the means employed by nature in purifying the atmosphere destined to the support of animal life, and that air vitiated by animal respiration was a pabulum to vegetable life; he had procured factitious air in a much greater variety of ways than had been known before, and he had been in the habit of substituting quicksilver in lieu of water, for the purpose of many of his experiments. In his paper before the Royal Society, in the spring of 1772, which deservedly obtained him the honour of the Copley Medal, he gives an account of these discoveries. In the same paper he announces the discovery of that singular fluid nitrous air,[50] and its beautiful application as a test of the purity or fitness for respiration of airs generally. In the same paper he shews the use of a burning lens in pneumatic experiments, he relates the discovery and properties of marine acid air; he adds much to the little of what had been heretofore known of the airs generated by putrefactive processes, and by vegetable fermentations, and he determines many facts relating to the diminution and deterioration of air, by the combustion of Charcoal, and the calcination of metals.
[50] Honestly referring to Dr. Hales and Mr. Cavendish for any idea that might have remotely led to this discovery (See Obs. on air 1st ed. v. 1 p. 108) the discovery however was completely his own.
Dr. Priestley seems always to have thought nitrous air as convenient a substance for eudiometrical experiments as any of the later substitutes, viz. the liquid sulphurets and the combustion of phosphorus. The foundation of Mr. Davy’s substitute, muriat or sulphat of iron saturated with nitrous air, was as Mr. Davy acknowledges first discovered by Dr. Priestley himself. See Nich. Journ. for Jan. 1802 p. 41. The different states of the solutions of iron in vitriolic acid have been ingeniously applied to the analysis of mixed gasses by Humboldt and Vauquelin.
Soon after this, in confirmation of Sir John Pringle’s theory of intermittents and low fevers being generally owing to moist miasma when people are exposed to its influence, he ascertained by means of his nitrous test that the air of marshes was inferior in purity to the common air of the atmosphere.[51]
He had obtained very good air from saltpetre in 1771, but his full discovery of dephlogisticated air, seems not to have been made until June or July, 1774,[52] when he procured it from precipitate per se, and from red lead. This was publicly mentioned by him at the table of Mr. and Madame Lavoisier, at Paris, in October 1774, to whom the phenomena were until then unknown. The experiments on the production of dephlogisticated air, he made before the scientific chemists at Paris about the same time, at Mr. Trudaine’s. This hitherto secret source of animal life and animal heat, of which Mayow had but a faint and conjectural glimpse, was certainly first exhibited by Dr. Priestley, and about the same time, (unknown to each other) by Mr. Scheele of Sweden. For the honour of science, it were much to be wished that the pretensions of Mr. Lavoisier were equally well founded. He has done sufficient and been praised sufficiently for what he has done, to satisfy a mind the most avaricious of fame; he is deservedly placed in the first rank among the philosophers of his day, and he ought not to have thrown a shade over his well earned reputation, by claiming for himself the honour of those discoveries which he had learned from another.
[51] Phil. trans. v. 54 p. 92.
[52] See Doctrine of Phlog. established p. 119.
From this brief account of the first stage of Dr. Priestley’s chemical labours, it appears that during the short period of two years, he announced to the world more facts of real importance, and extensive application, and more enlarged and extensive views of the œconomy of nature, than all his predecessors in pneumatic Chemistry had made known before.
In 1776 his observations on respiration were read before the Royal Society; in which he clearly discovered that the common air inspired, was diminished in quantity, and deteriorated in quality, by the action of the blood on it _through the blood vessels of the lungs_; and that the florid red colour of arterial blood, was communicated by the contact of air through the containing vessels. His experiments on the change of colour in blood confined in a bladder, took away all doubt of the probability of this mode of action. I cannot help thinking that the circumstance of Dr. Priestley’s mind being so much occupied with the prevailing theory of Phlogiston, was the reason why he did not observe that the diminution of the air, and the florid colour of the arterial blood was owing to the absorption of the pure part of the atmosphere, _rather_ than to any thing emitted from the blood itself. This part of the theory of respiration Mr. Lavoisier has certainly established; though it is by no means ascertained as yet whether the vital part of the atmosphere inspired, is wholly and alone absorbed, or whether in reality something is not contributed in the lungs to the formation of the fixed air found after expiration.[53]
[53] That azote is absorbed during respiration as Dr. Priestley supposed contrary to Mr. Lavoisier’s opinion, is made extremely probable by the experiments of Mr. Davy, whose accuracy is well known. Researches, p. 434. The formation of water in this process, is certainly no more than conjecture as yet. Dr. Bostock has lately published a very useful and laborious history of discoveries relating to respiration, both anatomical and pneumatical.