182 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



merest trace of a soluble dye will color millions of times its volume 

 of water. It is not surprising therefore that in the past there have 

 arisen schools who believed that matter was quite continuous and in- 

 finitely divisible. 



The upholders of this view said that if you took a piece of material, 

 lead, for instance, and went on cutting it into smaller and smaller 

 fragments with a sufficiently sharp knife, you could go on indefi- 

 nitely. The opposing school argued that at some stage in the opera- 



Th in nest Port 

 of a Bubble 



Oil Film 

 on Water 



Wove Length of Cadmium 

 Red Light 



6438.4702 All. 



-L-J I I » i i » i i t I I I » I... I . I 1 I i I I 1 



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Fig. 2. — Cubes 17 to 21 compared with minute objects to scale. 



tions either the act of section would become impossible, or the result 

 would be lead no longer. Bacon, Descartes, Gassendi, Boyle, and 

 Hooke were all partial to the latter theory, and Newton in 1675 tried 

 to explain Boyle's Law on the assumption that gases were made up 

 of mutually repulsive particles. 



The accuracy of modern knowledge is such that we can carry out, 

 indirectly at least, the experiment suggested by the old philosophers 

 right up to the stage when the second school is proved correct, and 

 the ultimate atom of lead reached. For convenience, we will start 

 with a standard decimeter cube of lead weighing 11.37 kilograms, 



