i8 



The Realm of Nature 



CHAP. 



The density of each kind of matter is very distinctive ; that 

 of quartz, for example, is 2-6, that of the diamond 3-5 ; and 

 by means of this difference diamond buyers at once detect 

 any attempts at fraud. The term specific gravity is often 

 used to express the ratio of the density of substances to 

 that of water. 



30. Form. The form which different kinds of matter 

 assume varies greatly, and can be easily changed. Pure 

 kinds of matter, i.e. elements and compounds ( 42, 45), 

 when allowed to solidify or separate out of solution fre- 

 quently assume a shape of beautiful symmetry metallic 

 bismuth, alum, or quartz, for example and these definite 

 forms are spoken of as crystals. Mixtures, and sometimes 

 pure kinds of matter, have no special form naturally, but 

 occur as they were moulded in the cavity or vessel contain- 

 ing them, or as they were broken off from larger pieces. 

 These are often spoken of as amorphous or formless. The 

 forms of crystals are so characteristic that the minutest 

 trace of some compounds may be recognised by their 

 appearance under the microscope. 



31. Angular Measurement. In considering the form 



and position of bodies re- 

 gard must be had to the 

 properties of space, and 

 especially to the nature 

 and use of angles. An 

 angle is the inclination of 

 two lines which meet at a 

 point and may be measured 

 by a certain definite amount 

 of turning done by a line ; 

 the angle APB in Fig. 2 

 is the amount of turning 

 in a line from the position 

 PA to the position PB. 

 If the line PA were drawn 



FIG. 2. Four right angles. 



on a piece of card pivoted to a table by a pin through P, and 

 if it were made to turn completely round, as shown by the 

 arrow, until it came back to its original position, the end A 



