ESSENTIAL AND PECULIAR, 



53 



Divisibility, however, does not destroy cohesion in every instance equally ; 

 though the farther it proceeds, the farther it loosens it. We are told by Mr. 

 Boyle, that two grains and a half of silk were, on one occasion, spun into a 

 thread not less than three hundred yards long, which is, notwithstanding, a 

 much shorter length than the spider is capable of spinning his web of the 

 same weight. Muschenbroek mentions an artist of Nuremburg, who drew 

 gold wire so fine that 500 inches of it only weighed one grain ; and Dr. Wol- 

 laston has obtained platinum wire as fine as oo^h of an inch.'^ The thick- 

 ness of tin-foil is about a thousandth part of an inch ;t that of gold-leaf is 

 less than a two hundredth thousandth part of an inch ; and the gilding of lace 

 is still thinner, probably in some cases not more than a millionth part of an 

 inch; and there are living beings visible to the microscope, of which a mil- 

 lion million would not make up the bulk of a common grain of sand. Yet it 

 is highly probable, from what has actually been ascertained of the anatomy 

 of minute and miscroscopic animals, that many of these are as complicated 

 in their structure as the elephant or the whale. 



Gravitation is the common basis upon which all the preceding properties 

 are built, except passivity ; the great principle into which all the rest resolve 

 themselves. Gravitation is the attraction by which bodies of all kinds act 

 upon each other, with a force regulated by the aggregate proportion of their 

 respective quantities of matter, and decreasing as the squares of the distances 

 increase. It is a law impressed on matter universally, and hence operates 

 alike on the minutest and on the largest masses ; produces what we call 

 weight on earth, or the tendency of heavy bodies to fall towards the earth's 

 centre; and governs the revolutions of the planets. The five principles 

 which regulate its mode of action, and constitute its magnificent code of 

 laws, are thus summed up by M. la Place.| 



1. Gravitation takes place between the most minute particles of bodies. 



2. It is proportional to their masses. 



3. It is inversely as the squares of the distances. 



4. It is transmitted instantaneously from one body to another. 



5. It acts equally on bodies in a state of rest, and upon those which, 

 moving within its range, seem to be flying off from its power. 



To a casual observer there are many substances that seem to fly away from 

 the earth, and consequently to oppose this general law. Thus smoke, when 

 extricated from burning bodies, and vapour, when separated from liquids, 

 ascend into the atmosphere ; and a piece of cork, plunged to the bottom of a 

 vessel of water, rises rapidly to the surface. But, in all these phenomena, 

 the bodies that seem to move upwards merely give way to bodies of a heavier 

 kind, or, in other words, which have a stronger tendency towards the earth. 

 Thus smoke and vapour only ascend, because the surrounding air, v/hich is 

 heavier than these, presses downwards and takes their place; 'and the cork 

 rises because lighter than the water into which it has been plunged : but 

 empty the vessel, and the cork will remain at the bottom, because heavier 

 than the surrounding air ; and let the smoke or the vapour be received into a 

 vacuum, and it will remain as much at the bottom as the cork. 



It was first systematically demonstrated by Sir Isaac Newton, that all the 

 motions of all the heavenly bodies depend upon the same power ; and the 

 principle thus struck out has of later years been still more extensively and 

 even more accurately applied to a solution of the most complicated pheno- 

 mena. This principle in astronomy is denominated the centripetal force, and 

 the term is sufficiently precise for all common purposes ; since, although 

 speaking with perfect strictness, the central point of no solid substance is 

 the actual spot in which its attractive power is chiefly lodged, yet it has been 

 abundantly proved by Sir Isaac, that all the matter of a spherical body, or a 

 spherical surface, may, in generally estimating its attractive force on other 

 matter, be considered as collected in the centre of such sphere. And hence, 

 as all the celestial bodies are nearly spherical, their action on bodies at a dis- 



* Wollaston in Phil. Trans, for 1813, p. 114. Thomson's Annals of Philos. No. III. p 234. 

 t Davy's Elern. vol. i. p. 379. t Exposition du Syst<imc du Monde. 



