52 



OF THE PROPERTIES OF MATTER, 



may occasionally be thrown from the moon, by the projectile power of such 

 volcanoes, to such a distance as that they should never return to her surface: 

 for if the momentum be only sufficient to cause the mass ejected to proceed 

 at the rate of about 8,200 feet in the first second of time,* and in a line passing 

 through the moon and the earth, such effect would necessarily be produced; 

 since, in this case, the propelled mass would quit the centripetal power of the 

 former, and be drawn into that of the latter, and would either become a satel- 

 lite to the earth, or be precipitated to its surface, according as the rectilinear 

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

 at their first meeting together. 



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

 four pound cannon ball Avould travel from the moon's surface : since its velo- 

 city 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. Olbers 

 first, and M. la Place has since, ascribed the origin of those wonderful aero- 

 lites, or stones, that are now known to have fallen from the air at some period 

 or other in every quarter of the globe ; believing 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. 



CoHEsiBiLiTY 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 common mass. 

 It includes the three modes which have often been regarded as three distinct 

 properties, of extension, density, and impenetrability. Extension is a term as 

 applicable to space as to matter : " The extension of body," observes Mr. 

 Locke, " being nothing but the cohesion or continuity of solid, separable, 

 moveable parts ; and the extension of space the continuity of unsolid, insepa- 

 rable, and immoveable parts." Hence extension applies 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 approxi- 

 mation between the different particles of which it consists ; so that the same 

 body, when in the exercise of this property, occupies a smaller portion of 

 space than before it was called into act. Hence density cannot be a property 

 of space, the parts of which, as I have just observed, are immoveable, 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 

 cohesibility. A wedge of iron, indeed, may force its way through the solid 

 fibres of the trunk of a tree ; but it can only do this by separating 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 phenomena that seem to 

 show that certain bodies are penetrable 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 but little doubt, since, if no evaporation 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 diffe- 

 rent metals affords, not unfreqently, similar instances of equal introsusception. 



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



* La Place, Exposition du Syst^rne du Monde 



