PRE 



PRE 



third power; and, by continuing the 

 multiplications by the same original num- 

 ber, we obtain the fourth, the fifth, the 

 sixth, &c. powers. Thus taking the 

 number 2,— 



2^ = 2, or 1" power; 

 2 X 2, or 22 = 4, or 2"^ power ; 



22 X 2, or 23 = 8, or 3"^ power ; 



23 X 2, or 24 = 16, or 4th power; 

 2* X 2, or 25 = 32, or 5^^ power; 



1. When it is said that 1 is the square 

 of 1, there is a logical error in the use 

 of the terms : it ought to be thus — 



1 square is the square of 1 linear ; thus 

 1 n is the square of — 



2. The numbers indicating the powers, 

 are called the exponents or indices ; and 

 it is evident that powers of the same 

 quantity are multiplied by adding their 

 exponents, and divided by subtracting 

 their exponents. 



3. With this subject is connected the 

 term dimension. Thus, a^, a^, a^, &c. 

 are said to be of one, two, three, &c. di- 

 mensions, respectively ; and, in general, 

 any product is said to be of n dimensions, 

 if the sum of the indices of its several 

 literal factors is equal to n. Thus ab, 

 that is, a'A^ is of two dimensions ; 3 aH^ 

 is of five dimensions ; and so on. 



4. A power in the denominator of a 

 fraction is also expressed by placing it in 

 the numerator, and prefixing the nega- 

 tive sign to its index ; thus 



a— ^, tt"~', flS"~^> o""", 



signify 



— , — , respectively. 



These are called the negative powers of a. 



PRACTICE. An arithmetical rule, 

 deriving its name from its wide practical 

 application, and teaching how to find the 

 value of any number of articles, having 

 given the value of one ; or to find the 

 value of any quantity, having given the 

 value of a single unit. 



PR^FLORA'TION {prce, before, flo- 

 reo, to flower). JEslivation. A term in 

 Botany, denoting the manner in which 

 the floral envelopes of plants are ar- 

 ranged previously to their expansion. 



PRASE {irpaaov, a leek). A mineral 

 substance of a leek-green colour, consist- 

 ing apparently of an intimate mixture of 

 common quartz and actonite, and found 

 in the island of Bute and in Borrow- 

 dale. 



PRECESSION OF EQUINOXES. 

 The term applied to the slow but regular 

 retrogradation of the equinox along the 

 270 



ecliptic, from east to west, or in the con- 

 trary direction to that in which the sun 

 appears to move in that circle. Thus, the 

 place of the equinox among the stars, at 

 every subsequent moment, precedes (with 

 reference to the diurnal motion) that 

 which was held the moment before. The 

 amount of annual retrogradation is only 

 50"- 10, or about a degree in seventy-two 

 years. The period in which the equinox 

 would perform a complete tour of the 

 ecliptic, is 25,868 years. 



PRECIPI'TATE {prccceps, headlong). 

 A solid substance precipitated, or thrown 

 down from a solution, by adding a re- 

 agent. Thus we have the red precipitate 

 or the peroxide of mercury; the white 

 precipitate, or the ammoniated chloride ; 

 the precipitate per se, or the purple, &c. 



PRECIPITA'TION {prceceps, head- 

 long). The process of throwing down 

 solids from solutions in which they are 

 contained. The substance so separated 

 is called a. precipitate ; and the substance 

 employed to produce this eifect, a pre- 

 cipitant. This process is the opposite to 

 that of chemical solution. 



PRE'DICABLE. In Logic, a term 

 which can be affirmatively predicated of 

 several others. Predicables are also 

 called common terms, from their be- 

 longing each to several others all 

 alike. 



PRE'DICAMENT. The predicaments 

 or categories of logicians are certain 

 general heads, or summa genera, under 

 which all abstract ideas, and their signs, 

 common words, may be arranged. The 

 classification of predicaments will be 

 found under the term Categories; it 

 was introduced by Archytas, and adopted 

 by Aristotle. 



PRE'DICATE. The predicate of a 

 proposition is that term which is affirmed 

 or denied of the other. Logically, the 

 predicate occupies the last place in a 

 proposition, though, not unfrequently, it 

 is placed first. 



PREHNITE, A zeolitic substance, 

 the grass-green variety of which, found 

 in South Africa, has been mistaken for 

 chrysolite, chrysoprase, and even eme- 

 rald. The variety which occurs in small 

 thin crystals is called koupholite. 



PREMISS. In Logic, a proposition 

 employed to establish a certain conclu- 

 sion. The major premiss is that in 

 which the major term is compared with 

 the middle ; the minor premiss, that in 

 which the minor term is compared with 

 the middle. The two premises of a syl- 



