FRE 



FRI 



small extent ; conchoidal, when one of the 

 separated surfaces is concave, the other 

 convex ; splintery, when the surface pre- 

 sents the appearance of thin-edged scales ; 

 hackley, when covered with very sharp 

 protruding points. 



FRA'GMENTARY ROCKS. Rocks 

 formed oi fragments of other rocks, gene- 

 rally of sufficient size to admit of being 

 distinguished into different kinds. When 

 the fragments are very small, they form 

 sandstones and tufas; when large and 

 rounded, they constitute conglomerates ; 

 when large and angular, breccice. The 

 fragments are commonly cemented to- 

 gether by an intervening substance. 



FRANGIBI'LITY. In Geology, the 

 degree of facility with which a rock 

 yields to the hammer. 



FRA'NKLINITE. A mineral re- 

 sembling oxidulous iron, occurring in 

 New Jersey, together with the red oxide 

 of zinc. 



FRAU'NHOFER'S SPECTRUM. 



Fraunhofer found that, in a perfect solar 

 spectrum, the coloured spaces are not 

 continuous, but are interrupted by dark 

 lines parallel to the slant edge of the 

 prism. These lines, which are called 

 Fraunhofer's, are more than 500 in num- 

 ber, and were employed by the discoverer 

 for accurately measuring the refraction, 

 the breadths of the individual colours, 

 and the intensity of the light of different 

 luminous bodies. 



FREE CHARGE. A term applied, in 

 experiments with the Leyden jar, or bat- 

 tery, to that portion of the induction 

 which is directed through the air to sur- 

 rounding conductors. 



FREEZING MIXTURES. The rapid 

 absorption of heat which bodies exhibit 

 in passing from the solid to the fluid 

 state, explains the operation of freezing 

 mixtures. If nitre be dissolved in water, 

 the temperature of the liquid falls above 

 16 degrees. If snow and a third of its 

 weight of common salt be mixed toge- 

 ther, they become liquid, and the tem- 

 perature of the mixture may fall to 0°. 

 These phenomena depend upon the affi- 

 nity which exists between these salts and 

 water; in order to satisfy this afiinity, 

 the bodies melt, and in so doing absorb 

 heat, which becomes latent in them, 

 from surrounding bodies. By this means, 

 considerable degrees of cold may be pro- 

 duced : the degree of cold depends on 

 the quantity of heat which passes from 

 a free to a latent state ; and this, again, 

 depends upon the quantity of the sub- 

 143 



stance liquefied, and the rapidity of the 

 liquefaction. 



FREEZING POINT. A fixed point 

 in the scale of the thermometer, at which 

 snow or ice melts. The length of the de- 

 grees depends on the system of gradu- 

 ation adopted ; in Fahrenheit's ther- 

 mometer, the freezing point is marked 

 32°. The space between the freezmg 

 and the boiling point is the fundamental 

 distance, or length of the scale. 



FRENCH WHITE. The common de- 

 signation of finely pulverized talc. 



FRIABFLITY (/no, to crumble). The 

 property by which a substance is capable 

 of being crumbled and reduced to 

 powder. 



FRICTION {frico, to rub). The act 

 of rubbing the surfaces of bodies upon 

 one another. In Mechanics, it is a hin- 

 derance of motion, and is termed a re- 

 tarding force. Friction is said to be 

 sliding, when the parts of one surface 

 move parallel with the other, as when an 

 axle moves in the nave of a wheel; and 

 rolling, when a round body turns about 

 the surface of some other body, so that 

 fresh points of both the surfaces are con- 

 tinually brought into contact with each 

 other. 



Measure of Friction. The force re- 

 quired to abrade the elevated parts of the 

 moving surfaces, or to raise them from 

 the depressions in which they had be- 

 come lodged. This is usually found by 

 trying what part of the weight of the 

 moving body must be exerted to maintain 

 its equilibrium, or to overcome the re- 

 sistance arising from this source. The 

 fraction expressing this ratio is called the 

 co-efficient of the friction. 



FRICTION WHEELS. A contriv- 

 ance for diminishing attrition by the sub- 

 stitution of a rolling motion. The ex- 

 tremities of an axle, instead of resting 

 in a cylindrical socket, are made to rest 

 on the circumference of two revolving 

 wheels, to which the friction is trans- 

 ferred, and consequently diminished in 

 the ratio of the radius of the friction 

 wheels to the radius of the axle. 



FRIESLAND GREEN. Brunswick 

 green. Ammoniaco-muriate of copper. 

 FRIGID ZONES. The two divisions, 

 or belts, of the surface of the earth, which 

 lie between the Polar Circles and the 

 Poles — one in each hemisphere. They 

 are so called from their excessive cold, 

 arising from the total absence of the sun 

 from them during a great part of the 

 year. 



