£:SSENTIAL A2iD PECULIAK. 



41 



force of the projectile was equal or inferior to the attractive force of the 

 «arth at their first meeting together. 



Yet this is, perhaps, but little more than the velocity with which a 

 twenty-four pound cannon-ball would trkvel from the moon's surface : 

 since its velocity on the earth's surface may be calculated at about 2,000 

 feet for the first second ; and it would rush nearly four times as rapidly if 

 not impeded by the resistance of the atmosphere. And hence it is to this 

 cause that M. Oibers first, and M. Laplace has since, ascribed the origin 

 of those wonderful aeroHtes, or stones, that are now known to have fallen 

 from the air at some period or other in every quarter of the globe ; be- 

 lieving them to be in every instance volcanic productions of the moon, 

 thrown by the impulse of the explosion beyond the range of her centripetal 

 influence. 



CoMEsiBiLiTY is the tendency which one part of matter evinces to unite 

 with another part of matter, so as to form out of different bodies one com- 

 mon mass. It includes the three modes, which have often been regarded 

 as three distinct properties, of extension^ density^ and impenetrahility. 

 Extension is a term as applicable to space as to matter : " The extension 

 of body," observes Mr. Locke, '•'being nothing but the cohesion or con- 

 tinuity of solid, separable, moveable parts ; and the extension of space the 

 continuity of unsohd, inseparable, and immoveable parts." Hence exten- 

 sion appUes to all directions of matter, for its continuity may take place in 

 all directions ; but in common language the longest extension of a body is 

 called its length, the next its breadth, and the shortest its thickness. 



Density is a property in matter to cohere with a closer degree of ap 

 proximation between the different particles of which it consists ; so that the 

 same body, when in the exercise of this property, occupies a smaller por- 

 tion of space than before it was called into act. Hence density cannot be 

 a property of space, the parts of which, as 1 have just observed, are im- 

 moveable, and cannot, therefore, either approach or recede. 



Impenetrability is the result of density, as density is of extension. It 

 is that property in matter which prevents two bodies from occupying the 

 same place at the same time. They are all branches of the common 

 property of cohesibiHty. A wedge of iron, indeed, may force its way 

 through the sohd fibres of the trunk of a tree ; but it can only do this by 

 separting them from each other : it cannot penetrate the matter of which 

 those fibres consist. In like manner, when a ship is launched, her hulk 

 cannot sink into the water without displacing the exact bulk of water 

 which existed in the space that the hulk below the surface now occupies. 



To a cursory survey, however, there are some phaenomena that seem to 

 show that certain bodies are penetral)le by others. Thus, if a cubic inch 

 of water be mixed with a cubic inch of spirit of wine or sulphuric acid, 

 the bulk of the compound will be something less than two cubic inches. 

 But in this case, one of the fluids appears to admit a part of the other fluid 

 into its pores ; a fact of which there can be httle doubt, since, if no evapora- 

 tion be allowed to take place, though the bulk of the mixture is somewhat 

 diminished, its weight is precisely equal to what it ought to be. The 

 combination of different metals affords, not unfrequently, similar instances 

 of equal introsusception. 



Divisibility is a power in matter directly opposed to its eohesibihty. 

 It is that property of a body by which it is capacified for separating into 

 parts, the union or continuity of which constituted its extension. 



Divisibility, however, does not destroy cohesion in every instance 



