ESSENTIAL AND PECULIAK. ' . 49 



■different or heterogeneous substances in itself, may be fairly contemplated 

 as a homogeneous mass when compared with those by which it is sur- 

 rounded ; and hence, too, we see the necessity for their having at first ex- 

 isted, from some cause or other, in a fluid state ; since otherwise, the dif- 

 ferent corpuscles which enter mto their make could not have assumed 

 that symmetrical arrangement which alone gives sphericity to the total bulk. 



We have equal proofs of the same peculiar attraction existing between 

 solid bodies^ though the proofs are not so common ; since, as 1 have just 

 observed, the particles of solid bodies have less power of movement, and, 

 consequently, of adaptation to each other, than those of liquids^ Thus, two 

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

 particle 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 may be 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 de- 

 scended 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 afterwards, to recover their former accuracy ; the at- 

 traction diminishing as the distance once more increases.* In like man- 

 ner, Studor remarks that beams of steel become sometimes erroneous by 

 acquiring magnetic polarity.] 



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

 produced that enter into the substance of the earth's solid crust. The 

 lowermost of these, as I shall have occasion to observe in an ensuing lec- 

 ture, are united by an intimate crystallization, which is the most perfect 

 form of aggregate 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 pecuhar 

 attraction, or, if I may be allowed the expression, physical partiality to the 

 rudimental 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 approxima- 

 tion, assisted by their own or a superincumbent pressure, in the same man- 

 ner as I 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 



* Reid, in Nicholson's JournaJ, vol. xxxiii. p. 92, 

 t Gi!b. xiii. 124, Young's Nat. Phil. ii. 159. 



