AFF 



AFF 



prepared by burning the fuciis vesiculo- 

 sus in the open air, and reducing it to a 

 black powder. 



^yXHOGEN {aWuyv, brilliant, yeivofxat, 

 to become). A compound of boron and 

 nitrogen, lately discovered by Mr. Bal- 

 main. It gives a brilliant phosphorescent 

 light when heated before the blowpipe. 



^'THRIOSCOPE {aiOpia, serene wea- 

 ther, o-KOTreci), to examine). An instru- 

 ment invented by Sir John Leslie for 

 indicating the power of the clouds in 

 preventing radiation. It consists of the 

 differential thermometer, having one of 

 the balls excluded from the light, and 

 the other placed in a polished metallic 

 cup. Exposed to a clear part of the sky, 

 the heat radiated from it escapes rapidly, 

 and the temperature falls ; exposed to a 

 cloud, the radiated heat is restored, and 

 there is no reduction of temperature. 



^TIO'LOGY {alria, a cause, \6yos, a 

 treatise). That branch of medical sci- 

 ence which treats of the causes of dis- 

 ease. 



AETI'TES LAPIS (uero?, an eagle). 

 Eagle-stone ; a variety of iron ore, so 

 called from the belief that it was found 

 in the nest of the eagle, where it was 

 supposed to prevent the eggs from be- 

 coming rotten, 



AFFINITY, CHEMICAL {affinitas, 

 relationship). That kind of attraction 

 by which certain substances, when placed 

 in contact, exhibit a tendency to combine 

 with each other, forming compounds dif- 

 fering in all their essential qualities and 

 actions from their constituent ingre- 

 dients, and constituting distinct species 

 of matter. The actual phenomena of 

 combination suggest the idea of peculiar 

 attachments and aversions subsisting be- 

 tween different bodies ; and it was in this 

 figurative sense that the terra affinity 

 was first applied by Boerhaave to a pro- 

 perty of matter. 



L Single affinity is the property by 

 which two elementary bodies combine, 

 as when oxygen combines with lead, 

 forming a white oxide. 



2. Single elective affinity is the property 

 by which a body exhibits a preference in 

 combining with another, rather than with 

 a third, a fourth, &c. Thus nitric acid 

 has an affinity for magnesia ; it has also 

 an affinity for lime ; but, on mixing 

 these three substances together, only one 

 of the affinities is obeyed : the nitric acid 

 combines with the lime, leaving the 

 magnesia altogether unaffected. 



3. Double elective affinity is the pro- 



11 



party by which two compounds are de- 

 composed, and a double affinity exhibited. 

 Thus, when carbonate of soda is added 

 to nitrate of lime, carbonate of lime is 

 instantly formed and precipitated, nitrate 

 of soda being formed at the same time 

 and remaining in solution. This is a 

 case of complex affinity, connected with 

 double decomposition. 



4. Reciprocal affinity is the property 

 by which bodies, which have no tendency 

 to unite, are made to combine by means 

 of a third, which is then called the m.e- 

 dium. Thus, on introducing a clean 

 plate of platinum into a mixture of oxy- 

 gen and hydrogen gases, the gases in 

 contact with the metallic surface in- 

 stantly unite and form water. This has 

 been termed disposing affinity, and the 

 affinity of intermedium. 



5. Quiescent and divellent affinities are 

 terms introduced by Kirwan. The former 

 is that property which prevents decom- 

 position, by maintaining the elements of 

 a compound body in their existing state. 

 The latter favours decomposition, by 

 tending to arrange the particles of a 

 compound in a new form. 



6. Elementary affinity is that which 

 takes place between the elements of a 

 body. Resulting affinity occurs only in 

 a compound, having no existence with 

 the elements of that compound. 



7. Inductive or current affinity is a term 

 expressive of the peculiar exhibition of 

 chemical affinity in the simple voltaic 

 circle. The idea of any thing like a cir- 

 culation of electricity in this case appears 

 to be abandoned, the phenomena admit- 

 ting of a perfectly intelligible explanation 

 when referred to ordinary chemical affi- 

 nity exerting its influence at a dis- 

 tance, by an inductive or circulating 

 agency. 



AFFIRMATIVE. A term employed 

 in logic to denote the essential quality of 

 a proposition : this quality is affirmative 

 when the copula asserts an agreement 

 between the predicate and the subject ; 

 negative, when it denies that agreement. 

 The essential quality of a proposition is 

 therefore determined by the copula. 



1. An affirmative quantity in algebra is 

 a positive quantity, or a quantity to be 

 added, as distinguished from a negative 

 quantity, or a quantity to be deducted. 



2. An affirmative sign, or a positive 

 sign, is a sign of addition, and is marked 

 + , signifying plus, or more. See Alge^ 

 braical signs. 



A'FFIX {affigo, to fasten to). Suffix. 

 B6 



