POS 



PO w 



land Beds. A series of limestone strata, 

 belonging to the upper part of the Oolite 

 group, found chiefly in England, in the 

 island of Portland, on the coast of Dor- 

 setshire. The great supply of the build- 

 ing stone used in London is from these 

 quarries. Portland Sand is an inter- 

 mixture of siliceous and calcareous sand, 

 containing green particles. Portland 

 Oolite is a calcareo-siliceous, more or less 

 oolitic freestone, of a yellowish-white 

 colour, containing nodules of flint in its 

 upper part. 



PORTU'NID^. Paddling Crabs; a 

 family of the brachyurous crustaceans, 

 nearly allied to the Cancerians. 



POSITION. An arithmetical rule, 

 also called the rule of Supposition, the 

 Rule of False, or the Rule of Trial and 

 Error. It consists in assuming numbers, 

 and operating upon them, in order to 

 discover the true number. Single Posi- 

 tion relates to those questions in which 

 the results are proportional to the sup- 

 positions, and in which, consequently, 

 one assumption only is required. Double 

 Position embraces questions in which 

 the results are not proportional to the 

 suppositions, and in which two assump- 

 tions are necessary in order to find the 

 true answer. 



POSITIVE QUANTITY. In Algebra, 

 this term denotes an afiirmative or addi- 

 tive quantity. Its symbol, +, which is 

 read plus, signifies that the quantity to 

 which it is prefixed, must be added ; and, 

 if no symbol is prefixed, the quantity is 

 still understood to be positive. 



POSITIVE TERMS. In Logic, those 

 terms which denote a certain view of an 

 object as being actually taken of it, are 

 called positive. They are thus distin- 

 guished from privative terms, which de- 

 note that this view might be taken of an 

 object, but is not ; and from negative 

 terms, which denote that such a view is 

 not and could not be formed of the object. 

 " A living man" is an illustration of the 

 first kind of terms ; •' a lifeless man," of 

 the second; "a lifeless carcase," of the 

 third. 



POSSIl^LE. This word, like the others 

 of kindred meaning, relates sometimes 

 to contingency, sometimes to power; 

 and these two senses are frequently con- 

 founded. In the former sense, we say, 

 "it is possible this patient may recover," 

 not meaning, that it depends on his 

 choice, but that we are not sure whether 

 the event will not be such. In the latter 

 sense it is " possible" to the best man to 

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violate every rule of morality ; since if it 

 were out of his power to act so if he 

 chose it, there would be no moral good- 

 ness in the case; though we are quite 

 sure that such will never be his choice 

 ( Whately). See Impossihility. 



POSTERIOR MARGIN. A term ap- 

 plied to that side of the bosses of acepha- 

 lous bivalves, which contains the liga- 

 ment. 



PO'STULATE {postulatum, a thing 

 demanded). A thing required to be 

 granted, for the purpose of reasoning. 

 The object of the postulates in Euclid is 

 to declare that the only instruments, the 

 use of which is permitted in Geometry, 

 are the rule and the compass. 



POTASH. Pofassa. The hydrated 

 protoxide of potassium, procured by the 

 incineration of vegetables, and named 

 from the circumstance that the water in 

 which the ashes are washed is evaporated 

 in iron pots. 



POTA'SSIUM. Kalium. A metallic 

 substance, white, with a brilliant lustre, 

 like mercury, hard and brittle at 32°, 

 soft and malleable at 50°, fluid at 150°, 

 and volatilized below a red heat. 



POTENTIAL MOOD. This mood, 

 according to the views of Crombie and 

 Greenlaw, is strictly subjunctive, and 

 in all those cases in which its signifi- 

 cation is potential, it is by ellipsis, 

 and not by its own proper power of ex- 

 pression. 



POT-METAL. A mixture of copper 

 and about a fourth its weight of lead. 



POTSTONE. Lapis Ollaris. A green- 

 ish-grey mineral, occurring in thick beds 

 in primitive slate, and fashioned into 

 culinary vessels in Greenland. It is a 

 sub-species of the rhomboidal mica of 

 Jameson. 



POTTER'S CLAY. A variety of clay 

 of various colours, disintegrating on ex- 

 posure to the air; w^hen mixed with 

 sand, it is made into bricks and tiles, 

 and is also used for coarse pottery. 



POWER. In Mechanics, this term de- 

 notes any force, animate or inanimate, 

 which, being applied to a machine, tends 

 to produce or to prevent motion ; in the 

 former case, it is termed a moving power 

 or force ; in the latter, a sustaining power 

 or force. Power is latent force. 



POWER OF NUMBERS. In Alge- 

 bra and Arithmetic, any given number 

 is said to be in its first power ; multiplied 

 by itself, it becomes the square, or second 

 power; this, again, multiplied by the 

 original number, becomes the cube, or 

 N 3 



