Frederick Guthrie on the Fracture of Colloids. 25 



were differently hard in opposite directions, so that they were 

 more easily scratched in the sense AB than BA ; and if they 

 possess one such unilateral property, they must surely have 

 others. One surely ought to expect that a current driven 

 through a pyroelectric crystal from analogous to antilogous 

 pole would heat it, and that a current in the reverse direction 

 would cool it, or heat it less. If, however, as I begin to fear, 

 this is a wrong scent, I should be very grateful to any one 

 who will kindly point the fact out. — Oliver J. Lodge. 



III. On the Fracture of Colloids. By Frederick Guthrie*. 



[Plates II. & ILL] 



§ 1. A PROMINENT property with regard to solid colloids 

 -£^- is that they have neither crystalline form nor planes 

 of cleavage. When such a body is broken it offers the so- 

 called conchoidal fracture. An agglomeration of crystals 

 may present in mass the conchoidal fracture usually associated 

 with colloids. This is the case with granite, and eminently 

 so with basalt, all of whose constituents are crystalline. 

 When the solid has resulted from the intersolution of two or 

 more crystalloids it may, like glass, present the colloidal 

 fracture in a most marked manner. And, indeed, even single 

 crystals themselves are often subcolloidal in fracture ; that is, 

 conchoidal fracture accompanies the crystalline. This state 

 is shown by the diamond, sugar-candy, quartz, &c. 



I assume here that every cohesionally homogeneous mass 

 of solid matter will break conchoidally when subjected to 

 pressure sufficient to cause fracture. 



Experiments. 



§ 2. The cracking of a glass plate by pressure offers no 

 special features of interest. A round plate placed on a thick 

 soft cloth and pressed in the centre by a round cork cracks 

 radially ; the cracks are generally slightly curved. Fig. 1 

 shows two examples of fracture of crown glass by pressure in 

 the centre. Similarly, if a round sheet of glass placed on a 

 thick soft cloth be pressed down at its circumference by means 

 of cardboard rings, the same class of crack is produced ; for, 

 indeed, the two conditions are essentially identical. 



§ 3. The internal strain caused by difference of temperature 

 causes fracture of great regularity and beauty. It rarely 

 happens that a sheet of glass of any shape breaks into only 

 two pieces when heated. If a circular piece of " crown " 



* Read before the Physical Society, March 22, 1879. 



