iv INTRODUCTION TO MECHANICS. 



closely united, the cohesive attraction of solid bodies is much greater 

 than that of fluids. 



The thinner and lighter a fluid is, the less is the cohesive attraction of 

 its particles, because they are further apart ; and in elastic fluids, such as 

 air, there is no cohesive attraction whatever. Air, however, is of the 

 same nature as other bodies in all its essential properties ; nor is it 

 probable that the particles of air are destitute of the power of attraction, 

 but they are too far distant from each other to be influenced by it ; and 

 the utmost efforts of human art have proved hitherto ineffectual in the 

 attempt to compress them, so as to bring them within the sphere of each 

 other's attraction, and make them cohere. 



It is owing to the different degrees of attraction of different substances, 

 that they are hard or soft ; and that liquids are thick or thin. This very 

 frequently, especially in bodies of the same nature, corresponds with what 

 we express by the term density, which denotes the degree of closeness 

 and compactness of the particles of a body : in these cases, whether in 

 solids or liquids, the stronger the cohesive attraction, the greater is the 

 density of the body. In philosophical language, however, density is said 

 to be^ that property of bodies by which they contain a certain quantity of 

 matter, under a certain bulk or magnitude. Rarity, though opposed to 

 density, as it denotes the thinness and subtlety of bodies, will admit 

 of the same definition ; for it implies merely a diminution of density : 

 thus we should say that mercury or quicksilver was a very dense fluid ; 

 ether, a very rare one, &c. 



We judge by the weight of the quantity of matter contained in a 

 certain bulk, and bodies of the same bulk are said to be dense in pro- 

 portion as they are heavy. Thus we say that metals are dense bodies, 

 wood comparatively a rare one, &c. But it may be objected, that when the 

 particles of a body are so near as to attract each other, the effect of this 

 power must increase as they are brought by it closer together : so that 

 one would suppose' the body would gradually augment in density, till it 

 was impossible for its particles to be more closely united. Now, we 

 know that this is not the case ; for soft bodies, such as cork, sponge, or 

 butter, never become, in consequence of .the increasing attraction of their 

 particles, as hard as iron. The answer is, that in such ^bodies as cork 

 and sponge, the particles which come in contact are so few as to produce 

 but a slight degree of cohesion : they are porous bodies, which, owing to 

 their peculiar arrangement, abound with interstices which separate the 

 particles ; and these vacancies are filled with air, the spring or elasticity 

 of which prevents the closer union of the parts. But there is another 

 fluid much more subtle than air, which pervades all bodies ; this is heat 

 Heat insinuates itself p 'more or less between the particles of bodies, and 

 forces them asunder; it may therefore be considered as constantly acting 

 in opposition to the attraction of cohesion, the one endeavouring to rend 

 a body to pieces, the other to keep its parts firmly united. 



The more a body is heated, then, the more its particles will be sepa- 

 rated ; consequently bodies generally swell or dilate by heat : this effect 

 is very sensible in butter, for instance, which expands by the application 

 of heat, till at length the attraction of cohesion is so far diminished that 

 the particles separate, and the butter becomes liquid. A similar effect 

 is produced by heat on metals, and all bodies susceptible of being melted. 

 Liquids are made to boil by the application of heat ; the attraction of 

 cohesion, then, yields entirely to the expansive power ; the particles are 

 totally separated, and converted into steam or vapour. But the agency 



