REFLECTION AND REFRACTION. 433 



in every portion of our globe, and will some day be proved to be the main- 

 spring of all life. 



Fracture in minerals is also to be noticed. Those substances which 

 we cannot laminate we arc obliged to break, and we may require to break 

 a mineral in a direction different from or opposed to its direction of 

 cleavage. Under such circumstances we must break it, disintegrate it, and 

 observe the fracture. Sometimes we shall find the surfaces very even, or 

 uneven, or what is -termed conchoidal. This is observable in the breaking 

 of flint. There are various ways in which minerals display fracture, and 

 the particular manner and appearance denotes the class to which the mineral 

 belongs. 



We may pass over the question of the specific gravity of minerals, as 

 we have in a former part explained this. It is important, however, to 

 ascertain the specific gravity. As a general rule, minerals containing heavy 

 metals are of high specific gravity. 



But the relation of minerals (crystals) with regard to light is of great 

 interest and importance. When we were writing of polarization, we men- 

 tioned the faculty a crystal has for double refraction, by which it divides a 

 ray of light into two prolonged rays taking different directions, the plane of 

 vibration of one being at right angles to that of the other. This property 

 is not possessed by all crystals. Some act as ordinary transparent media. 

 Some crystals transmit only one polarized ray, and tourmaline is called a 

 polarizer ; and if light be passed through it to another polarizer, it will be 

 transmitted if the latter be similarly held ; but if the second be held at right 

 angles to it the ray will be stopped. We can easily understand this if we 

 suppose a grating through which a strip of tin is passed ; but the strip will 

 be stopped by bars at right angles to it. The coloured rings in crystals 

 can be observed when a slice of a double refracting crystal is examined. 

 The rings are seen surrounding a black cross in some instances, and a white 

 cross in another. The effect when examined in the polariscope is very 

 beautiful. Selenite is probably the best crystal for exhibiting colours. 



Minerals sometimes reflect, sometimes refract light ; they are said to 

 possess lustre and phosphorescence. All these properties may be considered 

 as belonging to the crystals which are transparent, semi-transparent, trans- 

 lucent, or opaque, according to the degrees in which they permit light to 

 pass through them. All minerals are electric or non-electric, and the variety 

 can be ascertained by rubbing and placing the mineral near the electrometer. 

 But all do not exhibit magnetic properties. Taste and smell are strongly 

 marked in some minerals salts, for instance, and sulphur ; some are soapy to 

 the touch, some appear cold to the fingers. Chemistry is very useful to us in 

 determining the nature of the mineral, and the amount of it enclosed in the sub- 

 stance under examination. These delicate operations are termed qualitative 

 and quantitative analysis. The application of heat is increased by means of 

 the blowpipe, which is in effect a small bellows. We can thus, and particularly 

 by means of the oxy-hydrogen blowpipe, obtain a very intense heat with little 



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