bodies depend upon their structure In the times when chemistry was chiefly founded on various nonfenfical hypothefes, and was nevertheless applied with a blind zeal and the moft perfect confidence not only to the cure of all infirmities and difeafes, but even to the obtaining of immortality itfelf; then it could be productive of nothing but detriment and mischief in the science of phyfic. We have no knowledge of bodies a priori: every intelligence about them, must be acquired by proper obfervations and experiments. But to discover and purfue fuch experiments as really illuftrate the point we are in fearch of, requires not only fkill, and a peculiar application, but alfo the most impartial love of truth; in order not to be enfnared by the pleafing defire of drawing general conclufions from a few data of precarious certainty. It leflens no doubt our trouble, and flatters our vanity, to be able to disclose in a moment the whole course of nature. Man is befides naturally indolent, and much inclined to be captivated by imagination more than by reality. The confeffion, therefore, that we really know no more than what we know, is even in our days, when the experimental method is confidered as the only right and true method, very difficult and humiliating: but fince all fciences have their roads of error, their abufes and follies (which unfortunately always feduce the greater part of mankind), it is chiefly owing to this precipitation and fancy, that fuch monsters are produced. Every thing going forward in our body, is done either in a mechanical or in a chemical way; fince the operations of the imma terial foul having nothing to do with it. Our food is diffolved and changed on its road in paf fing from the mouth through the ftomach, entrails, and other inteftines, by means of the saliva, the liquor gastricus, the gall, the chyle, &c. fo that in feveral places various matters are prepared, all of which are neceffary for the increase and support of the machine! The lungs more over add by their conftant motion, various fubtle particles by means of the abforbing veffels, and again by the exhaling veels, carry off others. The leaft irregularity in all thefe natural functions, lays of courfe the foundation for many infirmities and diseases. Thefe ought to be prevented or cured by fuch medicines as have nothing noxious in other refpects capable of obftructing the main tendency, and which contain nothing fuperfluous; for if they were even in themselves harmlefs, they might however become hurtful by their quantity. It is therefore no. doubt doubt a great advantage, to be able to concentrate the powers of feveral ounces into a few grains. Did we exactly know, after chemical principles, the natural functions going forward in the body, the caufes of diseases, and the effects of medicines; chemistry would most certainly operate miracles. But I foresee already what may be objected to this affertion with some reason, viz. "Though fuch knowledge would be a molt excellent thing, it would however be rather the object of wishes than fuch as might be fuppofed or hoped for in the human body every operation goes forward very fecretly; nor is there any glafs window to admit the refearches of the inquifitive: it is by no means right to play with the lives of men, and it would be highly unjuftifiable to endanger them by uncertain experiments, &c." I reply, this is perfectly right; at the fame time it by no means juftifies us in neglecting the right means of obtaining information; and ftill lefs in treating them with contempt. Whatfoever is difficult, is not therefore impoffible always. The more investigation and trouble an invention cofis, the more honour may be obtained by it; efpecially if it has for its object, health, the most precious enjoyment a man can be bleffed with. If a man be only poffeffed of a truly fundamental knowledge, many things may be difcovered without the leaft danger. Higher chemiftry has difcovered many and various new manipulations, and by recalling to our minds one or the other inftance, we fhall be enabled to judge what more may with reason be expected. Several kinds of diseases spread devaftation over extenfive countries, over people of all ranks and denominations; without being however infectious in themselves. These cannot, properly speaking, be derived from the mode of living; which in those who are attacked with the evil, is widely different; there must therefore exift a general caufe, which af fects equally the richer and the poorer. The atmosphere is the fame for all; during feveral years its weight and temperature have been obferved; and this has furnished feveral explications, which however are not fatisfactory for the explanation of all its phænomena. Its conftituent parts therefore ought to be better known. The vapours and heterogeneous particles are different in regard to quantity, as well as to quality. Befides chemistry teaches us, that this elaftic fluid furrounding our globe, is at all times, and every where, a compound of three very different fubftances, viz. of good air, foul air, and aërial acid. The firft is called by Dr. Priestley, if not wrongly, however fomewhat improperly, phlogisticated air. Mr. Scheele calls it with more reafon empyreal air (fire air), fince this alone gives life to fire, when the other two kinds of air extinguifh the flame of a candle or fire. The last kind is commonly called fixed air: but I flatter myself to have fatisfactorily proved by experiments, that it is a peculiar acid. The nature of the firft kind, has as yet been very little inveftigated; however by analogy it feems to be hardly any thing but them; however it is not yet afcertained in what manner this air acts; but thus much is certain, that it deftroys all-irritability. I have, from animals killed by it, before they were grown cold, extracted the heart, in which it was imposible to raise the leaft irritability, either with the moft pow. but good air, which is corrupted either by a fuperfluity of inflam mable fubftance, or perhaps by a want of it. It will however be ftill difficult to decide which of the two above propofitions is the true one. Of thefe three divifions, the aërial acid always bears the leaft proportion, and fcarcely makes the fifteenth part of theerful menftrua, or the fire, or bulk of the atmosphere, at least on the furface of our globe: the foul air is always in the greatest proportion; and by far more, than the pure or good air. The various effects of each of these three fubftances upon animal bodies, are fill involved in impenetrable darkness. The good air fit for refpiration, ought however to be excellent in its operations, fince without it, it is impoffible to live. It has been be lieved, that it contained an indifpenfably neceffary vivifying food; which never has been proved, at least it seems not to be of an electric nature. It might per haps foon be poffible to decide by experiments, whether this good air carries off noxious, especially phlogistic particles; fince the air which has been expired from the lungs, is unfit for refpiration, and is fimilar to that which has been phlogisticated. We hall then likewife be informed, whether that which makes the greater part of the atmosphere, if it be infpired by itself, becomes fatal, for the reafon that it has been previously faturated with heterogeneous bodies, and is thence in-. capaciated for carrying off any particles from the lungs. Perhaps the aerial acid becomes an unfit vehicle for thefe articles, and is itself originally formed without VOL. XXIV. fcalpels. If mufcular fibres were the chief caufe of the motion of the lungs, it would not be difficult to find out the ultimate caufe of death; but fince its structure is. formed of quite different fubftances, the greatest difficulties. prefent themselves. By experi ments however it might be eafy to afcertain, whether the aerial acid and foul air act in the fa ne manner. It would therefore be neceffary to investigate, whether irritability (by all appearance fo very neceffary for he whole economy of animal bodies) is not likewife destroyed in fuch animais as have been fuffocated and killed inftantaneously by that air, which had been corrupted by refpiration, fire, or other means, Since the air returning from the lungs, by expiration, is a ways blended with e ial acid; it would be neceffary, in order to avoid all ambiguity, entirely to free the foul air of the aerial acid by means of lime-water, before any experiments were made with it. I hope foon to have an opportunity of making them. If then, contrary to expectation, the refult fhews, that they both act in the tame manner, it would feemingly follow from thence, that thefe two fluids, fo very diftin&t in their nature, act however by the fame common unfitness, either G for for carrying off from the lungs the noxious particles, or introducing into them a vivifying pabulum. But, however noxious aërial acid is for found lungs, it is nevertheless of great utility in the prime via. Purulent lungs not only bear aërial acid, but they are even cured by it: and no fooner is the cure performed, than its inhalation becomes again dangerous. But though there be ever fo much left for investigation, we may apply that for ufe which has already been difcovered. It is for inftance well known, that the air of the atmosphere may be examined in regard to its fitnefs for refpiration; a difcovery which bids fair to yield, in a very fhort time, the most important illuftrations. If fuch obfervations were made at the fame time in dwelling-rooms, infirmaries, and the open air, and were continued with fome degree of accuracy, we fhould certainly discover the causes of many phænomena, which hitherto have remained inexplicable. Thus much we know already; that wounds and ulcers are with difficulty kept from going into mortification in corrupted, foul air; whereas, on the other fide, the immenfely acute pains accompanying the cancer (that dreadful ailment) are in a few days not only mitigated and affwaged by the external ufe of aërial acid, but the loathfome aperture is alfo confiderably diminished. The compafs of a preface not only prevents me from entering into a fuller difcuffion of this important fubject, but also from producing more inftances of its utility; I cannot help however mentioning in a few words only, that the ob fervations on aërial acid have already fpread a new light on the method of affaying and imitating, by art, medicated waters; and on their application and ufe in the cure of the fcurvy, and other internal putrid diseases. The true analysis of the calculi from the kidneys and bladder, has enabled us to judge with precision of the beft remedies against the gravel. The discovery of the inner confti. tution of arfenic, made us better acquainted with the manner by which its dreadful effects are produced; and thereby the best means were pointed out, both to extinguish its poisonous quality, and likewife to mitigate and direct its effect for better purposes. -How fimplified are at present many compounded remedies; and how fafe and little perilous, are many preparations of the moft acrid and corrofive fubftances? How many abfurd mixtures, decompounding and counteracting themfelyes, have not of late been rejected? How many false theories of diseases, and their causes, are there not daily expofed, and fhewn to be without any foundation, by means of chemistry? Sugar was faid to contain lime, in order to account for certain ef fects afcribed to fugar; though it contains not an atom of it. The calculi of the kidneys and the bladder were faid to be calcareous; though they contain at the moft only one half of calcareous fubftance. And fo in many more inflances, which it would be too tedious to enumerate.. Laftly, how would it be pofhible to discover and to fubject to controul the various noftrums, quack-medicines, cheats, neglects, and other monsters of the the medical science, without the application of a chemical analyfis? The mufes are not without reason thought to be fifters; and they are a most excellent emblem of that harmonic union, by which all the fciences mutually affift one another; and without which they never can attain any degree of perfection. Next to health, there is nothing more preffing and neceffary than common food. In order to convince us of the utility produced by chemistry in regard to the neceffaries of life, we fhall only take a view of agriculture, that moft antient and noble occupation. -If we confult Columella, and many of the most antient writers on husbandry, we find to our humiliation and fhame, that they were as well inftructed as us, if not better; though in later times fo much pains have been taken, fo many encouragements have been given, and fo many rewards have been diftributed. This ought to be confidered in the following manner: The great Lord of nature has fo bountifully taken care of us, that corn grows without any great care and knowledge. The improvements which this art in general is capable of from experience and collected experiments, are likewife foon enough acquired; and in this ftate it remains till the fcience of nature throws a new light upon it. They are two very diftinct things, to raise corn on a certain piece of ground, and to raise as much corn as will poffibly grow there -The last defideratum is not always obtained by ploughing, ditching and manuring; for befides thefe mechanical operations, there are two things more required: fuch a mixture as will afford to the vegetables not only proper nourishment; but alfofuch a one as keeps humidity as long as the ufual drought makes it requifite; for nothing grows, even in the most exquifite foil, without water. The best compound, therefore, ought to be adapted to the nature of the foil, the expofure, the climate, and the common temperature of the weather; as I have proved more at large in another place. In the mean time it will be eafily found, that chemiftry is of the fame importance to agriculture, and all the more particular branches of rural economy, that astronomy is to navigation. The arts and mechanical trades, are occupied with the melioration of raw materials. One part of them, from beginning to end, is a feries of chemical operations; others are more mechanical; however there is hardly one of them which contains not one or more problems whofe more perfect folution depends upon chemistry only. What a number of years must have elapfed before they arrived by chance only, or by unpremeditated experiments, to their prefent perfection; or before they learned to avoid all occurring inconveniences. A perfect knowledge of the raw materials, often furnishes the best inftructions, in a direct way. And it has been hitherto a great miffortune, that the practical part of the arts has been kept very fecret: but fince the French Academy of Sciences has begun to remove this obftable, we have the moft certain expectations of their G 2 quick |