ANIMALCULE in a general sense, signifies a small ani mal; but here it is used to denote one so minute, that its form and parts cannot be distinguished, without the aid of the microscope.
If particles of animal or vegetable matter are a few days infused in the most limpid water, on applying the smallest portion of it to the microscope, innumerable animals of various shapes are discovered. These have been denominated Animalcula Infusoria by naturalists. But their only habitation is not in infusions thus artifi cially made; the mud of ditches, the scum of stagnant waters, pools, and marshes, which to vulgar eyes pass for the vilest matter, are the source of admiration to the philosopher, from the rare and 'wonderful beings they contain. In the sand deposited by common sewers is found an animal endowed with the incredible property of resisting death, in enjoying the privilege of a real and undoubted resurrection. It is to the nature of this, and some other remarkable animalcules, exhi biting peculiarities known to reside in no other part of animated existence, that our attention shall chiefly be directed, after briefly narrating the systematic arrange ment under which they have been brought.
Here we are guided by the labours of the indefatiga ble Muller, who has divided animalcules in two classes, consisting of seventeen genera, and these, from specific characters, subdivided into 378 species.
These, including a very few animalcules treated of by other naturalists, are all the species that have hith erto been enumerated. But we can hardly doubt that many more exist, and that sonic will be found peculiar to certain countries only ; for it must be wondered, that it is principally in the more northern regions where the properties of animalcules have been investigated.
It is probable, that, while future inquiries produce new animalcules to our notice, they may remove some of those already known into the insect tribes; and it is not unlikely that they will prove some of the generic distinctions, now admitted, to be founded on arbitrary or erroneous principles.
Accounting for the origin of animalcules is a point of extreme 'difficulty, because their existence seems solely dependent on the adventitious union of animal or vege table substances, and a simple fluid. Their germs, it
has been thought, are either suspended in the air, or are mixed with the macerating particles. Muller and Spallanzani, two philosophers of equal eminence, con ceive it more probable that they fall from the air. The latter took sixteen large glass vessels, of the same size, and divided them into four classes. Four were her metically sealed, four stopped with wooden stoppers, four with cotton, and the remaining four left open. Thus, the external air had no communication with the interior of some, very little with others, with the third class more, and with the rest it was as free as possible. Every class contained infusions of hemp-seed, rice, len tils, and pease ; which substances were boiled a full hour in the vessels before being closed up. In twenty five days the whole were examined, and were found to both large and small animalcules. The four open infusions seemed to teem with life ; about a third fewer were in those stopped with cotton ; still fewer were in those with the wooden stoppers, and in the vessels hermetically sealed were fewest of all. The result of the experiment was the same on changing the macerating substances. If, instead of using stoppers to all the vessels, the surface of the infusions were cover ed with oil, this still further diminished the number of animalcules. • It is a prevalent opinion, that animalcules may be dis covered in rain water by the microscope, and in that of the purest fountain. But this is an error. Sometimes, indeed, though they.may be found in either, they are ex tremely rare. They do not originate in vacuo; and as a vacuum in time becomes fatal to them, it is conjectured that they perhaps respire. Numbers appear where the rarefaction of the air k&ps thirteen inches of mercury in equilibrio; and they originate in vessels hermetically sealed, if sufficiently capacious; not invariably so, how ever, and there are always fewer. Thus, there is great reason to conclude, that their germs exist, not only in the air, but also in the macerating substances, or even in the fluid itself ; and are gradually unfolded, accord ing to the concurrence of the circumstances, which promote their expansion. Among these, heat and pu trescence seem the most indispensible.