4 INTRODUCTION TO PHYSIC& [ATTKACTION. 



The application of these interesting laws to the motions of the planets and other astronomical phenomena, will 

 be fully dealt with under the section of Astronomy. The consideration of the cause of the tides will also fall under 

 that head. 



Having thus attempted to give a general idea of the force of gravitation, we pass on to consider the aggregation 

 of atoms, and the causes of solidity, etc. It has already been remarked, that a mass is composed of very minute 

 particles. These are brought and held together by what is called the attraction of cohesion. 



COHESION. This force, unlike gravitation, is only exercised at extremely small distances ; and when particles 

 are violently separated, they generally remain apart, owing to the loss of their cohesive power. The ordinary 

 processes of pounding, cutting, cleaving, <to., are instances of cohesive attraction being destroyed between particles, 

 or, at all events, they cause its temporary suspension. In some instances, however, parts of bodies may be reunited, 

 as in the case of india-rubber, which, on being cut by a clean knife, may bo joined together again by simply pressing 

 the clean edges closely together. A leaden bullet, if divided by a sharp knife, may be re-formed by immediately 

 bringing the surfaces together by means of a kind of twisting motion, assisted by pressure. 



llighly polished surfaces will often adhere strongly if brought into close contact. Polished plate-glass affords 

 an excellent illustration of this ; for sometimes two sheets, if placed on each other, and having nothing between 

 them, will adhere with such force as to prevent their separation. Finely polished steel surfaces often present similar 

 results. Generally speaking, however, an extremely small distance is required to be attained before bodies will 

 adhere ; and hence the use of adhesive substances, such as glue and pitch, which, by touching the particles of two 

 bodies in many places, produce a temporary cohesion of great force. 



It may seem paradoxical, but at the same time it is true, that although the sense of touch leads us to imagine 

 that we actually come in contact with a body, it is extremely rare that such an event takes place. If two pieces of 

 convex glass, such as common lenses, are pressed together with great force by means of screws, there will still remain 

 between them a plate of air, which, by its coleur, indicates the distance still separating them. A needle resting on 

 the surface of water, has round it a film of air which prevents the liquid from at all touching it. If the hand be 

 pressed on any surface, there will generally exist three plates, which will prevent actual contact with it namely, 

 one of water, grease, and air. Again, if a little lycopodium, in powder, be cast on to the surface of some water, the 

 hand may be thrust into the liquid without becoming moistened. Some of these, and similar, effects have been 

 described as resulting from an active principle, which has been termed "repulsion ;" but they are easily explained, 

 by presuming tliat distance negatives the usual effect of cohesive attraction ; indeed, the slightest film on the surfaces 

 of the divided lead bullets above-named, will prevent their re-adhesion. 



The different states of bodies, such as the solid, liquid, and gaseous, are caused, it is believed, by the repulsive 

 power of heat. A solid may be supposed to have its particles close together. On the addition of heat thereto, its 

 particles are at once driven farther apart, and a fluid is produced. By a still further application of heat, the parti- 

 cles of the fluid may be separated to a greater distance, and a gas or vapour will be produced. The most common 

 illustration of these three separate states being found in one body, is that of water, which may be successively a 

 solid as ice, a fluid as water, and a gas in the form of vapour or steam. 



Masses may be reunited, after division, by various means ; solution and crystallisation being those most com- 

 monly employed. Sugar, for example, may be powdered, and no pressure will reunite the particles. On being 

 dissolved in water, the different parts are aggregated, and the solid mass may be restored. Under the section 

 on Crystallography, this mode of inducing cohesive attraction will be fully explained. 



CHEMICAL, ELECTRICAL, and MAGNETIC attraction will necessarily be spoken of under the respective branches 

 of Science to which they relate. It will therefore be sufficient to state that they exercise a varied influence on, and 

 modify or control, the form and condition of bodies. The former of these forces has a universal effect throughout 

 nature ; whilst the influence of Magnetism is, comparatively speaking, restricted to the metal iron, and its 

 compounds. 



CAPILLARY ATTRACTION. Tliis force seems to be a modification of that of cohesion, and is manifested whenever 

 a liquid is in contact with solids, which are placed at small distances from each other. On dipping a narrow glass 

 tube into water, it will be found that the fluid will rise lu'gher inside the tube than the general level of the aqueous 

 surface. A lump of sugar absorbs and raises above its level any water in which it may be placed. This force has an 

 astonishing effect both on animal and vegetable economy. By its action the sap is conveyed from the root of the 

 plant or tree to the highest branches ; and, generally speaking, liquids are raised by this force for a vast number of 

 purposes in nature. To' this end the pores or interstices of bodies highly contribute : and thus, whether we regard 

 the cells of animal and vegetable structure, or the particles of sand, earth, loam, <tc. , tlu's comparatively unobserved 

 but constantly acting force, is one of the chief agents in producing an infinite variety of effects. In daily We, 

 capillary attraction is constantly employed : as, for instance, in the conveyance of oil in lamps by the wick ; in the 

 absorption of perspiration by clothing ; and in other ways too numerous to mention. 



We have hitherto restricted our remarks to those properties of matter and forces in nature which are properly 

 called universal, because we can conceive of no body which can exist unaffected by them. There are, however, 

 others to which we must direct attention ; because, although not partaken of so generally, they are discovered in 

 most of the objects to which the researches of philosophers are directed. It will be found that some of these 

 properties are cliaracteristic of the body possessing them ; but in others, these properties may vanish when the 

 state of the body possessing them, is varied by the action of heat and other forces. 



ELASTICITY is that property by virtue of which bodies regain a form which they had temporarily lost The effect 

 of Elasticity is produced when two other properties, COMPRESSIBILITY and EXPANSIBILITY, are called into exercise. 

 Air, for instance, when compressed, will at once expand on the pressure being removed ; hence air is called an elastic 



