PRE 



PRE 



tei* has thus been accumulated all round 

 the equatorial parts than any where else 

 fen the earth, the sun and moon, when on 

 either side of the equator, by attracting 

 this redundant matter, bring the equator 

 sooner under them, in every return to* 

 wards it, than if there was no such ac- 

 cumulation. 



Sir Isaac Newton in determining the 

 quantity of the annual precession from 

 the theory of gravity, on supposition that 

 the equatorial diameter of the earth is t<> 

 the polar diameter, as 230 to 229, finds 

 the sun's action sufficient to produce a 

 precession of 9"i only ; and collecting 

 from the tides the proportion between the 

 sun's force and the moon's, to be as 1 to 

 4J, he settles the mean precession result- 

 ing from their joint actions at 50" ; which 

 is nearly the same as it has since been 

 found by the best observations. 



PRECISE, in botany, the name of the 

 twenty-first order inLinnaeus's Fragments 

 of a Natural Method: containing the prim- 

 rose, and a few other plants which agree 

 with it in habit and structure. 



PRECIPITATE, in chemistry, is any 

 matter or substance, which having been 

 dissolved in a fluid, falls to the bottom of 

 the vessel on the addition of some other 

 substance, capable of producing a decom- 

 position of the compound. The term is 

 generally applied when the separation 

 takes place in a floculent or pulverulent 

 form, in opposition to crystallization, 

 which implies a like separation in an an- 

 gular form. But chemists call a mass of 

 crystals a precipitate, when they sub- 

 side so suddenly, that their proper crys- 

 talline shape cannot be distinguished by 

 the naked eye, as in the instance of 

 Glauber's salt, when separated from its 

 watery solution by mixing with it a por- 

 tion of alcohol. 



PRECIPITATION, that process by 

 which bodies dissolved, mixed, or sus- 

 pended in a fluid, are separated from the 

 fluid, and made to gravitate to the bot- 

 tom of the vessel: this is one of the 

 great operations in chemistry, and is op- 

 posed to that of solution. In truth the 

 chief operations in the laboratory may be 

 resolved into solution and precipitation. 

 When a base is employed to precipitate 

 a soluble acid, the substance thrown down 

 is always a compound, consisting of the 

 acid united to the base employed. In 

 this case the acid is sometimes completely 

 separated, and sometimes not, according 

 to the energy of the base employed, and 

 the degree of insolubility of the salt form- 

 ed. The same explanation applies as in 



VOL. V. 



the first case. When a neutral salt is 

 employed as a precipitant, the substance 

 which falls is always a compound. It is 

 composed of one of the ingredients of the 

 precipitating salt united to one ingredient 

 of salt in solution. Such salts alone can 

 be employed as are known to form in- 

 soluble compounds with the acid or base 

 which we wish to throw down. In these 

 cases the separation is complete* when 

 the new salt formed is completely insolu- 

 ble. Neutral salts perform the office of 

 precipitants in general much more readily 

 and completely than pure bases or acids, 

 Thus the alkaline carbonates throw down 

 the earths much more effectually than the 

 pure alkalies, and sulphate of soda sepa- 

 rates barytes much more rapidly than 

 pure sulphuric acid. This superiority is 

 owing partly to the combined action of 

 the acid and base, and partly to the com- 

 paratively weak action of a neutral salt 

 upon the precipitate, when compared to 

 that of an acid or alkali. For the preci- 

 pitation takes place, not because the 

 salts are insoluble in water, but because 

 they are insoluble in the particular solu- 

 tion in which the precipitate appears. 

 Now if this solution happens to be capa- 

 ble of dissolving any particular salt, that 

 salt will not precipitate, even though it 

 be insoluble in water. Hence the reason 

 why precipitates so often disappear, when 

 there is present in the solution an excess 

 of acid, of alkali, &c. 



PRECONTRACT of marriage, in the 

 civil law, avoided the marriage ; but by 

 the statute 2 George I, c. 23, called the 

 marriage act, it is declared, that it shall 

 not be allowed, nor shall any contract of 

 marriage be enforced in the ecclesiastical 

 courts. The only remedy upon breach 

 of a promise of marriage is by action for 

 damages at common law. 



PREDIAL tithes, those which are paid 

 of things arising and growing from the 

 ground only, as corn, hay, fruit of trees, 

 and the like. 



PREENING, in natural history, the 

 action of birds dressing their feathers, to 

 enable them to glide the more readily 

 through the air, &c. For this purpose 

 they have two peculiar glands on their 

 rump, which secrete an unctuous matter 

 into a bag- that is perforated, out of which 

 the bird occasionally draws it with its 

 bill. 



PREGNANCY. See MIDWIFRUT. 



PREHNITE, in mineralogy, a species 

 of the flint genus. Its colours are green 

 in almost all its shades. It is sometimes 

 massive, sometimes 'crystallized. Exter- 



3 N 



