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



an unstable to a stable molecular condition is shown when the 

 yellow crystals of mercuric iodide change, on the touch of a 

 glass rod, to a scarlet colour, with a perceptible motion of their 

 particles. These and similar phenomena are illustrated by the 

 .change of unstable to stable configurations caused by vibration, 

 shock, and varying conditions of stress. Thus 56 changes 

 into 5 a, 66 into 6 a, and 8 c and 8 b into 8 a. 



(2) illustrations of Expansion on Solidification, as shown 

 by water, bismuth, antimony, cast iron, &c, are readily given 

 by the floating magnets. One volume of water at 0° C. ex- 

 pands, on freezing, into about one and one tenth volume of ice. 

 It happens that the area of 5 b is greater than the area of 5 a 

 by about one tenth ; so that the increase in area which takes 

 place when the pentagon of 5 a is changed into the square 5 b 

 may represent the increase in the volume of water when it 

 changes into ice. 



It will be observed, on an examination of the diagrams, that, 

 of two configurations formed of the same number of needles, 

 that configuration which has the larger area has a magnet . in 

 its centre. Thus 5 b exceeds in area 5 a, and 6 a is of greater 

 area than 6 b. To see the effect of a repulsive centre on a con- 

 figuration, compare the areas of the two squares 4 and 5 b, and 

 of the two pentagons 5 a and 6 a. The most marked effect of 

 a repulsive central magnet is seen on comparing 14 with 15. 

 The outside contour of each is formed of 9 magnets. The 

 nucleus of 14 is the peculiar flattened pentagon, which is ex- 

 panded into symmetry on the addition of another magnet, 

 while at the same time the outside contour of 15 conforms to 

 the regular pentagonal nucleus. These phenomena are so 

 suggestive, that I make bold to put the quescion, May it not 

 be that there is an actual centralization of atoms in the mole- 

 cule when a body expands in solidifying, and in the case where 

 of two or more isomeric bodies one has always the minimum 

 density ? I offer this as a suggestion which may be worthy of 

 the consideration of crystallographers. 



(3) Illustrations of Allotropy and Isomerism. — The most 

 interesting of our experiments with the floating magnets are 

 those illustrating the phenomena of allotropy and isomerism. 

 It is well known that an elementary substance may exist under 

 very different forms. By the action of heat, electricity, &c. 

 an element may have its physical and chemical properties so 

 changed that no one would suppose that the different bodies 

 thus made out of one and the same element were really all of 

 the same substance. Yet the body remains elementary under 

 the different appearances ; for it is impossible by any means of 

 subtraction to get any thing but the elementary substance 



