TEE 



[432] 



TEE 



TERO'XIDE. } "When oxygen is cora- 



TRITO'XIDE. j bined with any sub- 

 stance in the proportion of three 

 equivalents, it is called a teroxide, 

 or tritoxide. 



TE'RRA SIE'NNA. The name given to 

 an ochreous earth from its being 

 brought from Sienna : it is a sort 

 of brown bole, and is used as a 

 pigment. 



TE'RRAIIS- TERTIA'IRE. The name 

 given by the French geologists to 

 the tertiary strata. The Germans 

 call these, Tertiargebile. 



TE'RRE VERTE. Green earth. The 

 grim erde of Werner ; talc zogra- 

 phique of Hau'y ; chlorite baldog^e 

 of Brongniart. An earth of a 

 green colour, sometimes passing to 

 olive. It consists, according to 

 Klaproth, of silex 53, magnesia 2, 

 potash 10, oxide of iron 28, water 

 6. Yauquelin states that it con- 

 tains 7 '00 of alumine. It is found 

 in Germany, France, Italy, and 

 North America. It is ground with 

 oil, and used as a pigment. 



TERRE'STIAL REFRA'CTION. The power 

 which air possesses, in common 

 with all transparent media, of re- 

 fracting the rays of light, or 

 bending them out of their straight 

 course, rendering a knowledge 

 of the constitution of the atmosphere 

 important to the astronomer. When- 

 ever a ray of light passes obliquely 

 from a higher level to a lower one, 

 or vice versa, its course is not rec- 

 tilinear, but concave downwards ; 

 and of course any object seen by 

 means of such a ray, must appear 

 deviated from its true course, 

 whether that object be, like the 

 celestial bodies, entirely beyond 

 the atmosphere, or, like the sum- 

 mits of mountains, seen from the 

 plains, or other terrestial stations, 

 at different levels, seen from each 

 other, immersed in it. Every 

 difference of level, accompanied, as 

 it must be, with a difference of 

 density in the aerial strata, must 



also have, corresponding to it, a 

 certain amount of refraction. This 

 refraction bet ween terrestial stations 

 is termed terrestial refraction. The 

 refraction of a terrestial object is 

 estimated differently from that of 

 a celestial body. It is measured 

 by the angle contained between 

 the tangent to the curvilineal path 

 of the ray where it meets the eye, 

 and the straight line joining the 

 eye and the object. The quantity 

 of terrestial refraction is obtained, 

 by measuring contemporaneously 

 the elevation of the top of a moun- 

 tain above a point in the plain at 

 its base, and the depression of that 

 point below the top of the mountain. 

 The distance between these two 

 stations is the chord of the hori- 

 zontal angle ; and it is easy to 

 prove that double the refraction is 

 equal to the horizontal angle, in- 

 creased by the difference between 

 the apparent elevation and the 

 apparent depression. Whence it 

 appears that in the mean state of 

 the atmosphere, the refraction is 

 about the fourteenth part of the 

 horizontal angle. Mrs. Somerville. 



' Sir John Herschel. 



TER'TIARY. Third, a term applied to 

 those formations which have been 

 deposited subsequently to the chalk 

 formation. 



TE'RTIARY STRA'TA. A division of 

 sedimentary formations called ter- 

 tiary, as being of newer origin 

 than the secondary, and character- 

 ized by distinct species of fossil 

 animals and plants. They present 

 a most decided contrast with the 

 secondary and older strata in most 

 of their essential characters. The 

 most striking feature of these for- 

 mations consists in the repeated 

 alternations of marine deposites 

 with those of fresh water. We 

 are indebted to Cuvier and Brong- 

 niart, for the first detailed account 

 of the nature and relations of a 

 very important portion of the ter- 



