For example, in the first part of the work, after endeavouring to prove that animal heat cannot be owing to fermentation, the motion of the fluids, and other causes that have usually been assigned, he draws this conclusion:

"If none of these causes are sufficient to produce the effect; therefore, by dilemma,” says he, "it must be sought for in the nature and action of the nerves." This is a new species of dilemma:-If the author had proved, that the cause of heat in animals could not possibly exist any where, but either in fermentation, the motion of the fluids, &c. or in the nerves, after having disproved its existence in all the rest, his conclusion in favour of the nerves would have been just; but, as he has not so much as attempted this, the conclusion is not only false, but ridiculous.

HOWEVER, upon the authority of this dilemma, the author first gives what he calls a Compend of a new doctrine concerning the nerves, and then proceeds to inquire in what manner the nerves produce animal heat: He tells us, "That thought (cogitatio) and sensation depend upon impulses either on the extremities of the nerves, or the sensorium commune, and the consequent motions produced by these impulses: That these motions are A a 3

so quick, as to be almost instantaneous: That as all motion is mechanical, therefore thought, sensation, and muscular motion, must likewise be mechanical: That such quick motions cannot be produced without the intervention of some extremely elastic power; and as Sir ISAAC NEWTON has shown that the impulses which occasion the different sensations must be owing to an elastic power; therefore the muscular motions of animals must be occasioned by the oscillations of some elastic power." "But," says he, "as this elastic power cannot exist in the solid nervous fibres, nor in any inelastic fluid; therefore, by dilemma, it must exist in an elastic fluid; and hence also, by the former dilemma, this elastic fluid must be seated, either in the nerves, or in their medullary substance."

"

HERE again the author calls Sir ISAAC into his assistance." What confirms this opinion," says he, "is the Newtonian æther, which pervades all nature, and which, with a few variations in its modification, Sir ISAAC has shown to be the cause of cohesion, elasticity, gravity, electricity, magnetism, &c. in the following manner: 1. As the rays of light, when reflected, do not touch the solid parts of bodies, but are reflected a little be

fore they reach them, it is plain that the æther not only fills the pores of bodies, but likewise floats upon their surfaces and hence it becomes the cause of attraction and repulsion.-2. All metals, and inelastic fluids, are non-electrics; on the other hand, all solid bodies, metals excepted, are electrics, i. e. proper for accumulating æther. But æther, thus accumulated in such a variety of bodies, may produce various motions in the parts of these bodies, without inducing any change in the bodies themselves. Hence æther, with some variations in its modification, is sufficient to account for all the phenomena of electricity.-3. As iron, by accumulating æther around it, exhibits all the wonders of magnetism; so this magnetical æther is more analogous to the nervous æther of animals than other kind of it: For, as the magnetical æther passes along iron without changing any part of the iron; so the nervous æther, in like manner, passes along the medullary substance of the nerves, and excites motion in any part that is continuous with them, without inducing any change in the nerves.-4. The irritability and life of plants, which very much resemble those in animals, cannot be explained by any inelastic cause,

any

and must therefore be attributed to an ætherial one.-Lastly, As the common æther is differently modified in each of the substances above taken notice of, and also produces various motions or effects peculiar to each, it likewise varies, and has some peculiar qualities when residing in animal bodies; so that the nervous or animal æther is not exactly the same, but differs in some respects from those species of æther which give rise to cohesion, gravity, magnetism, electricity," &c.

HAVING thus explained the nature and qualities of æther, our author starts a very important question; viz. "Whence is æther derived? and whether does it leave any body after having once got possession of it?" In answer to this, he observes, "That certain bodies have the power of collecting the electrical matter from every circumjacent body, and of accumulating it in their pores and on their surfaces, but do not suffer it again to transmigrate into any other body. There are other substances of an opposite nature, which do not accumulate the electric matter, but instantly allow it to pass into others, unless prohibited by an electric. Hence," says he, "nothing more is necessary for substances of

the former kind but to be in such circumstances as allow them to accumulate the electric matter. In the same manner," proceeds our author, "the nervous æther, which is diffused through svery part of nature, flows copiously into the medullary part of the nerves, when no obstacle stands in its way; but, when once it has got there, it keeps firm possession, and never afterwards leaves it. Now," says he, "a quantity of æther probably consitutes one of the staminal parts of animal bodies, and increases in proportion to their age and growth: For nothing is more ridiculous than to suppose that what is commonly called the nervous fluid can be daily wasted by labour and exercise, and daily repaired by a new secretion from the brain. To refute this vulgar notion, nothing more is necessary than to say, That it is one of BOERHAAVES theories, and must be false, as all BOERHAAVES other theories have been proved to be ill-founded! But æther is of a more fixed and determinate nature; whenever it gets possession of any substance, it never forsakes it, unless the texture and constitution of the body itself be changed. Hence," continues our author," the æther of an acid body remains as long as the body continues to be