140 ATTRACTION. 



are examples of union where the minutest parts are of imperceptible 

 magnitude. 



Adhesion.* Two plates of glass or two of metal, or one of glass 

 and one of metal, when moistened or oiled, adhere, with considerable 

 force ; with still more force, two leaden hemispheres made by split- 

 ting a bullet, and pressing the surfaces together with a wringing or 

 twisting motion. If furnished with hooks, the parts of the bullet may 

 be suspended, and will support a considerable weight that may be 

 gradually increased for some time, before the hemispheres will part.f 



(b.) The cohesion of homogeneous^ particles is often termed aggre- 

 gation, and masses made up in that manner are said to be aggregates. 



(c.) The word adhesion may be used to denote the union between 

 surfaces of perceptible magnitude, whether similar or dissimilar in 

 their nature. 



(d.) Cohesion produces augmentation of volume, and frequent- 

 ly a change inform, but no change in properties. The dust of mar- 

 ble is the same substance with the stratum or mountain of marble 

 which afforded it ; it contains the same elements, and in the same 

 proportions. The elements are united by affinity or chemical attrac- 

 tion ; the compound particles produced by the union of the elements, 

 are united by cohesion. 



(e.) Mhesion of surfaces of perceptible extent produces no change 

 in properties. Generally the union of such surfaces is feeble. That 

 particular mode of corpuscular union which is called cohesion, is the 

 source of the different strength of materials, as of lead, iron, wood, &ic. 



(/.) The attraction which produces the union of particles is often 

 called corpuscular attraction. It is quite immaterial whether the par- 

 ticles be simple, as those of single metals, or compound as those of 

 metallic alloys or wood ; in either case, the state of the body results 

 from the union of minute particles, which are for this purpose regard- 

 ed as mechanically simple, whether chemically so, or noU 



The union of dissimilar particles, as will be hereafter seen, is re- 

 ferred to chemical action. Chemical union may first connect dis- 

 similar particles, as zinc and copper ; and the compound, which is in 

 that case called brass, is composed of panicles, that are regarded as 

 mechanically simple, and are called integrant particles ; while the 

 others are called constituent particles. 



* Jldhesion is merely a word of convenience ; the power that unites surfaces of 

 perceptible magnitude, and that which unites particles in aggregation, is doubtless 

 the same. 



t This effect evidently depends, in part, upon the furrows on the surface of the lead 

 which are brought into close contact by the twist that is given in pressing them to- 

 gether, with a screwing motion ; when polished, it is difficult to make thorn adhere. 



I Heterogeneous particles will also unite, but the result is not an aggregate ; it is 

 a new body, whose particles are connected not by mechanical but by chemical at- 

 traction. 



