NOTES. 473 



placing a slice of rock crystal in the polarised ray rs, fig. 64. The 

 uniform colour in the interior of the image depends upon the thickness of 

 the slice ; but whatever that colour may be, it will alternately attain a 

 maximum brightness and vanish with the revolution of the glass B. It 

 may be observed, that the two kinds of quartz, or rock crystal, mentioned 

 in the text, are combined in the amethyst, which consists of alternate 

 layers of right-handed and left-handed quartz, whose planes are parallel to 

 the axis of the crystal. 



NOTE 215, p. 193. Suppose the major axis A P of an ellipse, fig. 18, 

 to be invariable, but the excentricity C S continually to diminish, the 

 ellipse would bulge more and' more; and when C S vanished, it would 

 become a circle whose diameter is A P. Again, if the excentricity were 

 continually to increase, the ellipse would be more and more flattened till 

 C S was equal to C P, when it would become a straight line A P. The 

 circle and straight line are therefore the limits of the ellipse. 



NOTE 216, p. 194. The coloured rings are produced by the interference 

 of two polarised rays in different states of undulation, on the principle 

 explained for common light. 



NOTE 217, p. 225. According to Mr. Joule, that heat is produced 

 by motion, and that it is equivalent to it, Mr. Thompson of Glasgow 

 investigates from whence the sun derives his heat, since he shows that 

 neither combustion nor his primitive heat could have supplied the waste 

 during 6000 years. He concludes that the solar heat is maintained by 

 myriads of minute bodies that are revolving at the edge of his dense 

 nebulosity or atmosphere, some of which are often seen by us as falling 

 stars. These, vaporized by his heat, and drawn by his attraction, meet 

 with intense resistance on entering the solar atmosphere as a shower of 

 meteoric rain ; through it they descend in spiral lines to the sun's surface, 

 producing enormous heat by friction during their fall, and serving for fuel 

 on their arrival. 



NOTE 218, p. 252. The class Cryptogamia contains the ferns, mosses, 

 funguses, and sea-weeds ; in all of which the parts of the flowers are in 

 general too minute to be evident. 



NOTE 219, p. 254. Zoophytes are the animals which form madrepores, 

 corals, sponges, &c. 



NOTE 220, p. 254. The Saurian tribe are creatures of the crocodile 

 and lizard kind. 



NOTE 221, p. 266. If heat from a non-luminous source be polarised by 

 reflection or refraction at r, fig. 64, the polarised ray rs will be stopped 

 or transmitted by a plate of mica M I, under the same circumstances that 

 it would stop or transmit light ; and if heat were visible, images analogous 

 to those of figs. 65, 67, &c., would be seen at the point s. 



NOTE 222, pp. 275, 329, 357. The foot-pound, or unit of mechanical 

 force established by Mr. Joule, is the force that would raise one pound weight 

 of matter to the height of one foot ; or it is the impetus or force generated 

 by a body of one pound weight falling by its gravitation through the 

 height of one foot. 



