R A R 



RAT 



RA'LLID^ {rallus, the rail). The 

 Rail tribe ; a family of the Grallatores, 

 or Wading birds, characterized by the 

 extreme length of the toes. 



RAME'NTA {rado, to scrape off). 

 Filings, as of a metal. In Botany, the 

 term denotes the thin, brown, foliaceous 

 scales, which appear on the back of the 

 fronds of ferns, &c. 



RAMO'SE [ramosus, branched). A 

 term applied to those spines upon shells 

 ■which send out others in a lateral direc- 

 tion. 



RAMPHA'STID^ (ramphastos, the 

 toucan). The Toucans ; a family of 

 Scansorial birds, placed by Cuvier in his 

 third order, Grimpeurs, between the anis 

 (Crotophaga) and the parrots. 



RANCI'DITY. The change which 

 oils undergo by exposure to the air. It 

 depends on the combination of oxygen 

 with the extractive principle, which is 

 naturally united with the oily principle. 

 The result is, therefore, analogous to the 

 oxidation of metals. 



RA'NIDiE {rana, a frog). The Frog 

 tribe ; one of the principal divisions of 

 the Anoural order of Amphibious ani- 

 mals, which pass from the tadpole to the 

 perfect state, and are adapted for aquatic 

 life. See Bufoidce. 



RANUNCULA'CEiE. The Crowfoot 

 tribe of Dicotyledonous plants. Herba- 

 ceous plants with divided leaves, opposite 

 or alternate ; calyx of 3-6 petals; petals 

 5-15, hypogynous ; stamens hypogynous, 

 indefinite in number ; fruit distinct, 

 simple carpella, and albuminous seeds. 



RA'PHIDES {pacpk, a needle). Small 

 acicular crystals, found within the cells 

 of the parenchyma of certain plants. 



RAPTO'RES {raptor, a snatcher). 

 Raptrices. Plunderers, or Birds of Prey; 

 an order of birds, characterized by the 

 robustness and muscularity of the whole 

 body, by the strength of the legs and of 

 the bill and talons, by their rapacious 

 habits, solitary disposition, great quick- 

 ness of sight, and powerful flight. They 

 are distinguished into the diurnal, con- 

 taining the vultures, eagles, falcons, &c., 

 and the nocturnal, or the owls. 



RAREFACTION {rarus, thin, facio, 

 to make). The act of rendering a sub- 

 stance less dense ; also, the condition of 

 diminished density. The term is gene- 

 rally applied to elastic fluids, which ex- 

 pand on the application of heat, or in 

 consequence of partial exhaustion, and 

 thetxe become thinner or more rarefied. 

 RA'RITY {rarus, thin). A property 

 282 



of matter opposed to density, and denoting 

 a thinness or subtlety of bodies : mercury- 

 is a dense fluid, ether a rare one. The 

 term is generally applied to aeriform 

 bodies, those of dilatation and expansion 

 being employed in speaking of solids and 

 liquids. 



RAS ALGRATHA. A star of the 

 third magnitude in the northern constel- 

 lation Hercules. Mas Aliagus is the 

 principal star in the northern constella- 

 tion Serpentarius. 



RASO'RES {rado, to scratch). Ra- 

 drices. Scratchers ; an order of gallina- ^ 

 ceous Birds, so named from the general 

 habit which these graminiferous species 

 present of scratching up the soil to ob- 

 tain their food. The only general cha- 

 racter of the order is derived, according 

 to Macgillivray, from the digestive 

 organs. " A very large dilatation of the 

 oesophagus, forming a crop, lying, when 

 distended, equally on both sides of the 

 neck, and such an enlargement of the 

 coeca as to render their capacity at least 

 half of that of the intestine, occur toge- 

 ther in no other birds. It is very difla- 

 cult to assign general characters of any 

 other kind to them." 



RA'STABER. A star of the third 

 magnitude in the northern constellation 

 Draco. 



R A'THOFFITE. A species of garnet, 

 found in Sweden, accompanied by calc- 

 spar and hornblende. 



RA'TIO. When two subjects admit 

 of comparison, with reference to some 

 quality which they possess in common, 

 and which may be measured, this mea- 

 sure is their ratio, or the rate in which 

 one exceeds the other. With this term 

 is connected that of proportion, which 

 denotes the portions, or parts of one mag- 

 nitude which are contained in another. 

 In the language of Mathematics, the 

 term ratio has been adopted to express 

 what is more generally understood by 

 the terra proportion : thus, instead of 

 "the proportion which" one thing bears 

 to another, we say, "the ratio which" 

 one bears to the other, meaning its com- 

 parative magnitude ; instead of saying 

 that A is to B "in the proportion of 

 5 to 6," we say, "in the ratio of 5 to 6." 



1. When the ratio is commensurable 

 (that is, when it is reducible to numbers), 

 it is equivalent to proportion ; the latter 

 term is, however, usually employed in 

 the comparison of ratios, in which case 

 two ratios are said to be proportionals. 

 Thus, 3 has to 4 a certain ratio, or pro- 



