JET 



JUL 



designation of a triangle of which two 

 sides, or legs, are equal. 



16. Iso-stemonous {arfjuciv, a stamen). 

 A term applied by De Candolle to those 

 plants, the stamens of which are equal in 

 number to the petals. See Anisostemonous. 



17. Iso-thermal Lines {Oep/j-nt heat). 

 Lines of equal temperature ; a term ap- 

 plied, in physical geography, to lines con- 

 necting all those places on the surface of 

 the Globe which have the same mean 

 temperature. Lines drawn through places 

 having the same summer, and the same 

 winter temperatures, are termed, respec- 



tively, iso-theral {Oepos, summer), and 

 iso-cheimal (xf i/m"* winter) lines ; while 

 lines drawn through places having other 

 common temperatures, receive other ap- 

 propriate names. 



I'STHMUS. A narrow neck of land 

 lying between two seas, and connecting 

 two masses of land g:reater than itself. 



ITACONIC ACID. Another name 

 for the pyrocitric or citricic acid. 



I'TTNERITE. A rare mineral, con- 

 sisting chiefly of silica, alumina, and 

 soda, together with some hydrosulphu- 

 ret ; named after Ittner. 



JACULATRI'CES {jaculum, a dart). 

 The name given by Macgillivray to an 

 order of birds, from the peculiar form of 

 their bill, and their rapid flight. The 

 families composing the order are the 

 Alcedinse, the Galbulina?, and the Tro- 

 goninae, of which the last two have the 

 feet zygodactyle. 



JADE. Nephrite. A mineral sub- 

 stance found in granite and gneiss, in 

 Switzerland, and divisible into the two 

 varieties of common nephrite and ama- 

 zonian stone. The name has, however, 

 been applied to various minerals which 

 resemble one another in little else than 

 in colour. 



JA'MESONITE. A mineral consist- 

 ing principally of the sulphurets of anti- 

 mony and of lead, and named after Pro- 

 fessor Jameson. 



JA'RGON. Jargon of diamond is a 

 designation of one of the varieties of 

 zircon. 



JA'SPER. A siliceous substance of 

 various colours, occurring in veins in 

 trap rocks, in volcanic rocks, and in the 

 primary and secondary series. 



JA'TROPHIC ACID. Crotonic Acid. 

 An acid procured by the saponification of 

 croton oil. 



JE'FFERSONITE. A variety of py- 

 roxene ; a new mineral found in Frank- 

 linite and garnet, in New Jersey. 



JE'RVINE. A vegeto- alkali, found 

 in the root of veratrum album. 



JET, or PITCH COAL. A black vel- 

 vet-coloured bitumen, used for fuel, and 

 for making vessels, &c. In Prussia it 

 is called black amber, and is cut into 

 rosaries and necklaces. Ure says the 

 186 



word jet is derived from the river Gaga 

 in Lesser Asia. 



JEWELLERS' PUTTY. Ignited and 

 finely-levigated oxide of tin, used by 

 jewellers for polishing hard objects. 



JOINTS. In Geology, fissures or lines 

 of parting in rocks, often at right angles 

 to the planes of stratification. The part- 

 ings which divide columnar basalt into 

 prisms, are joints. 



JU'DGMENT. In Logic, an operation 

 of the mind, by which we compare two 

 ideas or notions which are the objects 

 of apprehension, whether complex or 

 incomplex, and pronounce upon their 

 agreement and disagreement. Judgment 

 is, therefore, either affirmative or nega- 

 tive. 



JU'GUM. The Latin term for a yoke, 

 and hence applied to a pair of opposite 

 leaflets on the petiole of a pinnate leaf. 

 Thus a leaf with one pair is called uni- 

 jugal; with two pairs, bijugal, &c. 



Juga in Umbelliferous Plants. The 

 term juga also signifies ridges, and is 

 hence applied to the elevated portions by 

 which the carpels of umbelliferous plants 

 are traversed; of these juga, five are 

 called primary, and four, alternating 

 with them, secondary. 



JULIAN CALENDAR. The mode 

 of reckoning the divisions of the year, 

 adopted by Julius Caesar, and afterwards 

 corrected by Gregory XIII. It is ex- 

 plained under the term Bissextile. The 

 Russians still retain the Julian style, 

 and their year consequently begins twelve 

 days later than ours. 



JULIAN PERIOD. A revolution of 

 7980 years, reckoned from 4713 years 



