MOTION AND LOCOMOTION 



83 



ter. Thus, one of the long bones of an ox, after being 

 thoroughly dried, yields about 60 per cent, of calcium 

 phosphate and about 10 per cent, of other inorganic 

 salts, while the remaining 30 per cent, consists of 

 combustible nitrogenous organic matter. On the 

 other hand, a piece of perfectly dry fir wood yields 

 on an average only about 2 per cent, of its weight 

 of incombustible ash, consisting mainly of salts of 

 calcium, potassium and sodium, while 

 all the remainder is composed chiefly 

 of compounds of carbon, hydrogen, 

 oxygen and nitrogen. 



Again, as to structure, we find that 

 wood consists of overlapping, spindle- 

 shaped fibres (Fig. 35), while bone con- 

 sists of "concentric lamellae surrounding 

 central spaces containing nerves, blood- 

 vessels, &c., the lamellae being, so to 

 speak, nailed together by fibres (Fig. 

 36). 



Taking these two series of facts into 

 account, let us next inquire whether FlG - 36 - 

 bone and wood form good building tudinal section 

 . , . ; . ^ of bone. (xoO.) 



materials Irom an engineering point ot 



view, and for that purpose let us contrast them, with 

 cast-iron and steel. Obviously, a good all-round 

 building material should be able to withstand equally 

 well a crushing force and a tearing force. From the 

 following table it will be seen at once that a bar of 

 cast-iron can withstand a crushing force extremely 

 well, but that it is very liable to snap if subjected 

 to a bending or tearing one. Steel, on the other 

 hand, can withstand both tearing and crushing 

 forces absolutely and relatively better than cast- 

 iron, hence its constant use as a material for 



Structure. 



