BOSCOVICH'S THEORY 



767 



Boscovich's 

 theory of 

 Natural 

 Philoso- 

 phy. 



v v - 

 Resistance 

 of fluids. 



Chemical 

 phenome- 

 na. 



The resistance of fluids to bodies in motion arises 

 partly from the motion impressed on the particles of 

 the fluid, since, according to the laws of collision, 

 the body impressing a motion on another will lose 

 some of its own. It also arises partly from the for- 

 ces exerted by the particles against those which ob- 

 struct their motion. To define accurately the laws 

 of resistance, is a matter by no means easy. We 

 should know the law of forces, the number and dis- 

 position of the particles. And after all the problem 

 would be too complicated for our analytical skill. 

 But it may be generally observed, that, in so far as 

 it arises from the inertia of the fluid itself, the resist- 

 ance is as the density and square of the velocity joint- 

 ly. As the density, because with an equal velocity, 

 the resistance will be as the number of particles mo- 

 ved. As the square of the velocity, because the 

 number of particles moved will be as the velocity, 

 and the motion impressed on each will be also as the 

 velocity. The resistance arising from the mutual for- 

 ces of the particles, will be constant, or as the times 

 only, if these forces be equal in all the particles ; but 

 as a greater velocity will produce a greater compres- 

 sion among the particles, and of course produce great- 

 er mutual forces, the resistance, therefore, is partly 

 constant, and partly also in some ratio of the veloci- 

 ty. And with this, experiments appear pretty well 

 to agree. 



The principles of chemical operations are all dedu- 

 cible from the same source, namely from the variety 

 of the particles. Were they subjected to the obser- 

 vation of our senses, there might, without doubt, be 

 a general reason given from the theory, for every che- 

 mical operation. But for this would be required an 

 intimate knowledge of the texture of every particle, 

 and its disposition in the general mass ; and a skill in 

 geometry and analysis, which far exceeds the powers 

 of the human mind. 



Some of these phenomena may be explained as fol- 

 lows. The particles of some solids have a less attrac- 

 tion for each other, than for those of some fluids. 

 Hence these particles are separated, and surrounded 

 on all sides by the fluid. The mixture is therefore 

 composed of globules, and retains its fluidity. In 

 this way we may have an idea of solution. But as 

 the attractive force will cease at a little greater dis- 

 tance, the solid particles will only be covered to a 

 certain depth ; and the fluid is then said to be satu- 

 rated. If another substance, whose particles are 

 more attracted by those of the fluid, be introduced 

 into the mixture, the particles which had been con- 

 glomerated round the former, will be drawn away 

 and accumulated round those of the latter, which 

 will be dissolved in their turn ; while the former by 

 its natural gravity, falls down in a fine powder. This 

 is called precipitation ; and perhaps rain is a precipi- 

 tation of this kind, when the aqueous particles are 

 abandoned by the air. The combination of two fluids 

 frequently forms a solid ; and it is often observed 

 that the specific gravity of the compound is greater 

 than the mean, or its mass less than the sum of the 

 component substances. This may be explained by 

 supposing that the particles, in the first case, come 

 into limits of cohesion ; and that in the other, by being 

 attracted more strongly, they come into smaller dis- 



tances. When a solid is combined with a solid, it is Boscovich's 



Theory of 

 Natural 

 Philoso- 

 phy. 



necessary to dissolve, or reduce one of them to pow- 

 der, so that their small particles may approach to 

 and join each other. This is principally done by fire, 

 which, by its vehement agitation, and the intestine 

 motion of its particles, may account for the pheno- 

 mena of fusion, liquefaction, and volatilization. By 

 this violent agitation, our author proceeds to account 

 for several other of the phenomena of chemistry, such 

 as fermentation, effervescence, and the like. But the 

 reader acquainted with modern chemistry will put 

 little value on such explanations as this. The facts, 

 at that time known were too few, or too imperfectly 

 described, to enable him to found a permanent theory. 

 And we have since seen many others fail in perfecting 

 the same structure, who had more numerous mate- 

 rials than Boscovich. 



Fire he conceives to be a sort of fermentation, F; rK . 

 which chiefly, if not solely, takes place in sulphure- 

 ous matter, when it meets with the matter of light 

 in sufficient abundance. This agitation separates the 

 parts of other bodies, and brings their particles into 

 new spheres of action, by which a small spark may 

 speedily propagate the motion throughout a great 

 mass ; as the foot of a small bird, alighting on the 

 top of a precipice, may move the sand, and that the 

 gravel and stones below it, until at length great rocks, 

 towards the bottom, tumbling into the sea, may pro- 

 duce a wide and lasting agitation. In the same man- 

 ner, if the limits of cohesion of any body, be suc- 

 ceeded by a powerful repulsive arc, the small motion 

 produced by a foreign body, may make some or all 

 of the points pass the limits of cohesion, and be re- 

 pelled from each other with great force and velocity ; 

 so that a volatilization may take place a deflagra- 

 tion, or sudden explosion. 



Light may be a sort of very fine fluid, or a kind Light, 

 of vapour, thrown off by vehement fiery fermenta- 

 tion. Its celerity may be accounted for, by suppo- 

 sing the repulsive arcs sufficiently powerful : its rec- 

 tilineal propagation, by its great velocity, which af- 

 fords little time for the forces of the points to pro- 

 duce any sensible effect ; and also by the equality of 

 actions on all sides, in a medium which is homogene- 

 ous. This homogeneity accounts for the free passage 

 through pellucid bodies ; whereas opacity will arise 

 from the unequal texture of heterogeneous particles, 

 producing unequal forces, which, acting on the light 

 in variCus directions, inflect it in various ways ; and, 

 if the substance be somewhat thick, totally prevent 

 its passage. The other phenomena of light may be v 

 explained in equal conformity with the principles of 

 the theory, as is done by our author, with great a- 

 cuteness and ingenuity. 



He next proceeds to our bodily sensations of taste, Bodily sen- 

 sound, smell, and feeling. He explains these much sations. 

 in the usual way, excepting, that for the immediate 

 contact of bodies, or of the particles emitted, he sub- 

 stitutes the attractions and repulsions, or oscillations 

 of the particles ; which, indeed, are particularly well 

 fitted for causing that motion in our nerves, which 

 the disciples of Hartley have supposed to take place 

 in the organs of sensation. But, indeed, we do Tiot 

 see how any attempt to account for these should have 

 been introduced into his theorv. 



