30 THE PRINCIPLES OF SCIENCE. 



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 gra- 

 vitating 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 

 find some effect, we cannot tell whether the effect is due 

 to one or the other, or to both jointly. A second reason 

 is that if no effect ensues we cannot safely conclude that 

 either of them is indifferent ; for the one may have neu- 

 tralized the effect of the other. In our logical formuke, 

 A (B -|- b) is identical with A (see vol. i. p. 112), and B may 

 be indifferently present or absent ; but A (BC -|- be) is not 

 identical with A, and none of our logical processes enabled 

 us to make the reduction. 



