ILLUSTRATIONS OF THE LOGIC OF SCIENCE. 479 



in the contempt which clings to the word hypothesis. To think that 

 we can strike out of our own minds a true preconception of how Nature 

 acts, is a vain fancy. As Lord Bacon well says: "The subtlety of 

 Nature far exceeds the subtlety of sense and intellect : so that these 

 fine meditations, and speculations, and reasonings of men are a sort of 

 insanity, only there is no one at hand to remark it." The successful 

 theories are not pure guesses, but are guided by reasons. 



The kinetical theory of gases is a good example of this. This theory 

 is intended to explain certain simple formula?, the chief of which is 

 called the law of Boyle. It is, that if air or any other gas be placed in 

 a cylinder with a piston, and if its volume be measured under the press- 

 ure of the atmosphere, say fifteen pounds on the square inch, and if then 

 another fifteen pounds per square inch be placed on the piston, the gas will 

 be compressed to one-half its bulk, and in similar inverse ratio for other 

 pressures. The hypothesis which has been adopted to account for this 

 law is that the molecules of a gas are small, solid particles at great dis- 

 tances from each other (relatively to their dimensions), and moving 

 with great velocity, without sensible attractions or repulsions, until 

 they happen to approach one another very closely. Admit this, and it 

 follows that when a gas is under pressure what prevents it from collaps- 

 ing is not the incompressibility of the separate molecules, which are 

 under no pressure at all, since they do not touch, but the pounding of 

 the molecules against the piston. The more the piston falls, and the 

 more the gas is compressed, the nearer together the molecules will be ; 

 the greater number there will be at any moment within a given distance 

 of the piston, the shorter the distance which any one will go before its 

 course is changed by the influence of another, the greater number of 

 new courses of each in a given time, and the oftener each, within a 

 given distance of the piston, will strike it. This explains Boyle's law. 

 The law is not exact ; but the hypothesis does not lead us to it exactly. 

 For, in the first place, if the molecules are large, they will strike each 

 other oftener when their mean distances are diminished, and will con- 

 sequently strike the piston oftener, and will produce more pressure 

 upon it. On the other hand, if the molecules have an attraction for 

 one another, they will remain for a sensible time within one another's 

 influence, and consequently they will not strike the wall so often as 

 they otherwise would, and the pressure will be less increased by com- 

 pression. 



When the kinetical theory of gases was first proposed by Daniel 

 Bernoulli, in 1738, it rested only on the law of Boyle, and was there- 

 fore pure hypothesis. It was accordingly quite naturally and deserved- 

 ly neglected. But, at present, the theory presents quite another as- 

 pect ; for, not to speak of the considerable number of observed facts of 

 different kinds with which it has been brought into relation, it is sup- 

 ported by the mechanical theory of heat. That bringing together 

 bodies which attract one another, or separating bodies which repel one 



