622 SCIENCE PROGRESS 



similar panels are to be made, in which case costly machinery 

 can be installed for the purpose. The surface of a well-made 

 aluminium panel is also better than that of one made of steel, 

 whilst wood, owing to the shrinkage which it undergoes, the 

 amount of paint it absorbs and the difficulty of working it, is no 

 longer used for motor-car work. 



The advantages accruing from the use of aluminium for the 

 purposes mentioned are not sufficiently marked, however, to 

 induce makers to employ it unless the price of the metal be 

 very low. It has been seen already that when the price rises 

 appreciably, aluminium is discarded in favour of steel but at 

 prices obtaining at the time of writing, aluminium panels are 

 being used to a large extent. 



The case of aluminium castings for engine parts is very 

 different, as the advantages the metal has are very conspicuous 

 and be the price high or low, very little else than aluminium 

 is used. In the first instance, the saving of weight is very 

 considerable, as such castings are of necessity bulky and if 

 made in gun-metal or iron would burden the machinery to an 

 excessive extent. The saving of cost is also very large indeed. 



Pure aluminium is not used for this class of casting. When 

 unalloyed, aluminium does not run at all well and in conse- 

 quence small passages in the mould may not be well filled. 

 Moreover, it often happens that portions of the molten metal 

 which meet in the interior of the mould do not unite owing 

 to the skin of oxide which covers their surfaces. Aluminium 

 itself also lacks the necessary rigidity and the shrinkage of 

 the metal on solidification (r8 per cent.) makes the production 

 of sound castings difficult. Recourse is usually had to alloys 

 containing about 10 to 12 per cent, of zinc and 2 to 3 per 

 cent, of copper. These alloys have the properties which alu- 

 minium itself lacks and are more suitable even than other 

 metals for the production of castings of intricate pattern. If 

 the percentage of zinc be increased to excess, the castings are 

 apt to break when exposed to continual vibration. In earlier 

 days great trouble was experienced on this account but when 

 the enormous number of castings in daily use is borne in 

 mind, the number of breakages now occurring must be considered 

 trifling. 



