The function of respiration was still more obscure, and much farther beyond the reach of common ob servers. The ancients seem to have thought that it was intended to moderate the heat of the (1.4u7or vrue, or, as Virgil calls it, the " igneus vigor." Yet Cicero appears to have entertained a different opinion. He imagined that xther, or the principle of fire, was, either in a sensible or latent state, intimately blended with all bodies ; he says expressly, that the coldest water owes its fluidity to the heat which it contains ; that even air, which, according to the Stoics, is the coldest of bodies, possesses no inconsiderable portion of this igneous principle ; and that its combination with this principle, is what makes it necessary to the preservation of animal life.* He adds, that the air, on entering the body, is warmed by the breath, and still more by the agitation of the lungs, whence it is sent through the heart to the arteries.t This language would almost induce us to believe, that Cicero supposed respiration to be the principal cause of animal heat. But his random expres sions are, in general, so vague, that we must, in justice, ascribe this discovery to a later period. Our country man, Mayow, was the first who entertained accurate ideas on the subject of respiration. From a number of well-conducted chemical e xperiments, he proved, that air is a compound, only one of the ingredients of which sup ports life ; he proved, likewise, that the same ingredient alone maintains combustion, and therefore naturally con cluded, that respiration is the principal cause of animal temperature. Observing that the same aerial ingredient is contained in nitre, and that nitre changes the colour of the blood from black to red ; he very easily explained the cause of that remarkable difference of colour, which Lower had seen between the pulmonic and systomic fluid, while passing through the lungs. From this change of colour, he was led to conclude, that the blood, in passing through the lungs, was mixed, or somehow, combined, with these nitro-aerial particles ; and that the principal object of respiration is, to furnish a con stant and regular supply of this ingredient, which is necessary to life, and to the due performance of the other functions. This theory, however, was opposed by an opinion, then prevalent, that the alternate movement of the lungs was chiefly designed to promote the cir culation of the blood. To obviate this objection, he observed, that the circulation of the fcetus is carried on without such assistance, and that, though our breathing be suppressed for a while, the pulse is felt at the wrist as before.—See MAvow tie ReRtiratione, p. 260-1.
This aerial fluid, discovered by Mayow, was after wards obtained in a separate state by the celebrated Dr Priestly, who gave it the name of oxygen gas; and Dr Black, by the discovery of latent heat, has been enabled to explain in what manner it occasions animal temperature.
These facts sufficiently show how much physiology is indebted to the chemists ; and encourage us to hope, that much valuable information may yet be derived from their labours. Let them be careful, however, to con fine themselves within those bounds which nature has prescribed to their researches: let them remember, that every science has its limits, beyond which, it serves only to mislead ; that chemical knowledge can no more explain the properties of a circle, than mathematical reasoning the effervescence of acid and alkali : let them beware, in short, of proceeding, as the n.athematicians and older chemists did once before, on the vain suppo sition, that their science can explain every thing—as if a person, acquainted with only a few districts, or coun t•ies, should think himself qualified to give a map of the world.
A caution of this kind is the more necessary, as some chemists have begun to deny the existence of a vital principle, and to maintain, that all the phenomena of living bodies are the effect of chemical combinations, and that a more perfect combination might produce those phenomena in nature which we ascribe to the providence, wisdom, and unbounded power of the Great Author of all things. Such speculations, founded merely on gra tuitous hypotheses, and tending ultimately to subvert not only the religion but the morals of mankind, cannot be too strongly reprobated, and are indeed more like the ravings of fanatical enthusiasts, than the sober contem plations of philosophers.
It will not be difficult to spew, that something more than chemical affinities is necessary to account for the phenomena of the animal structure. When we consider the number of bones, joints, cartilages, veins, arteries, lymphatics, glands, nerves, ligaments, and membranes, all differing from one another, yet all of determined shapes, and all growing at the same time—when we see the number of chemical processes by which they are formed, differing in kind, but not interfering, or similar in kind but producing works of a different form, and although contiguous, always distinct in their opera tions—when we sec the same chemical processes in the other half of the body performing the like variegated work in the same way and in the same time—when we see them change and succeed one another according to the stated periods of life, and yet all their actions regu larly cooperating, all depending on one another, all contributing to one end, and constructing one whole, subservient to one will, and obedient to one mind—can we suppose that this whole is produced without a plan, or that this plan could have been formed without great wisdom and foresight ; qualities which we must now ascribe to chemical affinities, if they be the sole agents in nature ? In this case we must suppose, that these chemical affinities vary their work at pleasure ; that they construct on different plans the numerous species of animals and vegetables, preserve the regular diffe rence of sex, and the stated proportions of males and females, with all the desires, appetites, and passions which accompany that distinction. It will likewise be 'VT_ • _ Yr necessary to be liege, that they are acepi,onn_d v, ith dif ferent countries, know the various seasons of ach, and that this power occasionally eanaols through a sp. cc oi sonic thousand miles, drawing animals Iroin distant cli mates, repelling (Ali( us in an opposite direct;on, and at last sending all back at a sttttd period to their homes, as we see to be the case with birds and Iishc a, and, in smaller degrees, with quadrupeds of passag, .
Since they occasionally form r.ut II «mibinstious as a I Tomer, a Virgil, a Milton, or a Newton, we mu- I al,n conclude, that they possess the faculties of memory, imagination, and reason, in a ninth .1.g.ier degree than could possibly belong to those fabrics of tit I own ( ok struction. When we have brought ourselves to believe all this, it cannot surely be difficult. to imagine, that all our association of ideas, and their .,ions with arid teary signs, are only a kind of chemical s ; no:• need we be surprised to see hereafter, the processes lit the laboratory exhibiting all the symptoms of passion, desire, and appetite, discussing moral and religious sub jects, pretending to great skill in philosophy, and form new models of empires.