360 THE WORLD OF LIFE 



CHAP. 



Each of the seven metals (and a few others now in 

 common use) has very special qualities which renders it use- 

 ful for certain purposes, and these have so entered into our 

 daily life that it is difficult to conceive how we should do 

 without them. Without iron and copper an effective steam- 

 engine could not have been constructed, our whole vast 

 system of machinery could never have come into existence, 

 and a totally distinct form of civilisation would have 

 developed — perhaps more on the lines of that of China and 

 Japan. Is it, we may ask, a pure accident that these metals, 

 with their special physical qualities which render them so 

 useful to us, should have existed on the earth for so many 

 millions of years for no apparent or possible use ; but 

 becoming so supremely useful when Man appeared and 

 began to rise towards civilisation ? 



But an even more striking case is that of the substances 

 which in certain combinations produce glass. Sir Henry 

 Roscoe states that silicates of the alkali metals, sodium and 

 potassium, are soluble in water and are non-crystalline ; those 

 of the alkaline earths, calcium, etc., are soluble in acid and 

 are crystalline ; but by combining these silicates of sodium 

 and calcium, or of potassium and calcium, the result is a 

 substance which is not soluble either in water or acids, and 

 which, when fused forms glass, a perfectly transparent 

 solid, not crystallised but easily cut and polished, elastic 

 within limits, and when softened by heat capable of being 

 moulded or twisted into an endless variety of forms. It can 

 also be coloured in an infinite variety of tints, while hardly 

 diminishing its transparency. 



The value of cheap glass for windows in cold or change- 

 able climates cannot be over-estimated. Without its use in 

 bottles, tubes, etc., chemistry could hardly exist ; while 

 astronomy could not have advanced beyond the stage to 

 which it had been brought by Copernicus, Tycho Brahe, 

 and Kepler. It rendered possible the microscope, the 

 telescope, and the spectroscope, three instruments without 

 which neither the starry heavens nor the myriads of life- 

 forms would have had their inner mysteries laid open 

 to us. 



One more example of a recent discovery of one of the 



