in] of the New Physics. 81 



the help of hypotheses concerning the size and shape of the 

 particles to be secreted, and of the orifices or spaces through 

 which they had to pass. This is what he says : 



" For we have shewn that the fluidity of a liquid cannot be 

 "conceived of without its mass being actually divisible into 

 " very minute hard and consistent particles of a definite shape, 

 u united together not by firm bonds but by simple contact, so 

 " that some of them can be agitated, can flow, can move about 

 " while others are at rest or are moving in another direction. 

 " In no other way can be preserved that fluidity through which 

 ' ; fluid parts flow along, mix and fuse together. 



" Moreover it cannot be doubted that the different natures 

 " and properties of fluids depend on the different consistency, 

 " structure, configuration and motion of the molecules composing 

 " the fluids. Thus the molecules composing water are all homo- 

 "geneous and like each other but different from those com- 

 " posing oil or a fluid of another nature, and indeed it is agreed 

 '• that the particles of the said fluids differ in structure, size 

 " and shape. 



"And indeed if the molecules of two heterogeneous fluids 

 " were equally mobile so that they could be mixed by simple 

 "contact, then a mixture of them, a mixture for instance of 

 11 oil and water, might be compared to a mixed heap of millet 

 "and barley. And since we see that these can be separated 

 " by a sieve, so in like manner water and oil are able to pass 

 " through the pores of skin and of wood but air cannot, while 

 " mercury can pass through the pores of gold but water, oil or 

 "air cannot. Consequently the said fluids can be separated 

 "(just as vegetable grains maybe) from other different fluids 

 " with which they may be mixed by means of a sieve of an 

 "appropriate structure without any fermentation; for just as 

 " grains pass through a sieve uninjured, so oil and water can 

 "pass through the pores of skin or wood, intact, without any 

 "change. Wherefore it must be confessed that it follows by 

 " mechanical laws that the reason why fluids of the one kind 

 " do pass through and those of another kind do not is without 

 'doubt because the shapes of the molecules of the said fluids 

 " match and are fitted to the shapes of the minute pores 

 p. l. 6 



