432 Introduction to the Study of Science 



fourths as heavy as silver, and a little more than two thirds as 

 heavy as lead ; but about two and one half times as heavy as 

 aluminum. Its relatively light weight for a unit volume makes 

 it invaluable in large, tall structures where strength and light 

 weight are necessary. 



As is well known, iron and steel are rigid. A steel frame for 

 a bridge, a high building, a machine, or the hull of a ship 

 possesses a rigidity which no other material can give. A steel 

 frame for an aeroplane is more rigid than one of wood ; and 

 rigidity of such a structure is indispensable, as the slightest 

 yielding might lead to disaster. 



Many familiar appliances show the advantage of the relatively 

 high melting point of iron. Consider its importance in such 

 appliances as ranges, stoves, furnaces, boilers, or anything to 

 which intense heat may be directly applied. Its use in fire- 

 proofing buildings is also due to this property. 



One property of iron, not mentioned above, needs special 

 consideration. This is its capacity for magnetism, which is the 

 basis of all progress in the generation and transformation of 

 electricity (page 324). Without iron the electric generator and 

 the transformer would be impossible. This magnetic property 

 of iron was recognized ages ago by such peoples as the Chinese 

 and the Greeks, who made use of it in various ways (pages 370 f .). 

 Some ores are naturally magnetic, as those in Arkansas. These 

 possess a strong attractive force for suitable objects that come 

 within their range. 



II. IRON ORE 



207. Distribution of iron. Iron is found abundantly in 

 different compounds in almost every part of the earth. In 

 smaller amounts it occurs in sand, clays, and rock.' It is 

 contained in solution in water, and is an important constituent 

 of plant and animal organisms. It is present in the hemoglobin 

 of human blood, making it possible for the blood to unite with 

 oxygen and to transport it to the bodily tissues. 



