68 SCIENCE PRIMERS. [MATERIAL 



upon suppositions or hypotheses, and our success or 

 failure in practical affairs depends upon the legitimacy 

 of these hypotheses. You believe a man on the 

 hypothesis that he is always truthful; you give him 

 pecuniary credit on the hypothesis that he is solvent. 



Thus, everybody invents, and, indeed, is compelled 

 to invent, hypotheses in order to account for phe- 

 nomena of the cause of which he has no direct 

 evidence ; and they are just as legitimate and neces- 

 sary in science as in common life. Only the scientific 

 reasoner must be careful to remember that which is 

 sometimes forgotten in daily life, that a hypothesis 

 must be regarded as a means and not as an end ; that 

 we may cherish it so long as it helps us to explain the 

 order of nature ; but that we are bound to throw it 

 away without hesitation as soon as it is shown to be 

 inconsistent with any part of that order. 



48. The Hypothesis that Water is com- 

 posed of Separate Particles (Molecules), 



It has been pointed out that we cannot see, and 

 indeed that there is not much hope of our ever being 

 able to see, the separate particles of water, even if water 

 is composed of such particles. But it is perfectly 

 legitimate to suppose that water is made up of such 

 particles, if that hypothesis will enable us to explain 

 the properties ot water. 



Let us suppose then that any portion of fluid water is 

 really composed of a prodigious number of particles less 

 (and probably much less) than a millionth of an inch 

 in diameter. We may call theso particles molecules. 1 



1 Diminutive of moles, a mass. 



