422 THE PRINCIPLES OF SCIENCE. [CHAP. 



the poles is of no importance, being merely the path 

 through which the electric force reaches the liquid acted 

 upon. Poles of water, charcoal, and many diverse sub- 

 stances, even air itself, produced similar results ; if the 

 chemical nature of the pole entered at all into the question, 

 it was as a disturbing agent. 



It is an essential part of the theory of gravitation that 

 the proximity of other attracting particles is without effect 

 upon the attraction existing between any two molecules. 

 Two pound weights weigh as much together as they do 

 separately. Every pair of molecules in the world have, as 

 it were, a private communication, apart from their rela- 

 tions to all other molecules. Another undoubted result of 

 experience pointed out by Newton l is that the weight of 

 a body does not in <he least depend upon its form or 

 texture. It may be added that the temperature, electric 

 condition, pressure, state of motion, chemical qualities, and 

 all other circumstances concerning matter, except its mass, 

 are indifferent as regards its gravitating power. 



As natural science progresses, physicists gain a kind of 

 insight and tact in judging what qualities of a substance 

 are likely to be concerned in any class of phenomena. The 

 physical astronomer treats matter in one point of view, 

 the chemist in another, and the students of physical optics, 

 sound, mechanics, electricity, &c., make a fair division of 

 the qualities among them. But errors will arise if too 

 much confidence be placed in this independence of various 

 kinds of phenomena, so that it is desirable from time to 

 time, especially when any unexplained discrepancies come 

 into notice, to question the indifference which is assumed 

 to exist, and to test its real existence by appropriate 

 experiments. 



Simplification of Experiments. 



One of the most requisite precautions in experimentation 

 is to vary only one circumstance at a time, and to main- 

 tain all other circumstances rigidly unchanged. There are 

 two distinct reasons for this rule, the first and most ob- 

 vious being that if we vary two conditions at a time, and 



1 Principia, bk. iii. Prop. vi. Corollary i. 



