98 SCIENCE PROGRESS 



practice. The most familiar technical examples occur amongst 

 the "white metals" used for the lining of bearings. The 

 essential qualities of such an alloy are sufficient hardness to 

 resist the rubbing action of the shaft and sufficient plasticity to 

 enable the lining to become adapted to the rubbing surface and 

 thus to correct any slight error of alignment or want of accuracy 

 in the shaping of the bearing originally present. These two 

 requirements are best met by an alloy in which primary crystals 

 of some hard material are embedded in a comparatively soft and 

 plastic ground-mass. The hard crystals are generally either of 

 antimony or of a compound ol tin and antimony, SnSb, which 

 forms very well-defined crystals of apparently cubical shape. 

 The plastic mass is an eutectic, generally, although not always, 

 containing lead as one of its components. Bearing-metals 

 usually contain more than two metals and a hard and brittle 

 compound of copper and tin is frequently present in small 

 quantities. 



A different plan is adopted in the manufacture of " plastic 

 bronzes," which also find considerable application as bearing- 

 metals. In these alloys copper hardened by the addition ot 

 either nickel or sulphur or of both forms a sponge the inter- 

 stices of which are filled with lead. Some tin is added to 

 produce partial miscibility in the liquid state but even with 

 this addition the alloy needs to be cast under specified 

 conditions to avoid separation into two layers. Crystalline 

 outlines are entirely absent from the micro-sections and the 

 structure is merely that of a meshwork of the harder metal 

 holding globules of the soft lead alloy. Such emulsion-like 

 solids are quite unmistakable when seen under the microscope. 



Non-metallic elements only enter into consideration as 

 essential structural constituents in a few cases. The most 

 familiar of these is graphite in grey cast-iron or pig-iron. The 

 graphite is seen in the form of thin plates, usually curved and 

 appearing as lines where cut by the plane of the section. The 

 size of the plates and their distribution through the iron give 

 much information as to the mechanical properties that may be 

 expected from the material. A very finely divided variety of 

 graphite is met with in malleable castings as a product of de- 

 composition of the carbide. It is often regarded as amorphous 

 carbon but has been shown to be chemically identical with 

 graphite. Phosphorus is not visible in steel or in ordinary 



