THE SCIENTIFIC METHOD 137 



And lastly, Kaufmann has shown by a delicate ex- 

 periment that the apparent mass of an electron is a 

 function of its velocity. This conclusion has been con- 

 firmed by others, although in minor points there is a 

 considerable difference in results and opinions. 



It must not be lost sight of that all these experiments 

 deal with quantities of matter, supposing it to exist, 

 too small to be appreciable by either chemical analysis 

 or mechanical apparatus, such as the balance. They 

 are ultimately measured by the force of electrical at- 

 traction of an electrical charge. We are, therefore, 

 experimenting with matter which appeals to us through 

 only one of its attributes. Is it not almost inevitable 

 that an exclusive attention paid to this single attribute 

 is likely to exalt it into an undue prominence? We 

 have had, in the past, examples of much the same sort 

 of reasoning. When the phenomena of light were 

 predominantly discussed, physicists drifted into the 

 opinion that this property of matter could be explained 

 only by creating a light substance. Again, this process 

 of reasoning occurred when heat was first investi- 

 gated; we had the creation of caloric. And now we 

 are asked to do the same thing with electricity. It 

 is safe to predict that history will be repeated again, 

 and that electrical charges and their forces will also 

 sink into the condition of an attribute of matter. 



It might certainly be true that two experiments 

 showing equal electrical charges would, if we could 



