152 THE THEORY OF GRAVITATION. 



does not the very sensible weight which they impart to these envelopes 

 demonstrate the contrary, that is that all substances arrest the passage 

 of a great number of corpuscles'? 



XX. 



To this the Epicureans would have been forced to respond that the 

 atoms doubtless traverse very freely ' all heavy bodies ; as freely, for 

 example, as light passes through diamond and magnetic matter through 

 gold, though one of these bodies is the hardest and the other the 

 heaviest of all known bodies (which shows that they are less porous 

 than most substances). Thus the number of atoms which are inter- 

 cepted by the first layers of a heavy body would be absolutely insensible 

 relatively to the number of those which pass through the last layers. 2 

 Nevertheless, the relatively small number intercepted would produce a 

 sensible action upon the body, since they have, in virtue of an immense 

 velocity, 3 the force of impact which they would lack by reason of their 

 small mass. 



1 Several ancient physicists recognized the pores in bodies. It may he seen, for 

 example, in the eighth chapter of the first hook of Aristotle on Generation and Cor- 

 ruption, that Empedocles, Leucippus, and Democritus had made a great deal of use 

 of them to explain sensations and mixtures. Galen reports in his works on the 

 Natural Properties, that Erasistratus (the grandson, it is believed, of Aristotle), a 

 celebrated corpuscular physician who denied attraction, believed in the existence of 

 a vacuum and attempted to reduce all natural properties from the size of the pores. 

 Coelius Aurelianus speaks of them also in connection with Asclepiades, of Bithynia, 

 a physician of the time of Pompey. And Sextus Empiricus assures us that not only 

 Asclepiades but also other physicians and physicists of the sect of Epicureans 

 made many applications of the pores. Finally, in the first book of Lucretius there 

 are ten or twelve lines upon the great permeability of bodies, concluding as follows : 

 Usque adeo, in rebus, solidi nil esse videtur. 



2 However considerable we assume the number n of horizontal layers going to 

 compose a body of uniform density, the number (and consequently the effectiveness) 

 of the gravitational atoms is diminished in passing each one of them, because some 

 atoms are intercepted by the solid material composing the layer. The number of 

 atoms transmitted by a layer, and remaining effective to produce weight in the next 

 lower one, will bear the same ratio to the number reaching the first that the volume 

 of the spaces or pores in the layer bears to its total volume. Assuming the body to 

 be of uniform density, this ratio will be constant, and since the weight of each layer 

 is proportional to the number of atoms available to collide with its substance, this 

 ratio represents the relative weight of any layer to that next above it. However 

 nearly equal we may suppose the numbers a and b, which express the ratio which is 

 assumed between the weight of the highest layer of the body and that of the lowest 

 (the two layers being supposed equal in volume and density), it is possible to express 

 in numbers the ratio of the entire volume to that occupied by the pores as y a to ty ^ 

 Such a ratio may be obtained by experiments with several sorts of tissues, as, for 

 example, by means which Newton indicates in his Optics (Book II, Part III, Prop. 

 8), the number of the orders of pores being the excess of the logarithm of y/ a over 

 that of i/o y/h divided by the logarithm of two. 



3 The movement of the atoms is so rapid, according to Epicurus (in his letter to 

 Herodotusj, that they traverse the greatest imaginable spaces in a time inconceivably 

 short. 



