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n has these observations。
〃Bodies are much more rare and porous； than is commonly believed。
Water is nineteen times lighter； and by consequence nineteen times rarer； than gold； and gold is so rare； as very readily； and without the least opposition； to transmit the magnetic effluvia； and easily to admit quicksilver into its pores； and to let water pass through it。 From all which we may conclude； that gold has more pores than solid parts； and by consequence that water has above forty times more pores than parts。 And he that shall find out an hypothesis； by which water may be so rare； and yet not capable of compression by force； may doubtless； by the same hypothesis； make gold； and water； and all other bodies； as much rarer as he pleases； so that light may find a ready passage through transparent substances＇43＇。〃
＇43＇ Newton； Optics； Book II； Part III； Prop。 viii。
Again： 〃The colours of bodies arise from the magnitude of the particles that reflect them。 Now； if we conceive these particles of bodies to be so disposed among themselves； that the intervals； or empty spaces between them； may be equal in magnitude to them all； and that these particles may be composed of other particles much smaller； which have as much empty space between them as equals all the magnitudes of these smaller particles； and that in like manner these smaller particles are again composed of others much smaller； all which together are equal to all the pores； or empty spaces； between them； and so on perpetually till you come to solid particles； such as have no pores； or empty spaces within them： and if in any gross body there be； for instance； three such degrees of particles； the least of which are solid； this body will have seven times more pores than solid parts。 But if there be four such degrees of particles； the least of which are solid； the body will have fifteen times more pores than solid parts。 If there be five degrees； the body will have one and thirty times more pores than solid parts。 If six degrees； the body will have sixty and three times more pores than solid parts。
And so on perpetually＇44＇。〃
＇44＇ Ibid。
In the Queries annexed to the Optics； Newton further suggests an opinion； that the rays of light are repelled by bodies without immediate contact。 He observes that：
〃Where attraction ceases； there a repulsive virtue ought to succeed。 And that there is such a virtue； seems to follow from the reflexions and inflexions of the rays of light。 For the rays are repelled by bodies； in both these cases； without the immediate contact of the reflecting or inflecting body。 It seems also to follow from the emission of light； the ray； so soon as it is shaken off from a shining body by the vibrating motion of the parts of the body； and gets beyond the reach of attraction； being driven away with exceeding great velocity。 For that force； which is sufficient to turn it back in reflexion； may be sufficient to emit it。 It seems also to follow from the production of air and vapour： the particles； when they are shaken off from bodies by heat or fermentation； so soon as they are beyond the reach of the attraction of the body； receding from it and also from one another； with great strength； and keeping at a distance； so as sometimes to take up a million of times more space than they did before； in the form of a dense body。〃
Newton was of opinion that matter was made up； in the last resort； of exceedingly small solid particles； having no pores； or empty spaces within them。 Priestley； in his Disquisitions relating to Matter and Spirit； carries the theory one step farther； and； as Newton surrounds his exceedingly small particles with spheres of attraction and repulsion； precluding in all cases their actual contact； Priestley is disposed to regard the centre of these spheres as mathematical points only。 If there is no actual contact； then by the very terms no two particles of matter were ever so near to each other； but that they might be brought nearer； if a sufficient force could be applied for that purpose。 You had only another sphere of repulsion to conquer； and； as there never is actual contact； the whole world is made up of one sphere of repulsion after another； without the possibility of ever arriving at an end。
〃The principles of the Newtonian philosophy；〃 says our author； 〃were no sooner known； than it was seen how few in comparison； of the phenomena of nature； were owing to solid matter； and how much to powers； which were only supposed to accompany and surround the solid parts of matter。 It has been asserted； and the assertion has never been disproved； that for any thing we know to the contrary； all the solid matter in the solar system might be contained within a nutshell＇45＇。〃
＇45＇ Priestley； Disquisitions； Section II。 I know not by whom this illustration was first employed。 Among other authors； I find； in Fielding （Joseph Andrews； Book II； Chap。 II）； a sect of philosophers spoken of； who 〃can reduce all the matter of the world into a nutshell。〃
It is then with senses； from the impressions upon which we are impelled to draw such false conclusions； and that present us with images altogether unlike any thing that exists out of ourselves； that we come to observe the phenomena of what we call the universe。 The first observation that it is here incumbent on us to make； and which we ought to keep ever at hand； to be applied as occasion may offer； is the well known aphorism of Socrates； that 〃we know only this； that we know nothing。〃 We have no compass to guide us through the pathless waters of science； we have no revelation； at least on the subject of astronomy； and of the unnumbered inhabitable worlds that float in the ocean of ether； and we are bound therefore to sail； as the mariners of ancient times sailed； always within sight of land。 One of the earliest maxims of ordinary prudence； is that we ought ever to correct the reports of one sense by the assistance of another sense。 The things we here speak of are not matters of faith； and in them therefore it is but reason； that we should imitate the conduct of Didymus the apostle； who said； 〃Except I put my fingers into the prints of the nails； and thrust my hand into his side； I will not believe。〃 My eyes report to me an object； as having a certain magnitude； texture； and roughness or smoothness； but I require that my hands should confirm to me the evidence of my eyes。 I see something that appears to be an island at an uncertain distance from the shore； but； if I am actuated by a laudable curiosity； and wish to possess a real knowledge； I take a boat； and proceed to ascertain by nearer inspection； whether that which I imagined to be an island is an island or no。
There are indeed many objects with which we are conversant； that are in so various ways similar to each other； that； after having carefully examined a few； we are satisfied upon slighter investigation to admit the dimensions and character of others。 Thus； having measured with a quadrant the height of a tower； and found on the narrowest search and comparison that the report of my instrument was right； I yield credit to this process in another instance； without being at the trouble to verify its results in any more elaborate method。
The reason why we admit the inference flowing from our examination in the second instance； and so onward； with less scrupulosity and scepticism than in the first； is that there is a strict resemblance and analogy in the two cases。 Experience is the basis of our conclusions and our conduct。 I strike against a given object； a nail for example； with a certain degree of force； because I have remarked in myself and others the effect of such a stroke。 I take food and masticate it； because I have found that this process contributes to the sound condition of my body and mind。 I scatter certain seeds in my field； and discharge the other functions of an agriculturist； because I have observed that in due time the result of this industry is a crop。 All the propriety of these proceedings depends upon the exact analogy between the old case and the new one。 The state of the affair is still the same； when my business is merely that of an observer and a traveller。 I know water fr