Page. — Apparent Relation between Physical Properties of Solids. 26y 



In the closely related compounds, sugar, starch, and cellulose, the insolu- 

 bility and strength increase together very clearly, sugar being freely soluble 

 and weak, starch semi-soluble and comparatively strong, cellulose insoluble 

 and very strong. 



In the numerous commercial glasses, all those easily fusible are soft ; 

 hardness and in fusibility increase together. Very much the same is true of 

 the resins. 



All the solid hydrocarbons are very easily melted, and all are very soft 

 and weak. 



Ordinary phosphorus is readily fusible, soluble, and very soft, while the 

 change which converts it into red phosphorus makes it difficultly fusible, 

 and at the same time insoluble and relatively hard. 



There are a few prominent cases which appear at first sight to be excep- 

 tions to the general rule. All the forms of carbon are highly infusible and 

 insoluble, but some are quite soft. The explanation appears to be that 

 the soft forms are not compact ; the particles are not close enough for 

 cohesion to have full play. Any process which makes the carbon more dense 

 and compact increases very largely its strength and hardness. This is very 

 well seen in the manufacture of the glow-lamp filament. Ordinary charcoal 

 does not seem a promising material to make into a strong, hard, elastic wire ; 

 yet by simply increasing its compactness, bringing a greater proportion of 

 the molecular particles within reach of each other, the required strength and 

 elasticity result. Similarly, retort carbon is hard and strong. The softness 

 of charcoal is due to its spongy, open character. Alumina and its important 

 compound clay are very insoluble and difficultly fusible, and yet both are 

 soft and unctuous to the touch. That this also is due to the loose, open 

 structure, to the molecules being too far away to hold each other firmly, is 

 pretty clear. Clay, when strongly heated, shrinks very much, the particles 

 come within closer reach of each other, and the mass becomes both hard and 

 strong. The change is well seen in the manufacture of bricks and pottery. 

 If the heating be very prolonged, the clay may actually crystallize, as in 

 the mineral andalusite, which is harder even than quartz. Alumina can be 

 melted in the electric arc, and then forms a compact solid, with the strength 

 and hardness of the ruby. 



Quartz is much more infusible than any glass, and after fusion is propor- 

 tionately stronger — so strong that thin crucibles of quartz can be plunged 

 into cold water while red-hot without injury. 



In practically every case, then, with resistance to solution, or fusion, 

 or volatilisation comes resistance to mechanical rupture. Hence strength 

 and hardness may be to a great extent foretold in compact solids if the 

 solubility or fusibility be known, and vice versa. 



The matter has been brought before the Institute in order to make a 

 little more clear and definite a relation long felt more or less vaguely, and 

 utiUsed more or less consciously by many practical men. 



