MATTER AND ITS PHYSICAL PROPERTIES. 23 



the same class arise from the agency of the same cause. However probable 

 this assumption may be, it is altogether unnecessary. All the objects of sci- 

 ence, the enlargement of mind, the extension and improvement of knowledge, 

 the facility of its acquisition, are obtained by generalization alone, and no good 

 can arise from tainting our conclusions with the possible errors of hypotheses. 



It may be here, once for all, observed, that the phraseology of causation and 

 hypotheses has become so interwoven with the language of science, that it is 

 impossible to avoid the frequent use of it. Thus we say, " The magnet attracts 

 iron :" the expression attract intimating the cause of the observed effect. la 

 such cases, however, we must be understood to mean the effect itself, finding 

 it less inconvenient to continue the use of the received phrases, modifying 

 their signification, than to introduce new ones. 



Force, when manifested by the mutual approach or cohesion of bodies, is 

 also called attraction, and it is variously denominated, according to the circum- 

 stances under which it is observed to act. Thus the force which holds to- 

 gether the atoms of solid bodies is called cohesive attraction. The force which 

 draws bodies to the surface of the earth, when placed above it, is called the 

 attraction of gravitation. The force which is exhibited by the mutual approach 

 or adhesion of the loadstone and iron, is called magnetic attraction, and so on. 



When force is manifested by the remotion of bodies from each other, it is 

 called repulsion. Thus, if a piece of glass, having been briskly rubbed with 

 a silk handkerchief, touch, successively, two feathers, these feathers, if brought 

 near each other, will move asunder. This effect is called repulsion, and the 

 feathers are said to repel each other. 



The influence which forces have upon the form, state, arrangement, and mo- 

 tions, of material substances, is the principal object of physical science. In its 

 strict sense, MECHANICS is a term of very extensive signification. According 

 to the more popular usage, however, it has been generally applied to that part 

 of physical science which includes the investigation of the phenomena of motion 

 and rest, pressure, and other effects developed by the mutual action of solid 

 masses. The consideration of similar phenomena, exhibited in bodies of the 

 liquid form, is consigned to HYDROSTATICS, and that of aeriform fluids to PNEU- 

 MATICS. 



DIVISIBILITY. 



Observation and experience prove that all bodies of sensible magnitude, even 

 the most solid, consist of parts which are separable. To the practical sub- 

 division of matter there seems to be no assignable limit. Numerous examples 

 of the division of matter, to a degree almost exceeding belief, may be found in 

 experimental inquiries instituted in physical science ; the useful arts furnish 

 many instances not less striking ; but perhaps the most conspicuous proofs 

 which can be produced, of the extreme minuteness of which the parts of mat- 

 ter are susceptible, arise from the consideration of certain parts of the organ- 

 ized world. 



The relative places of stars in the heavens, as seen in the field of view of a 

 telescope, are marked by fine lines of wire placed before the eyeglass, and 

 which cross each other at right angles. The stars appearing in the telescope 

 as mere lucid points without sensible magnitude, it is necessary that the wires 

 which mark their places should have a corresponding tenuity, 

 wires, being magnified by the eyeglass, would have an apparent thickness, 

 which would render them inapplicable to this purpose, unless their real dimen- 

 sions were of a most uncommon degree of minuteness. To obtain wire i( 

 this purpose, Dr. WoUaston invented the following process : A piece of i 



