NATURAL PHILOSOPHY MATTER, MOTION, AND HEAT. 



are contrasted with them as solid. But experi- 

 ment and reflection lead us to the conclusion that 

 all bodies are porous. We have seen that bodies 

 are made up of indefinitely small molecules ; and 

 the fact that all bodies admit of compression and 

 expansion, makes us believe, that in no case do 

 these molecules fill the whole space occupied by 

 the body, but have interstices of greater or less 

 size between them ; so that when a body is com- 

 pressed, its molecules are only more closely 

 packed. There is nothing, then, that is not porous, 

 in this sense ; and one body is more dense or 

 solid than another, only because it is less porous. 

 Density thus means the comparative closeness of 

 the molecules of a body ; and a dense body con- 

 tains, bulk for bulk, more molecules, that is, more 

 matter, than one that is less dense, or, in other 

 .words, more porous. As weight depends upon 

 the quantity of matter, density and weight thus go 

 together. 



In comparing the densities of different sub- 

 stances, that of distilled water is taken as a 

 standard, and called I. If a cubic inch, then, of 

 any substance weigh twice as much as a cubic 

 inch of water, its comparative density is expressed 

 by 2 ; and this is generally called its specific 

 gravity. The following table exhibits the specific 

 weights of a few of the more familiar substances : 



Platinum, coined. .22-100 



Gold, coined 19-325 



Mercury 13-598 



Lead n-352 



Silver io-474 



Copper, hammered 8-878 

 Iron, wrought 7-788 



cast 



.... 7-207 

 Diamond 3 - 5 2 



Flint Glass 3-375 



Boxwood I '33 



Oak, old 1-170 



Milk. 



.1-030 



Sea-water 1-026 



Water, distilled i 



Alcohol, absolute... -800 



Beech, dry -590 



Cork -240 



Attraction and Repulsion. The term attraction 

 is applied to a great many phenomena, which we 

 must regard as of different kinds, or produced by 

 different causes. The force, whatever it is, that 

 makes a stone fall to the earth, is called the at- 

 traction of gravitation, because it is the cause of 

 gravity or weight. Sir Isaac Newton demonstrated 

 that the same force acts on the moon, drawing it 

 towards the earth ; and on the earth, drawing it 

 towards the sun : or rather, that the attraction 

 between any two heavenly bodies is mutual, mak- 

 ing them approach each other. It is now estab- 

 lished as a fundamental law, not only of our globe, 

 but of the universe, that every atom of matter is 

 attracted towards every other atom. The effects 

 of this law, in causing the fall of bodies and 

 weight, will be considered under Terrestrial Gravity 

 in this number. See also ASTRONOMY. 



When a loadstone or a magnetic needle is 

 brought near to a piece of iron, the two bodies, if 

 free to move, will come together and adhere to 

 each other. Also, if a stick of sealing-wax is 

 rubbed with silk, it will draw a light feather to it. 

 These two forms of attraction the magnetic and 

 the electric will be particularly described in the 

 numbers on ELECTRICITY and MAGNETISM. 



The kinds of attraction already mentioned act 

 between bodies at a distance, as well as near. 

 But there are forces at work in matter which act 

 only at insensible distances, and between the 

 adjacent atoms and molecules ; they are hence 

 called atomic forces and molecular forces. They 

 are spoken of under the names of Cohesion, Adhe- 

 sion, Repulsion, and Chemical Attraction. 



Cohesion is that force that binds together 

 particles or molecules of the same kind of matter 

 so as to form masses or bodies. Without some 

 force to hold the molecules together, we could not 

 have bodies, but mere heaps, as of sand. Cohesion 

 acts only when the particles are at distances so 

 minute as to be insensible to us : when removed 

 beyond that distance, it has no influence what- 

 ever ; and when the molecules of a solid body are 

 once separated, it is in most cases impossible to 

 bring them near enough again to make them 

 cohere. 



The three states of aggregation, as they are 

 called that is, the solid, liquid, and aeriform 

 are owing to differences in the strength and man- 

 ner of acting of cohesion. It is commonly said 

 that its force is greatest in solids, less in liquids, 

 and altogether wanting in gases. But this account 

 does not explain all the differences of these states. 

 If the smallest quantity of air, or any other gas, is 

 admitted into the exhausted receiver of an air- 

 pump, it does not remain at the bottom, but 

 spreads itself instantly and uniformly through the 

 whole space, as if its particles wished to remove 

 from one another as far as possible. There seems 

 no limit to the space over which the smallest por- 

 tion of gas will thus spread itself, so that it shall 

 be found in every part of it We cannot help 

 inferring from this, that the molecules of a gas, 

 instead of attracting, actually repel one another 

 with a force sufficient to overcome their own 

 weight for gases have weight as well as solids 

 and liquids. The dilatation of solid bodies by 

 heat, and the recoil of elastic bodies after com- 

 pression, would also seem to imply some repulsive 

 force at work. 



Again, some gases, when compressed with great 

 force, have their molecules forced so near that 

 they become liquid ; and in this condition they 

 are seen to cohere and form drops, thus shewing 

 that they are not destitute of attractive force. The 

 natural conclusion from these and other observed 

 facts is, that among the molecules of all bodies 

 there are two opposing forces at work an attract- 

 ive force, and a repulsive force that when attrac- 

 tion considerably predominates over repulsion, a 

 solid body is the result ; when there is almost a 

 balance of the two forces, we have a liquid ; and 

 when the repulsive force has the upper hand, we 

 have a gas. Many substances are seen to assume 

 all three forms in turn. Liquid water turns at one 

 time into solid ice, at another into vapour or 

 steam. Greater extremes of cold and heat have 

 the same effects on mercury. Several gases, by 

 applying great pressure and cold, have been ren- 

 dered liquid, and one at least even solid ; and 

 thus it becomes probable that all substances are 

 capable of existing in any one of the three states 

 under certain conditions. 



From the fact that the increase of heat regularly 

 increases the energy of repulsion, heat and the 

 repulsive force may be considered one and the 

 same thing. The subject of HEAT will be more 

 fully considered under a separate head. 



In solids, the cohesion is exerted in such a way 

 as not only to keep the atoms from separating, 

 but also to retain them in the same relative posi- 

 tion ; in liquids, on the contrary, the atoms, while 

 still kept from separating, are allowed to move or 

 slide freely upon one another in all directions. 

 This free motion of the molecules upon one 



