10 THE PRINCIPLES OF SCIENCE. 



ganized succession of small and large drops, oscillating in 

 form. The drops fall so rapidly that their impressions 

 upon the eye run into each other, and in order to see the 

 separate drops we require some device for giving an in- 

 stantaneous view, such as illumination by the electric 

 spark, or the use of the revolving disc called the phena- 

 kistiscope. 



One insuperable limit to our powers of observation 

 arises from the impossibility of following and identifying 

 the ultimate atoms of matter. One atom of oxygen is pro- 

 bably undistinguishable from another atom ; only by keep- 

 ing a certain volume of oxygen safely enclosed in a bottle 

 can we assure ourselves of its identity ; allow it to mix with 

 other oxygen, and we have lost all power of identification . 

 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 

 condition from observing the behaviour of distinct gases 

 which we can distinguish in their course, and by reasoning 

 on the grounds of molecular theory %. 



External Conditions of Correct Observation. 



Before we proceed to draw inferences from any series o 

 recorded facts, we must take great care to ascertain per- 

 fectly, if possible, the external conditions under which the 

 facts are brought to our notice. Not only may the ob- 

 serving 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 events or objects of different kinds 

 existing may in any degree differ from the numbers which 

 we are able to record. This difference must if possible 

 be taken into account before we make any inferences. 



6 Maxwell, 'Theory of Heat/ p. 301. 



