Animalcule, in its general acceptation, merely signifies a little animal, but is usually applied to those living objects, which are invisible to the naked eye, and can be discoverable only by the assistance of glasses.

By the invention of the microscope, we have become acquainted with a variety of animals, which, from their minuteness, would otherwise have escaped our observation; and there is reason to believe that myriads of them exist, both in the atmosphere and on the earth, which elude the human eye, even when assisted by this instrument. They are of various kinds, and to be met with in different natural bodies. By the assistance of magnifying glasses, they may be seen in water, vinegar, beer, milk, etc.; they are also found in corn, paste, flour, and other farinaceous substances.

In the year 1677, M. Lewen-hceck first discovered their existence in the human semen, and that of the lower animals; their number is inconceivable. On, viewing with a microscope the milt or seed of a male cod-fish, he found them in such swarms, and of so diminutive a size, that he supposed 10, 000 of them, at least, capable of being contained in the bulk of a grain of sand; whence he concludes, that the semen of this fish produces more animalculae than there are found living persons in the whole world. They appear to be very vigorous and tenacious of fife, as they continue to move long after the animal, from which they are taken, is dead. They also have this peculiarity, that they are in constant motion, without intermission, provided there be sufficient fluid, in which they may swim.

Great numbers of animalculae, some of which are of an oval fi-gure, and others resemble eels, are to be found in the whitish matter that adheres between the human teeth; but they have never hitherto been discovered, either in the blood, saliva, urine, bile, or chyle.

Animalculae are generated by putrefaction, and are supposed to produce many diseases, such as the plague, typhus, marsh miasma, etc. The small-pox, measles, and other cutaneous eruptions, are also by many conjectured to owe their origin to this source.

The existence of animalculae in the semen, has by several authors been denied, and among others by Mr. Needham, who, in an inquiry into the generation or production of animals, observes that seeds macerated in water, first disunite into small, motionless, and apparently inert particles, but that these afterwards possess power of motion, and seem alive, though in reality they are not so. He asserts, that there are no pre-existent gems formed for the production of animals, or vegetables, but that matter, organized in a peculiar manner, in its minute assemblages, produces them. In this opinion he is supported by M. Buffov, Reaumur, MAUPERTUIS, and other French Naturalists.—See Generation and Microscope.