xvin.] OBSERVATION. 407 



keeping a certain volume of oxygen safely inclosed in 

 a bottle can we assure ourselves of its identity ; allow it 

 to mix with other oxygen, and we lose all power of iden- 

 tification. Accordingly we seem to have no means of 

 directly proving that every gas is in a constant state of 

 diffusion of every part into every part. We can only 

 infer this to be the case from observing the behaviour 

 of distinct gases which we can distinguish in their course, 

 and by reasoning on the grounds of molecular theory. 1 



External Conditions of Correct Observation. 



Before we proceed to draw inferences from any series of 

 recorded facts, we must take care to ascertain perfectly, 

 if possible, the external conditions under which the facts 

 are brought to our notice. Not only may the observing 

 mind be prejudiced and the senses defective, but there 

 may be circumstances which cause one kind of event to 

 come more frequently to our notice than another. The 

 comparative numbers of objects of different kinds existing 

 may in any degree differ from the numbers which come to 

 our notice. This difference must if possible be taken into 

 account before we make any inferences. 



There long appeared to be a strong presumption that 

 all comets moved in elliptic orbits, because no comet had 

 been proved to move in any other kind of path. The 

 theory, of gravitation admitted of the existence of comets 

 moving in hyperbolic orbits, and the question arose 

 whether they were really non-existent or were only 

 beyond the bounds of easy observation. From reason- 

 able suppositions Laplace calculated that the probability 

 was at least 6000 to I against a comet which comes 

 within the planetary system sufficiently to be visible at 

 the earth's surface, presenting an orbit which could be 

 discriminated from a very elongated ellipse or parabola in 

 the part of its orbit within the reach of our telescopes. 2 

 In short, the chances are very much in favour of our 

 seeing elliptic rather than hyperbolic comets. Laplace's 

 views have been confirmed by the discovery of six 



1 Maxwell, Theory of Heat, p. 301. 



2 Laplace, Essai Philosophise, p. 59. Todhunter's History, 

 pp. 491494- 



