24 The Realm of Nature CHAP. 



tension, or the tendency all liquid surfaces have to become 

 as small as possible. A small portion of a liquid when 

 thrown off as a drop shrinks into a little sphere, because 

 a . sphere has the smallest surface possible containing a 

 given volume. A soap-bubble blown on the wide end 

 of a glass funnel contracts and creeps up to the narrowest 

 part of the tube when left to itself. Surface tension 

 accounts for such phenomena as the rapid spreading of a 

 film of oil over a wide surface of water, and the extra- 

 ordinary gyrations of a piece of camphor floating on clean 

 water. 



40. Analysis and Synthesis. If we wish to find out 

 for ourselves of what parts a piece of mechanism, such 

 as a watch, is composed, we must begin by unloosening 

 the parts from one another and taking the watch to pieces. 

 So when we wish to find of what parts a piece of matter, 

 such as a rock, is made up, we must unloosen its parts and 

 take it to pieces. This process is called by the Greek 

 name of analysis. There is another process sometimes 

 employed : we might imagine a watch so strongly made 

 that it could not be taken to pieces, but if we had seen 

 the parts put together to .make it, we would know of what it 

 was composed. This putting together is called synthesis, 

 and the process is sometimes used for investigating kinds 

 of matter. 



41. Mixtures. We may take a piece of granite as 

 typical of a pure kind of matter which is easily recognised 

 by its characteristic appearance. On examining it with 

 the eye we see that it is made up of three different 

 substances. One of these is clear and glassy, breaking 

 with a sharp edge, and hard enough to scratch glass. It 

 is called quartz. Another is milky and opaque, whitish 

 or pinkish in colour, too soft to scratch glass, and when it 

 is broken it splits into regular smooth -sided blocks of 

 similar shape. It is called felspar. The third ingredient 

 is silvery or black in appearance ; it forms flakes which are 

 soft enough to be scratched by the nail, and flexible, split- 

 ting up into thin transparent scales. It is called mica. 

 Granite, then, is a mixture of quartz, felspar, and mica, 



