PAG 



C 335 ] 



P A C 



pure water; and is locked up to 

 an immense amount in the various 

 rocks, which are little else than a 

 mass of oxidized substances. Plants 

 give out oxygen, animals absorb it. 

 It is to Dr. Priestley we owe the 

 knowledge of the former of these 

 two facts ; and he it was who first 

 discovered oxygen, in 1774. Oxy- 

 gen has neither taste nor smell. It 

 is a trifle heavier than atmospheric 

 air, 100 inches weighing 33.88 

 grains. The combining proportion, 

 or equivalen number of oxygen, 



that of hydrogen being taken as 

 unity, is 8 ; its symbol, 0. 



OXYQENA'TION. "This word," says 

 Dr. Tire, "is often used for oxi- 

 dation, and frequently confounded 

 with it ; but it differs, in being of 

 more general import, as every union 

 with oxygen, whatever the pro- 

 duct may be, is an oxygenation; 

 but oxidation takes place only when 

 an oxide is formed." 



O'YSANITB. A name given by Lameth 

 to pyramidal titanium, or ana- 

 tase. 



PACHYDE'EMATA. (from va^vv, thick, 

 and Seppa, skin, Gr.) Thick-skinned 

 animals. The seventh order of the 

 class Mammalia, in Cuvier's ar- 

 rangement. This order Cuvier di- 

 vided into two families, namely, 

 Proboscidian a, or those pachyder- 

 matous animals, which have tusks 

 and a proboscis, as the elephant 

 and mastodon; and pachydermata 

 ordinaria, in which are included 

 the hippopotamus, anoplotherium, 

 palaeotherium, tapir, &c. 



Several genera of the order Pa- 

 chydermata have become extinct, 

 their fossil remains alone proving 

 that such ever existed. Amongst 

 these are the mastodon, the ano- 

 plotherium, the palaeotherium, the 

 lophiodon, the anthracotherium, 

 the cheropotaraus, adapis, &c. Of 

 these there are about forty species, 

 all extinct, and to which there are 

 none analogous in the living world, 

 except two tapirs and a daman. 

 Of the existing genera of pachyder- 

 mata, many species which existed 

 during the older and newer pliocene 

 periods also seem to have become 

 extinct, and, in fact, the living 

 species bear no sort of proportion 



to the extinct. Those shades which 

 approximate genera to each other, 

 those intermediate forms, those 

 steps from one genus to another, 

 so common in the other families of 

 the animal kingdom, are here 

 wanting. It was reserved for the 

 science of fossil osteology to recover 

 them from the entrails of the eatth, 

 among the races which completed 

 the grand system of animated na- 

 ture, and whose destruction has 

 produced such wide and striking 

 intervals. 



The pachydermata appear to be, 

 as it were, only the remnants of a 

 very extensive order, which for- 

 merly inhabited the earth, but have 

 now almost entirely disappeared. 

 They feed upon grass, but they do 

 not ruminate. They are, for the 

 most part, huge and unwieldly 

 animals, with thick integuments; 

 solidity and strength appear to be 

 the objects chiefly regarded in their 

 construction. 



PACHY'MIA. A genus of fossil bivalve 

 shells described by Mr. Sowerby 

 as obliquely elongated, equivalve, 

 thick, sub-bilobed, with beaks near 

 the anterior extremity; the liga- 



