Z AM 



ZE A 



in sidereal time. The reason of this dif- 

 ference is, that, as the sun's apparent 

 annual motion among the stars is per- 

 formed in a contrary direction to the 

 apparent diurnal motion of both sun and 

 stars, it comes to the same thing as if the 

 diurnal motion of the sun were so much 

 slower than that of the stars, or as if the 

 sun lagged behind them in its daily 

 course. When this has gone on for a 

 whole year, the sun will have fallen be- 

 hind the stars by a whole circumference 

 of the heavens ; or, in other words, in a 

 year, the sun will have made fewer diur- 

 nal revolutions by one than the stars. 

 The proportion between the mean solar 

 and the sidereal day is, when reduced to 

 a decimal fraction, that of 1.00273791 to 1. 



2. The anomalistic year is the time 

 which elapses between the sun's leaving 

 his apogee and his return to it, consisting 

 of 365d. 6h. 13m. 49s. 3. The difference 

 between this and the tropical or civil 

 year is owing to the orbit of the earth 

 representing an ellipse, of which the 

 major axis has a slow motion of 11".8 per 

 annum in advance. 



3. Historical and ecclesiastical year. 

 In England, the historical year was, for 

 a very long period, begun on the 1st of 

 January. But the civil, ecclesiastical, 

 and legal year began, until the end of the 

 thirteenth century, at Christmas, In and 

 after the fourteenth century, it com- 

 menced on the 25th of March, and so 

 continued until the 1st of January, 1753. 

 Much confusion arose from these two 

 modes of computing dates ; for the legis- 

 lature, the ecclesiastics, and the civilians, 

 referred every event which happened be- 

 tween the 1st of January and the 25th of 

 March to a different year from historians. 



4. To avoid, so far as possible, the mis- 

 takes which this custom produced, it was 

 usual to add the date of the historical to 

 that of the legal year, when speaking of 



any day between the 1st of January and 

 the 25th of March ; thus,— 



8 ri. e. the civil and legal 

 Jan. 30, 164 -j year, 



9 \i. e. the historical year ; 

 or, thus : 



January 30, 1648-9. 



YEAST. Fermentum. A substance 

 generated during the vinous fermenta- 

 tion of vegetable juices and decoctions, 

 rising to the surface in the form of a 

 frothy, flocculent, and somewhat viscid 

 matter. It is used for promoting fer- 

 mentation. See Catalysis. 



Artificial Yeast may be made by boil- 

 ing malt, pouring off the water, and 

 keeping the grains in a warm place to 

 fprment, repeating the process till a suf- 

 ficient quantity is procured. 



YELLOW EARTH. A massive mine- 

 ral of an ochre-yellow colour, consisting 

 of silica, alumina, iron, and lime. It 

 occurs in Upper Lusatia, associated with 

 clay and clay-ironstone. When burnt, 

 it is sold by the Dutch as a pigment, 

 under the name of English red. 



YENITE. Another name for lievrite, 

 a double silicate of lime and iron, found 

 in the Isle of Elba and in Norway. It 

 was named yenite by Leli^vre, its disco- 

 verer, in honour of the battle of Jena. 



YOU-STONE. Chinese jade; a zeo- 

 litic substance referred to prehnite. 



YTTRIA. A new earth, dis overedby 

 Gadolin, in a mineral from Yiterby in 

 Sweden. Its metallic base is yttrium, of 

 which it is considered to be a protoxide. 



YTTRO-CERITE. A mineral sub- 

 stance containing yttria, oxide of cerium, 

 and fluoric acid ; it is found at Finbo in 

 Sweden, imbedded in quartz, or incrust- 

 ing pyrophysalite. 



YTTRO-TANTALITE. An ore of 

 tantalum, also called yttrious oxide of 

 tantalum, found in reniform masses at 

 Ytterby in Sweden. 



ZAFFRE. An impure oxide of cobalt, 

 being the residuum of the native arseni- 

 uret of cobalt, after the sulphur, arsenic, 

 and other volatile matters of this mineral 

 have been expelled by calcination. The 

 substance brought from Saxony, and 

 commonly sold under this name, is a 

 mixture of oxide of cobalt with vitrifiable 

 earth. 



ZAMITE. The name given to the 

 fossil species of zamia, the principal 

 381 



forms of which have been found in the 

 lias and oolitic formations. They differ 

 from the other species in the exsertion 

 and venation of their leaves. 



ZANTHOPI'CRITE. A crystalline 

 substance extracted from the bark of the 

 zanthoxylum of the Caribbee islands. 



ZEA'GONITE. A Vesuvian mineral 

 referred to the harmotone or cross-stone, 

 and to the potass division of this zeolitic 

 substance. 



