an appellation given to all bodies whose particles easily yield to the least partial pressure, or force impressed. For the

Laws and Properties of Fluids. See HYDROSTATICS.

There are various kinds of animalcules to be discerned in different fluids by the microscope. Of many remarkable kinds of these, a description is given under the article ANIMALCULE. All of these little creatures are easily destroyed by separating them from their natural element. Naturalists have even fallen upon shorter methods. A needle point, dipped in spirit of vitriol, and then immersed into a drop of pepper-water, readily kills all the animalcules; which, though before frisking about with great liveliness and activity, no sooner come within the influence of the acid particles, than they spread themselves, and tumble down to all appearance dead. The like may be done by a solution of salt; only with this difference, that, by the latter application, they seem to grow vertiginous, turning round and round, till they fall down. Tincture of salt of tartar, used in the same manner, kills them still more readily; yet not so, but there will be apparent marks of their being first sick and convulsed. Inks destroy them as fast as spirit of vitriol, and human blood produces the same effect. Urine, sack, and sugar, all destroy them, though not so fast; besides, that there is some diversity in their figures and appearances, as they receive their deaths from this poison or that. The point of a pin dipped in spittle, presently killed all the kinds of animalcules in puddle-water, as Mr Harris supposes it will other animalcules of this kind.

All who are acquainted with microscopic observations, know very well, that in water, in which the best glasses can discover no particle of animated matter, after a few grains of pepper, or a fragment of a plant of almost any kind, has been some time in it, animals full of life and motion are produced; and those in such numbers, as to equal the fluid itself in quantity.—When we see a numerous brood of young fishes in a pond, we make no doubt of their having owed their origin to the spawn, that is, to the eggs of the parents of the same species. What are we then to think of these? If we will consider the progress of nature in the insect-tribes in general, and especially in such of them as are most analogous to these, we shall find it less difficult to give an account of their origin than might have been imagined.

A small quantity of water taken from any ditch in the summer-months, is found to be full of little worms, seeming in nothing so much as in size to differ from the microscopic animalcules. Nay, water, without these, exposed in open vessels to the heat of the weather, will be always found to abound with multitudes of them, visible to the naked eye, and full of life and motion. These we know, by their future changes, are the fly-worms of the different species of gnats, and multitudes of other fly-species; and we can easily determine, that they have owed their origin only to the eggs of the parent-fly there deposited. Nay, a closer observation will at any time give ocular proof of this, this; as the flies may be seen laying their eggs there, and the eggs may be followed through all their changes to the fly again. Why then are we to doubt but that the air abounds with other flies and animalcules as minute as the worms in those fluids; and that these last are only the fly-worms of the former, which, after a proper time spent in that state, will suffer changes like those of the larger kinds, and become flies like those to whose eggs they owed their origin? Vid. Reaumur. Hist. Insect. vol. iv. p. 431.

The differently medicated liquors made by infusions of different plants, afford a proper matter for the worms of different species of these small flies; and there is no reason to doubt, but that among these some are viviparous, others oviparous; and to this may be, in a great measure, owing the different time taken up for the production of these insects in different fluids. Those which are a proper matter for the worms of the viviparous fly, may be soonest found full of them; as, probably, the liquor is no sooner in a state to afford them proper nourishment, than their parents place them there; whereas those produced from the eggs of the little oviparous flies, must, after the liquor is in a proper state, and they are deposited in it in the form of eggs, have a proper time to be hatched, before they can appear alive.

It is easy to prove, that the animals we find in these vegetable infusions were brought thither from elsewhere. It is not less easy to prove, that they could not be in the matter infused any more than in the water in which it is infused.

Notwithstanding the fabulous accounts of salamanders, it is now well known, that no animal, large or small, can bear the force of fire for any considerable time; and, by parity of reason, we are not to believe, that any insect, or embryo insect, in any state, can bear the heat of boiling water for many minutes. To proceed to inquiries on this foundation: If several tubes filled with water, with a small quantity of vegetable matter, such as pepper, oak-bark, truffles, &c. in which, after a time, insects will be discovered by the microscope; and other like tubes be filled with simple water boiled, with water and pepper boiled together, and with water with the two other ingredients, all separately boiled in it; when all these liquors come to a proper time for the observation of the microscope, all, as well those which have been boiled, as those which have not, will be found equally to abound with insects; and those of the same kind, in infusions of the same kind, whether boiled or not boiled. Those in the infusions which had sustained a heat capable of destroying animal-life, must therefore not have subsisted either in the water, or in the matters put into it, but must have been brought thither after the boiling; and it seems by no way to probably, as by means of some little winged inhabitants of the air depositing their eggs or worms in these fluids.

On this it is natural to ask, how it comes to pass, that while we see myriads of the progeny of these winged insects in water, we never see themselves? The answer is equally easy, viz. because we can always place a drop of this water immediately before the focus of the microscope, and keep it there while we are at leisure to examine its contents; but that is not the case with regard to the air inhabited by the parcut-flies of these worms, which is an immense extent in proportion to the water proper for nourishing these worms; and consequently, while the latter are clustered together in heaps, the former may be dispersed and scattered. Nor do we want instances of this, even in insects of a larger kind. In many of our gardens, we frequently find vessels of water filled with worms of the gnat kind, as plentifully, in proportion to their size, as those of other fluids are with animalcules. Every cubic inch of water in these vessels contains many hundreds of animals; yet we see many cubic inches of air in the garden not affording one of the parent flies.

But neither are we positively to declare that the parent flies of these animalcules are in all states wholly invisible to us: if not singly to be seen, there are some strong reasons to imagine that they may in great clusters. Every one has seen in a clear day, when looking steadfastly at the sky, that the air is in many places disturbed by motions and convolutions in certain spots. These cannot be the effects of imagination, or of faults in our eyes, because they appear the same to all; and if we consider what would be the case to an eye formed in such a manner as to see nothing smaller than an ox, on viewing the air on a marsh fully peopled with gnats, we must be sensible, that the clouds of these insects, though to us distinctly enough visible, would appear to such an eye merely as the moving parcels of air in the former instance do to us: and surely it is thence no rash conclusion to infer, that the case may be the same, and that myriads of flying insects, too small to be singly the objects of our view, yet are to us what the clouds of gnats would be in the former case.

Nervous Fluid. See Nervous Fluid.

Elastic Fluids. See Air, Gas, Vapour, &c.