54 ON THE PROPERTIES OF MATTER, 



tance may be held the same as if the whole of the matter of which they con- 

 sist were condensed into their respective centres. 



To what extent in the heavens the power of gravitation ranges it is impos- 

 sible to determine ; there can be little doubt, however, that it extends from 

 one fixed star to another, although its effects are too inconsiderable to be 

 calculated by man. It may possibly influence the progressive motion of 

 several of the stars, and, as I had occasion to observe in a preceding lecture, 

 is the cause to which Dr. Herschel ascribes the origin of the material 

 universe, which he supposed at one time, though he seems afterward to 

 have modified his opinion, as we shall notice in our next study, to have 

 issued from an immense central mass of matter, peculiarly volcanic in its 

 structure, and to have been, consequently, thrown forth in different quanti- 

 ties, and at different times, by enormous explosions ; each distinct mass, thus 

 forcibly propelled, assuming, from the common law of projectiles, an orbicu- 

 ]ar path, and endowed with the common property of the parent body, ejecting 

 in like manner, minuter masses at different periods of time, which have 

 equally assumed the same orbicular motion, and ultimately become planets 

 to the body from which they have immediately issued, and which constitutes 

 their central sun. 



To produce such an effect, however, and in reality to produce any of the 

 motions which occur to us in the celestial bodies, the PASSIVITY of matter is 

 just as necessary as its gravitation. I have already observed that, owing to 

 its passivity, or vis INERTLZE, matter has a tendency to persevere in any given 

 state, whether of motion or of rest, till opposed by some exterior power ; and 

 that the path it assumes must necessarily be that of a right line, unless the 

 power it encounters shall bend it into a different direction. A projectile, 

 therefore, as a planet, for example, thrown forth from a volcano, would travel 

 in a right line for ever, and with the exact velocity with which it was thrown 

 forth at first, if there were nothing to impede its progress, or to alter the 

 course at first given to it. But the attraction of the volcanic sphere from 

 which it has been launched does impede it, and equally so from every point 

 of its surface : the consequence of which must necessarily be, that every 

 step it advances over the parent orb it must be equally drawn back or reined 

 in, and hence its rectilinear path must be converted into a curve or parabola, 

 and a tendency be given to it to escape in this line, which may be contem- 

 plated as a line of perpetual angles, instead of in a direct course ; and as soon 

 as the projectile or planet has acquired the exact point in which the two an- 

 tagonist powers precisely balance each other the power of flying off from 

 the centre, communicated to it by the volcanic impulsion, and which is de- 

 nominated its CENTRIFUGAL FORCE, and the power of falling forwards to the 

 centre, communicated by the attractive influence of the aggregate mass of 

 matter, which the parent sphere contains in itself, and which is called its 

 CENTRIPETAL FORCE it will have reached its proper orbit; and, through the 

 influence of this constant antagonism of the two properties of passivity and 

 gravitation, of a centrifugal and centripetal force, persevere in the same to the 

 end of time. 



Of the immediate cause of gravitation, or the nature of that power which 

 impels different bodies to a union, we are in a very considerable degree of 

 ignorance ; or rather, perhaps, may be said to know nothing at all. It is 

 necessary, however, to notice one very singular phenomenon concerning it, 

 and to give a glance at two out of various theories by which gravitation has 

 been attempted to be accounted for. 



The phenomenon is, that although owing to this power, all bodies have a 

 tendency to come into contact, they never come into actual contact : some 

 kind of pore or open space being still left between the corpuscles of bodies 

 that approach the nearest to each other. Thus, a plate of heated iron, solid 

 as it appears to be, and altogether destitute of pores, becomes contracted in 

 every direction by cold. So, too, as I have already observed, equal measures 

 of water and alcohol, or of water and sulphuric acid, have their bulk sensibly 

 diminished. In like manner, Newton has remarked, that when two plates of 



