36 THE PRINCIPLES OF SCIENCE. 



We can hardly realize now the importance of the in- 

 vention of the air-pump, previous to which it was exceed- 

 ingly difficult to make any experiment except under the 

 ordinary pressure of the atmosphere. The Torricellian 

 vacuum had been employed by the philosophers of the 

 Accademia del Cimento to show the behaviour of water, 

 smoke, sound, magnets, electric substances, &c., in vacuo, 

 but their experiments were often unsuccessful from the 

 difficulty of excluding air . 



Among the most constant circumstances under which 

 we live is the force of gravity, which does not vary, 

 except by a slight fraction of its. amount, in any part 

 of the earth's crust or atmosphere to which we can 

 attain. Now this force is often sufficient to overbear and 

 disguise various actions ; for instance, the mutual gravi- 

 tation of small bodies. It was an interesting experi- 

 ment of Plateau to withdraw substances from the action 

 of gravity by suspending them in liquids of exactly the 

 same specific gravity. Thus a quantity of oil poured 

 into the middle of a suitable mixture of alcohol and 

 water, assumes a spherical shape which, on being made to 

 rotate, becomes spheroidal, and then successively sepa- 

 rates into a ring and a group of spherules. Thus we 

 have at least an illustration of the mode in which the 

 planetary system may have been produced?, though it is 

 to be remembered that the extreme difference of scale 

 prevents our arguing with confidence from the experiment 

 to the conditions of the nebular theory. 



It is possible that the so-called elements are elementary 

 only to us, because we are restricted to temperatures at 

 whi|h they are fixed. Lavoisier carefully defined an 

 element as a substance which cannot be decomposed by 



' Essayes of Natural Experiments made in the Accademia del Cimento. 

 Englished by Richard Waller, 1684, p. 40, &c. 



P Plateau, Taylor's ' Scientific Memoirs,' vol. iv. pp. 16-43. 



