200 



Aggregation may also take place between portions of a 

 body merely softened by a solvent, which is afterwards with- 

 drawn, as in the familiar instance of Indian Rubber, softened 

 by naptha for the manufacture of waterproof cloths ; where 

 the former, after being moulded or united in any way re- 

 quired, is left in its pristine condition by the evaporation of 

 the naptha from amongst its particles. But the cases of 

 aggregation of solids, without such elevation of temperature, 

 or the presence of solvents, are so rare, that but two or three 

 have as yet been observed. Of these the most remarkable 

 is that recorded by Pouillet, of the gradual, but complete, 

 adhesion of surfaces of clean plate-glass, when left to re- 

 pose on each other for a considerable time. It has also been 

 stated, that clean plates of lead or of tin, if pressed together 

 by a considerable force when cold, require a proportionably 

 great force to separate them. The case presented to the 

 Academy, therefore, is another added to these rare instances 

 of molecular aggregation in solids, independent of solution 

 or fusion : the author therefore thought it worth while to 

 examine with a little care the properties both of the original 

 brass, and of the mass thus curiously formed from it, or, as 

 he thenceforth called them, o^ the normal and the anomalous 

 alloy. 



The normal alloy is of a bright gold colour, and sub-cryscal- 

 line in structure, and of great toughness ; its cohesive force is 

 equal to 21.8 tons per square inch, which is above the average 

 strength of any of the alloys of copper and zinc, or copper 

 and tin, as found by my experiments on the cohesive power 

 of these alloys, published in the Proceedings of the Academy, 

 and elsewhere. The cohesive force of the anomalous alloy 

 is only 1.43 tons per square inch, or only about one-fifteenth 

 that of the former. 



The specific gravity of the normal alloy is = 8.600 ; that 

 of the anomalous only z: 7.581. 



