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Tery different proportion of fossil 

 recent species. The newer pliocene, 

 the latest of the four, contains from 

 ninety to ninety-five per cent, of 

 recent fossils; the older pliocene 

 contains from thirty-five te fifty 

 per cent, of recent fossils ; the 

 miocene contains eighteen per cent, 

 of recent fossils ; the eocene contains 

 only three and a-half per cent, of 

 recent fossils. The newer pliocene 

 period is that which immediately 

 preceded the recent era ; the older 

 pliocene period is that which inter- 

 vened between the miocene and the 

 newer pliocene. The newer plio- 

 cene formations occur in Sicily and 

 Tuscany; the older pliocene at Nice, 

 Perpignan, Norfolk, Suffolk, and 

 near Sienna. Both the newer plio- 

 cene and the older pliocene exhibit 

 marine as well as fresh-water de- 

 posites. 



PLUMB A.' GO. (plumbago, Lat.) Gra- 

 phite. Commonly called black-lead. 



PLU'MULE. (from plumula, Lat. a 

 little feather.) In botany, that part 

 of the seed which grows into the 

 stem and axis of the future plant. 

 In the bean, horse-chesnut, &c., 

 the plumule is distinctly visible, 

 but in plants generally, it is scarcely 

 perceptible without the aid of a 

 magnifying glass ; and in many it 

 does not appear till the seed begins 

 to germinate. The first indication 

 of development is the appearance 

 of the plumule, which is a collec- 

 tion of feathery fibres, bursting from 

 the enveloping capsule of the germ, 

 and which proceeds immediately to 

 extend itself vertically upwards. 



PLTJTO'NIC. (from Pluto, one of the 

 heathen deities.) A name given 

 to certain rocks elaborated in the 

 deep recesses of the earth, from 

 the opinion they were formed by 

 igneous action at great depths; 

 whereas the volcanic, although they 

 may have risen up from below, 

 have cooled from a melted state 

 upon or near the surface. 



PLTJTO'NTC EOCKS. TJnstratified crys- 

 talline 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. 

 Plutonic rocks differ from volcanic,' 

 not only by their more crystalline 

 texture, but also by the absence of 

 tuffs and breccias, which are the 

 products of eruptions at the earth's 

 surface. They differ also by the 

 absence of pores or cellular cavities, 

 which the entangled gases give rise 

 to in ordinary lava. Lyell. 



PNETJMA'TICS. (from Trvevpcnucj), Gr.) 

 That branch of science which 

 relates to the equilibrium or move- 

 ments of aerial 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 unknown 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 equilib- 

 rium of an elastic fluid, like air, 

 may be considered to originate in 

 the mutual repulsion of its particles, 

 has been investigated by Newton, 

 and the actual state 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, caeteris paribus, propor- 

 tioned to the pressure it supports," 

 has been verified by direct experi- 

 ment. This law contains the 

 principal of solution of every dy- 

 namical question that can occur re- 

 lative to the equilibrium of elastic 

 fluids, and is therefore to be re- 

 garded as one of the highest axioms 

 in the science of pneumatics. 



