262 THE POPULAR SCIENCE MONTHLY. 



be what is called crystalline fracture. These crystals are, in fact, 

 everywhere. If you break a sugar-loaf, you find the surface of fract- 

 ure to be composed of small, shining, crystalline surfaces. In the 

 fracture of cast-iron you notice the same thing ; and next to his great 

 object of squeezing out the entangled gas from his molten metal, 

 another object of your celebrated townsman, Sir Joseph Whitworth, 

 when he subsequently kneads his masses of white-hot iron as if they 

 were so much dough, is to abolish this crystalline structure. The 

 shining surfaces observed in the case of crystalline fracture are sur- 

 faces of weak cohesion; and, when you come to examine large and 

 well-developed crystals, you soon learn why they are so. I try the 

 crystal of sugar referred to at the beginning of this lecture in various 

 directions with the edge of my knife, and find it obdurate ; but I at 

 length come upon a direction in which it splits clearly before the 

 knife, revealing two shining surfaces of cleavage. Such surfaces are 

 seen when you break cast-iron, and the metal is strengthened by their 

 abolition. Other crystals split far more easily than the sugar. 



In the course of scientific investigation, then, as I have tried to 

 impress upon you, we make continual incursions from a physical world 

 where we observe facts, into a super or sub-physical world, where the 

 facts elude all observation, and we are thrown back upon the picturing 

 power of the mind. By the agreement or disagreement of our picture 

 with subsequent observation, it must stand or fall. If it represent a 

 reality, it abides with us ; if not, it fades like an unfixed photograph 

 in the presence of subsequent light. Let me illustrate this. You 

 know how very easy it is to cleave slate-rock. You know that Snow- 

 don, Honister Crag, and other hills of Wales and Cumberland, may be 

 thus cloven from crown to base. How was the cleavage produced ? 

 By simple bedding or stratification, you may answer. But the answer 

 would not be correct ; for, as Henslow and Sedgwick showed, the 

 cleavage often cuts the bedding at a high angle. Well, here, as in 

 other cases, the mind, endeavoring to find a cause, passed from the 

 world of fact to the world of imagination, and it was assumed that 

 slaty cleavage, like crystalline cleavage, was produced by polar forces. 

 And, indeed, an interesting experiment of Mr. Justice Grove could be 

 called upon to support this view. I have here, in a cylinder with glass 

 ends, a fine magnetic mud, consisting of small particles of oxide of 

 iron suspended in water. You can render those suspended particles 

 polar by sending round the cylinder an electric current ; and their 

 subsequent action may be rendered evident. At present they are 

 promiscuously strewed in the liquid. But the moment the current 

 passes they all set their lengths parallel to a common direction. Be- 

 fore the current passes, the strongest beam of light can hardly strug- 

 gle through the turbid medium. But, the moment it passes, light is 

 seen to flash out upon the screen. Now, if you imagine the mud of 

 slate-rocks to have been thus acted on, so as to place its particles with 



