14-2 Royal Irish Academy. 



It frequently happens, notwithstanding the polish of both metallic 

 surfaces, and the application of oil, that the friction due to their 

 rapid passage over each other, while exposed to undue or irregular 

 pressure, produces a considerable rise of temperature, and the brass 

 becomes abraded. Its particles have no coherence, and much re- 

 semble the " bronze powder " used by painters. 



In an instance, however, which some time since came under the 

 author's notice, a different result took place. The minute particles 

 of abraded brass were by the motion of the shaft, during a few hours, 

 impacted into a cavity, at the junction of the two semicylinders of 

 the bearing, where they became again a coherent mass, and when 

 removed presented all the external appearance of an ingot or piece 

 of brass which had been poured in a state of fusion into the cavity. 

 On more minute examination, however, the mass was found to differ 

 much in properties from the original brass, out of which it was 

 formed. 



The mass or ingot of brass, thus formed by the union of particles 

 at a temperature which had never reached that of boiling water, and 

 a fragment of which was presented, possessed on that side which had 

 been in contact with the shaft, a bright polished metallic surface, 

 like that of the original metal from which it had been formed : its 

 other surfaces bore the impress of the cavity in which it was found. 

 It was hard, coherent, and could be filed or polished like ordinary 

 brass. It was, however, perfectly brittle ; and when broken, the 

 fracture, in place of possessing a sub-crystalline structure and me- 

 tallic lustre, like that of the normal brass or alloy, was nearly black, 

 and of a fine grained earthy character, and without any trace of 

 metallic lustre or appearance. 



Examined with a lens, some very minute pores or cavities are 

 found throughout its substance, which is uniformly of a very dark 

 brown or nearly black colour, and devoid of all metallic character, 

 except when cut or filed — that is, in minerological language, its co- 

 lour is earthy black, and its streak metallic. 



The author remarked that the observed cases of aggregation in 

 solid particles, without the intervention either of a solvent or of 

 fusion, are extremely rare, and as bearing upon the little understood 

 subject of cohesive attraction, are of much interest. 



The property of welding, which is possessed by all bodies, whether 

 metallic or not, which pass through an intermediate stage of soft- 

 ness or pastyness previous to fusion, and is not found in any sub- 

 stance which readily crystallizes, and hence passes per saltum from 

 the solid to the liquid state by heat, forms a " frontier instance " of 

 cohesive forces, being enabled to act in the aggregation of bodies, 

 by only an approach to liquidity, or by a very small degree of inter- 

 mobility. 



Aggregation may also take place between portions of a body 

 merely softened by a solvent, which is afterwards withdrawn, as in 

 the familiar instance of Indian rubber, softened by naphtha for the 

 manufacture of waterproof cloths ; where the former, after being 

 moulded or united in any way required, is left in its pristine condi- 



