92 THE LIMITATIONS OF SCIENCE 



parts are made of closely packed and intricately ar- 

 ranged particles, which can move with respect to each 

 other but very slightly, if at all. The spaces between 

 the constituent parts of a liquid are greater and contain 

 a larger proportion of the subtle elements than solids ; 

 while gases have their particles so separated that they 

 approach fire in their freedom of motion. As he 

 quaintly puts it : a body is a fluid when it is divided in 

 many little parts which are free to move separately in 

 various ways, and it is hard when all its parts are so 

 interlaced that there is no action capable of separating 

 them. Cohesion in hard bodies is caused by the lack 

 of motion of their constituent parts, for no cement 

 could be harder than rest, which is the quality the most 

 contrary to motion. He does not tell us how to inter- 

 lace and twist a number of spheres and irregular 

 fragments together, so as to form compact and in- 

 extricable units of matter. To anyone but a great 

 philosopher they would remain ropes of sand. 



Descartes had spent much time investigating the ex- 

 perimental laws and phenomena of light and with 

 notable success. One of his greatest ambitions was 

 evidently to explain the nature of light and the mode of 

 its transmission, for he discusses these in great detail 

 in his Principia and refers to his solution of the prob- 

 lem constantly in his letters with much complacency. 



As noted, the substance of the sun and stars is like 

 fire in respect to its motions; for there is nothing 



