ESSENTIAL AND PECULIAR. 



59 



solid bodies, though the proofs are not so common ; since, as I have just ob- 

 served, the particles of solid bodies have less power of movement, and, con- 

 sequently, of adaptation to each other, than those of liquids. Thus, two 

 plates of lead, whose opposite surfaces correspond so exactly that every par- 

 ticle of each surface shall have a bearing- upon the particle opposed to it, when 

 once united by pressure, assisted by a little friction, cohere so powerfully as 

 to require a very considerable force to separate them. And it maybe shown, 

 either by measuring this force, or by suspending- the lead in the vacuum of an 

 air-pump, that the pressure of the atmosphere is not materially concerned 

 in producing- this effect. A cohesion of this kind is sometimes of practical 

 utility in the arts; little ornaments of laminated silver remaining attached to 

 iron or steel, with which they have been made to connect themselves by the 

 powerful pressure of a blow, so as to form one mass with it. And it is now 

 a well-known fact, and of a most curious nature, that one of the causes by 

 which eight-day clocks go at times irregularly, and monthly clocks, whose 

 weights are much larger and heavier, often amounting to not less than thirty 

 pounds, stop suddenly, proceeds from the attraction which takes place between 

 their leaden weights and the leaden ball of the pendulum, when the weights 

 have descended just so low as to be on a level, and, consequently, very nearly 

 in a state of contact, with the pendulum-ball. And hence the reason why 

 both these kinds of clocks, if the pendulum have not actually stopped, seem 

 gradually, a few days afterward, to recover their former accuracy; the 

 attraction diminishing as the distance once more increases.* In like manner, 

 Studor remarks that beams of steel become sometimes erroneous by acquiring 

 magnetic polarity.f 



It is by the same means that the greater number of rocks seem to be pro- 

 duced that enter into the substance of the earth's solid crust. The lower- 

 most of these, as I shall have occasion to observe in an ensuing lecture, are 

 united by an intimate crystallization, which is the most perfect form of aggre- 

 gate or homogeneous attraction that can exist between solid bodies, and 

 which must have commenced while such bodies were in a fluid state. 

 Some of the upper kinds or families are united by a particular cement, 

 which is nothing more than a substance possessing a peculiar attrac- 

 tion, or, if I may be allowed the expression, physical partiality to the rudi- 

 mental corpuscles of which the rock consists ; and others by nothing more 

 than the law of aggregation or homogeneous attraction in its simplest state ; 

 whence earths unite to earths in consequence of mutual approximation, 

 assisted by theii own or a superincumbent pressure, in the same manner as 

 1 have just stated that plates of lead or other metals unite to metals. 



II. But there are substances that are unlike in their nature, as solids and 

 fluids, for instance, that under particular circumstances are often found to 

 exhibit a mutual attraction; whence this mode of union is called heteroge- 

 neous attraction, and from its occurring most p-alpably between liquids and 

 solid substances possessing small capillary or hair-tubes, capillary at- 

 traction. 



The cause of this attraction is obvious ; and it. is still more clearly a mere 

 modification of the general attraction of gravitation, than the preceding 

 power of homogeneous attraction. It is the common attractive property of ma- 

 terial substance for material substance ; the liquid, or that whose particles are 

 easily separable, pressing toward the solid, whose parts'are by any action of their 

 own altogether inseparable. Hence the reason why water or any other liquid 

 hangs about the sides of a wine-glass: hence, partly, the reason why a wine- 

 glass, when somewhat more than brim-full of a liquid, does not overflow ; the 

 co-operative reason being, as I have already stated, the homogeneous attraction 

 of the corpuscles of the fluid for each other, which prevents them from sepa- 

 rating readily : and hence also the reason why a liquid contained in a narrow- 

 necked and inverted phial does not obey the common attraction of gravita- 

 tion, and fall to the earth, although the stopper be removed to allow it, till we 



Raid, in Nicholson's Journal, vol. xxxiii. p. 92. 



t Gilb. xiii. 124. Young's Nat. PLiL u. 130. 



