106 Prof. A. M. Mayer on the Laws of the 



crystallizes in two forms, differing in density — viz. as calc spar, 

 with a specific gravity of 2*72, and as arragonite, with a 

 specific gravity of 2*93. Configuration 6 a may stand for the 

 molecular structure of calc spar, while 6 b may stand for that 

 of arragonite. The relative densities of these two configura- 

 tions are as 208 to 247. 



A striking example of isomerism is given in titanic acid, 

 which crystallizes in three distinct forms : — as anatase, specific 

 gravity 3*82 ; as Brookite, specific gravity 4*02 ; and as rutile, 

 specific gravity 4*25. These three isomers may be illustrated 

 by 8 c, 85, and 8 a, which have respectively the densities of 

 382, 364, and 360. 



It will, of course, be understood that the above parallelisms 

 are given merely as illustrations of how our experiments may 

 serve to explain and illustrate the phenomena, on the assump- 

 tion of the atomic hypothesis, and on the supposition that the 

 actions which, in the experiments, take place in a plane, may 

 similarly take place among repelling and attracting points 

 situate in space of three dimensions. 



Other forms of the Experiments. — Instead of floating the 

 magnets, they may be suspended by fine silk fibres. In this 

 method of experimenting the attractive action of the super- 

 posed magnet is replaced by the action of gravity, which 

 draws the mutually repellant needles towards the vertical. 



An advantage of this form of the experiment is that the 

 configuration can be transported, and may thus serve in illus- 

 tration of a moving molecule as set forth in the kinetic 

 theory of gases. It is interesting to watch the mutual actions 

 of two or more approaching configurations, and to observe 

 the motions in the exterior and in the contour of a suspended 

 configuration on its impact against a resisting or a yielding 

 surface. 



Professor 0. N. Rood suggested to me to replace the sus- 

 pended magnets by gilded pith balls, hung by silk fibres and 

 similarly electrified. 



Professor Frederick Guthrie, under date of May 21, 

 writes : — " If the corks are made somewhat wider than in your 

 larger needles, the needles move and arrange themselves very 

 quickly if they are turned over and floated on perfectly 

 pure and freshly filtered mercury. Those wjbich reach the 

 edge incline with their corks in the capillary trough." 



Method of projecting the magnified Images of the experiments 

 on a Screen. — To exhibit these experiments before a large 

 audience it is best to use short magnets made as follows : — 

 Magnetize rather large sewing-needles, with their points all of 

 the same polarity ; then take each needle between the flat jaws 



