268 Transactions. 



The energy required to break down and completely overcome the 

 cohesion of a given mass of solid should be the same in amount whether 

 that breaking-down be accomplished by crushing, by solution, or by fusion 

 and volatilisation. 



Measurements are made of these three kinds of change in solids in various 

 ways, the more important of which are as follows : — 



Case 1 : Mechanical Rupture. — This is measured by — (a) tensile strength ; 

 (6) crushing strength ; (c) hardness, &c. 



In isotropic solids the force to be overcome is the same for all of these 

 methods, and the results expressed in suitable terms should be comparable. 

 But most solids are not isotropic ; mica, selenite, and asbestos may be cited 

 as extreme , cases where the strength in different directions is totally dif- 

 ferent. In all such cases different methods of measurement may give quite 

 different values for the mechanical strength. For present purposes the 

 determination of hardness, although by no means precise or exact, is 

 perhaps the most useful. It is very easy to measure roughly, so that where 

 the hardness differs much in different directions the maximum value is 

 easily found. 



Case 2 : Solution in a Solvent. — This method of breakmg down a solid is 

 measured usually in terms of the weight of solid which dissolves in a given 

 weight of solvent at a definite temperature. In a given solvent at a given 

 temperature the solubility depends upon two principal factors — first, the 

 attractive force or adhesion between solid and solvent ; a.nd, second, the 

 cohesion of the solid. Assuming the first to be nearly constant, the solubility 

 should increase as the cohesion and the mechanical strength become less. 



Case 3 : Fusion and Volatilisation. — This, the third method given for 

 separating molecules, is a question of two opposing forces — molecular 

 cohesion versus molecular velocity. When the average kinetic energy of 

 the moving molecules is greater than the energy of cohesion, the molecules 

 escape from each other and volatilise. With any given solid the energy of 

 the moving molecule increases with the temperature proportionately, while 

 the cohesion remains the same. Hence the greater the cohesion and 

 mechanical strength, the higher the temperature required for volatilisation, 

 or fusion and volatihsation. 



In the absence of suitable quantitative measurements, which, as already 

 seen, are particularly difficult to get in the case of mechanical strengths, no 

 exact correspondence can at present be shown between strength, solubility, 

 and volatility or fusibility in solids, but evidence enough is available to show 

 distinctly that some such correspondence exists. 



In a chemical laboratory hundreds of artificially prepared pure solids are 

 met with. Owing to the limitations of manufacturing processes, these are 

 almost entirely confined to compounds readily soluble, or fusible, or volatile, 

 and in every case the crystals are mechanically soft and weak — in fact, 

 softness and weakness are the most striking characteristics of artificially 

 prepared crystals. Quite recently the high temperatures of the electric arc 

 have come into commercial use, and one of the most notable results is the 

 formation of new and difficultly fusible crystals, such as carborundum, 

 which is little short of a diamond in hardness. 



A few more definite examples may be given. If we arrange six of the 

 more common salts of calciuni in order of increasing hardness, thus — chloride, 

 carbonate, sulphate, phosphate, fluoride, silicate — it will be found that this 

 is also the order of increasing insolubilitv. 



