CRYSTALLINE AND MOLECULAR FORCES. 261 



cellent observer, but he is no philosopher, and can never reach those 

 principles which bind the facts of science together. True, the specu- 

 lative faculty may be abused like all good things, but it is not men 

 of science that are most likely to abuse it. When he accounted for 

 the heat of chemical combination by referring it to the clash of atoms 

 falling together, a townsman of your own described an image pre- 

 sented to his mind but entirely beyond the reach of his senses. It 

 was, however, an image out of which grew memorable consequences ; 

 among others this one of a personal nature : The walls of this Free- 

 Trade Hall, or rather its predecessor, have rung with the speeches of 

 Cobden, and Bright, and Wilson. But, at the time when their words 

 rolled round the world, a scientific worker was silently and studiously 

 engaged in your city grappling with the problem how out of heat is 

 extracted mechanical force, and by implication with far higher prob- 

 lems. He grappled with it successfully, bringing it into the full light 

 of experimental demonstration. And I venture to affirm that in the 

 coming time, not even the great orators and politicians just named, 

 not even the greatest of your manufacturing princes, will enjoy a 

 purer, a more permanent or enviable fame there is not a man among 

 them of whom Manchester will be more justly proud than of her mod- 

 est brewer, but renowned scientific worker, James Prescott Joule. 



You will pardon this momentary deflection from my subject. We 

 have now to track still further the growth of our notions of force. 

 We have learned that magnetism is a polar force ; and experience also 

 hints that a force of this kind may exert a certain structural power. 

 It is known, for example, that iron- filings strewed round a magnet ar- 

 range themselves in definite lines, called, by some, "magnetic curves," 

 and, by Faraday, " lines of magnetic force." In these observed results 

 of magnetic polarity we find the material for speculation, in an ap- 

 parently distant field. You can readily make an experiment or two 

 for yourselves with any magnet. My excellent assistant, Mr. Cottrell, 

 places two magnets before me, and over them a sheet of paper. Scat- 

 tering iron-filings over the paper and tapping it, the filings arrange 

 themselves in a singular manner. There is a polar force here in action, 

 and every particle of iron on the paper responds to that polar force, 

 and the consequence is, a certain structural arrangement if I njay 

 use the term of the iron-filings. Here is a fact of experience which, 

 as you will see immediately, furnishes further material for the mind to 

 operate upon, rendering it possible to attain intellectual repose and 

 satisfaction while speculating upon apparently remote phenomena. 



You cannot enter a quarry and scrutinize the texture of the rocks 

 without seeing that it is not perfectly homogeneous. If the quarry be 

 of granite, you find the rocks to be an agglomeration of crystals, of 

 quartz, mica, and felspar. If the rocks be sedimentary, you find them, 

 for the most part, composed of crystalline particles derived from older 

 rocks. If the quarry be marble, you find the fracture of the rocks to 



