P L U 



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POL 



then deities.) A name given to certain 

 rocks elaborated in the deep recesses of 

 the earth. 



PLUTO'NIC ROCKS. Unstratified crystal- 

 line rocks, such as granites, greenstones, 

 and others, of igneous origin formed at 

 great depths from the surface. Plutonic 

 rocks are distinguished from those which 

 are called volcanic, although they are 

 both igneous ; plutonic rocks having 

 been elaborated in the deep recesses of 

 the earth, while the volcanic are solidified 

 at or near the surface. 



PNEUMA'TICS. (from Trvtv/taruc/}, Gr.) 

 That branch of science which relates to 

 the equilibrium or movements of aerial j 

 fluids under all circumstances of pressure, 

 density, and elasticity. The weight of 

 the air, and its pressure on all the bodies 

 on the earth's surface, were quite un- 

 known to the ancients, and only first 

 perceived by Galileo, on the occasion of 

 a sucking-pump refusing to draw water 

 above a certain height. The manner in 

 which the observed law of equilibrium of 

 an elastic fluid, like air, may be consider- 

 ed to originate in the mutual repulsion of 

 its particles, has been investigated by 

 Newton, and the actual statement of the 

 law itself, as announced by Mariotte, 

 " that the density of the air, or the 

 quantity of it contained in the same space, 

 is, cseteris paribus, proportional to the 

 pressure it supports," has recently been 

 verified by direct experiment. This law 

 contains the principle of solution of every 

 dynamical question that can occur re- 

 lative to the equilibrium of elastic fluids, 

 and is therefore to be regarded a* one of 

 the highest axioms in the science of pneu- 

 matics. Herschel. 



POIKILI'TIC. (from TrouaXoc, Gr. various, 

 variegated.) To the new red sandstone 

 group, M. Brongniart has applied the 

 name of Terrain Poecilien. Mr. Cony- 

 beare has proposed to extend the term 

 Pcecilitic to the entire group of strata 

 between the coal formation and the lias, 

 comprising the new red conglomerate, 

 the magnesian limestone, the variegated 

 sandstone, the shell limestone, and the 

 variegated marl. Some common appel- 

 lative, says Dr. Buckland, for all these 

 formations has been long a desideratum 

 in geology; but the word poecilitic is, in 

 sound, so like pisolite, that it may be 

 better to adhere more literally to the 

 Greek root, and apply the common name 

 of Poikilitic group to the strata in ques- 

 tion. 



PO'LLEN. (pollen, Lat. fine flour.) In 

 botany, the fecundating powder or dust 

 contained in the anther. In dry and 

 warm weather the anther bursts, and the 

 pollen is thrown out. 



POLYGA'STRICA. (from TroXvc, many, and 

 yaor^p, a stomach, Gr.) The lowest 

 class of animals, belonging to Cyclo- 

 neura, or Radiata. 



When we place, says Professor Grant, 

 a drop of any decayed infusion of animal 

 or vegetable matter under a powerful 

 microscope, and throw a light through 

 that drop, and through the microscope 

 to the eye, we discover in the drop of 

 water various forms of living beings ; 

 some of a rounded, some of a lengthened 

 form, and some exhibiting ramifications 

 shooting in all directions but all appa- 

 rently of a soft, transparent, gelatinous, 

 and almost homogeneous texture. In 

 these minute animals there are numerous 

 cavities or stomachs, in some of them being 

 two hundred in number. There is every 

 reason to believe that polygastrica exist 

 in every drop of water. They form the 

 food of other classes, more especially the 

 zoophytes. Almost all the known genera 

 of polygastric animalcules possess eyes : 

 they are also found to possess an acute 

 sense of taste ; they distinguish, pursue, 

 and seize their prey, and, although so 

 excessively minute that five millions 

 have been calculated as being contained 

 in one drop of water, they avoid in- 

 fringing on one another while swimming. 

 All their movements appear to be as well 

 directed, regular, methodical, and spon- 

 taneous, as those of the higher classes of 

 swimming animals. These movements 

 are effected by means of very minute, 

 hair-like, tapering, transparent, vibratile 

 filaments disposed frequently around the 

 mouth, where they are generally largest 

 and longest. There is no proper skeleton 

 in the whole order polygastrica, nor any 

 secretion of shell on the surface, yet 

 there are parts destined to give support. 

 Some of the polygastric animals exude 

 on their surface a secretion which agglu- 

 tinates foreign particles floating in the 

 waters which surround them, and thus 

 form for themselves a partial covering. 

 In the majority of polygastric animals 

 there is an alimentary canal, with an 

 oral and an anal orifice, which traverses 

 the body. No teeth for mastication, nor 

 any glandular organs to assist in diges- 

 tion, have been discovered in them. 

 Notwithstanding their extreme minute- 

 ness, they appear to be the most nu- 

 merous, the most prolific, the most active, 

 and the most voracious of all living 

 beings. Lectures on Comp. Anatomy, 

 passim. 



PO'LYGON. (from TTO\VC, many, and yow'a, 

 angle, Gr.) A geometrical figure of many 

 angles. 



POLY'GONAL. (polygone, Fr.) Having many 

 angles and sides. 



