Action of a Galvanic Coil on an external small Magnet. 181 



perfect fluidity, and of the same density every where when at 

 rest, and when in motion varying in density always and at all 

 points in exact proportion to variations of its pressure. Also 

 the size of the atoms is supposed to be so small that even in 

 dense bodies they fill a very small portion of a given space. 



2. These hypotheses, which I have enunciated on several pre- 

 vious occasions, are repeated here for the purpose of directing 

 attention to what especially characterizes them. They involve 

 no assertion that is not comprehensible by the indications of com- 

 mon sensation and experience. It is because they possess this 

 character that the physical theories I have founded on them 

 differ from those generally maintained by contemporary physi- 

 cists, which rest for the most part on experimental data con- 

 joined with arbitrary hypotheses not in the same manner intel- 

 ligible. It does not, however, follow from the dissimilarity of 

 the hypotheses that the two modes of philosophizing are con- 

 tradictory to each other. This I think I shall be able to show 

 by pointing out the distinction between their fundamental prin- 

 ciples, and the consequent relation in which they stand to each 

 other. 



3. For this purpose reference will be more particularly made to 

 the physical theories of magnetism and galvanism, as proposed 

 by Gauss and Ampere, or illustrated and extended by other 

 physicists who have adopted their views. The object of all in- 

 vestigations of this class is to deduce from the results of certain 

 fundamental experiments, by the intervention of arbitrary or 

 provisional hypotheses, mathematical expressions of the laws of 

 the phenomena. Accordingly natural philosophy is not thereby 

 advanced beyond a stage analogous to that to which physical 

 astronomy was brought by the results of Kepler's observations. 

 Newton's hypothesis of a gravitating force varying inversely as 

 the square of the distance, and his discovery of the mode of cal- 

 culating its effects by mathematics, were steps necessary for 

 completing that science, inasmuch as they gave reasons for 

 Kepler's laws. In the empirical theories I am referring to, the 

 consideration of physical force is included, and from certain 

 hypothetical modes of action of the forces mathematical expres- 

 sions of the laws of the phenomena are deduced. But con- 

 fessedly the intrinsic nature of the forces, and the reasons for 

 the facts and hypotheses on which the investigations rest, are 

 left undetermined. 



4. The final stage of physical investigation is reached when 

 explanations of phenomena and of their laws have been given by 

 means of mathematical deductions from hypotheses satisfying 

 the condition of being intelligible from sensation and experience. 

 Till this is done, we can hardly be said to have arrived at theory 



