In regard to the degree of heat to which infusions may be exposed, and still produce animalcules, there is a remarkable difference. The smaller species still ori ginate, after infusions have been subjected to 212°, in close vessels. None of the larger species appear after 95°. Some are seen when it has been 93°, and they are numerous at 88°. Hence it is inferred, that the germs of the lesser animalcules are more capable of with standing the influence of heat than those of the larger ones.
We have already intimated, that the animalcule tribes are not confined to artificial infusions alone. Milk, blood, urine, and other animal fluids, abound with them after standing a certain time ; for none are naturally there, contrary to the general belief. In urine, the same race always continues, though kept for several months; nor do the animalcula of stale urine die, when put into that which is recent. .There is no certain law with re gard to the peculiar species produced by any particular infusion. In general, several different species will be exhibited, which disappear, and are succeeded by others; and sometimes, where there are myriads of one kind, a single solitary animalcule, of a remote genus, is found among them. Vinegar is full of minute eels, which are al found in paste. Muller conceives that the sea abounds in animalcula peculiar to itself; and Spallanzani observes, that vegetable substances, dis solving in sea water, produce swarms of animalcula. It is possible, that, on carefully attending to the different seasons of the year, the existence of various species of animalcula may be found dependent on them.
The extraordinary minuteness of animalcula sur passes the conception of the human mind. Leeuwenhoek calculates, that the size of some, is to that of a mite, as a bee is to a horse ; an hundred others will not exceed the thickness of a single hair; and ten thousand of a different species may be contained in the space occu pied by a grain of sand. The most powerful micro scopes can only discover points in motion among the fluid, gradually decreasing, until they become imper ceptible to the view.
The shape of animalcula is infinitely diversified. Let one suppose himself transported to a region, where the appearance, figure, and motion of every animal is un known, and he will form some idea of the variety pre sented by a drop of an infusion presented to the micro scope. One animalcule is a long slender line ; another is coiled up like an eel, or a serpent; some are circu lar, elliptical or globular ; others, a triangle, or a cylin der. Some resemble thin flat plates, and some may be
compared to a number of articulated reeds. One is like a funnel, another like a bell ; and the structure of many cannot he compared to any object familiar to our senses. Certain annnalcula, such as the proteus diffluens, can change their figure at pleasure ; beinz sometimes extended to immoderate length, and t contracted to a point. One moment they are ir‘fl• into a sphere, next completely flaccid, and then var, eminences will project from the surface, altering ti apparently into animals entirely different. Neithc, the peculiar motion of animalcula less. remarkable. In several species, it consists of incessant gyration on the head, as a centre, or around a particular point, as if one of the foci of an ellipse. The progression of others is by means of leaps or undulations; some swim with the velocity of an arrow, the eye can hardly follow them ; some drag their unwieldy bodies along with painful exertion; and others again seem to persist in perpetual rest. These observations lead to an important conside ration, namely, the inconceivable minuteness of the or gans, and the component parts of these organs, by which such motions are performed.
The food of the different species of animalcula is not. yet indisputably ascertained. Probably it consists both of animal and vegetable matter; and they also prey on each other. The latter circumstance, indeed, has been denied; for it is maintained by several authors, that, if the vortex which many are endowed with the faculty of creating in the fluid, does ingulph other animalcula along with vegetable substances, that the former are rejected, and escape in safety. Other authors maintain the opposite opinion with equal confidence. Leeuwen hock, in speaking of the wheel-animal, indicates his belief of the fact. Ellis says the same of an animalcula, denominated by him volvox vorax. Goeze has seen the trichoda cimex devour the lesser animalcula vora ciously; and Spallanzani, speaking of this subject, ob serves, that animalcule feed so greedily upon each other, as to become larger. Here the pursuit is no longer interesting ; they prove indolent and sluggish. On the contrary, if reduced to abstinence, by being kept in distilled water, they are full of spirit, and eagerly de vour the minute animalcula supplied to them ; and the transparency of their bodies allows us to perceive them within, still retaining motion after hcing swallowed. Thus, the opinion of the authors who deny the fact must yield to such distinct observations.