CONCEPTS OF PHYSICAL SCIENCE. 57 



mathematical and the entire superstructure is erected upon the three 

 fundamental quantities L, M and T and certain definitions; just as 

 geometry arises from its axioms and definitions. 



Of many of those physical quantities, for which we are not as yet 

 able to give the dimensional formula, our knowledge is precise and 

 definite, but it is incomplete. In the case, for example, of one im- 

 portant group of quantities, those used in electric and magnetic meas- 

 urements, we have to introduce, in addition to L, M and T, a constant 

 factor to make the dimensional formula complete. This, the sup- 

 pressed factor of Eiicker,* is p, the magnetic permeability, when the 

 quantity is expressed in the electromagnetic system, and becomes Tc, the 

 specific inductive capacity, when the quantity is expressed in terms 

 of the electro-static system. 



Here the existence of the suppressed factor is indicative of our 

 ignorance of the mechanics involved. If we knew in what way a 

 medium like iron increased the magnetic field or a medium like glass 

 the electric field, we should probably be able to express fx and h in 

 terms of the three selected fundamental dimensions and complete the 

 dimensional formulae of a large number of quantities. 



Where direct mechanical knowledge ceases the great realm of phys- 

 ical speculation begins. It is the object of such speculation to place 

 all phenomena upon a mechanical basis; excluding as unscientific all 

 occult, obscure and mystical considerations. 



Whenever the mechanism, by means of which phenomena are pro- 

 duced, is incapable of direct observation either because of its remote- 

 ness in space, as in the case of physical processes occurring in the 

 stars, or in time, as in the case of the phenomena with which the 

 geologist has to do, or because of the minuteness of the moving parts, 

 as in molecular physics, physical chemistry, etc., the speculative ele- 

 ment is unavoidable. Here we are compelled to make use of analogy. 

 We infer the unknown from the known. Though our logic be without 

 flaw and we violate no mathematical principle, yet are our conclusions 

 not absolute. They rest of necessity upon assumptions, and these are 

 subject to modification indefinitely as our knowledge becomes more 



complete. 



A striking instance of the uncertainties of extrapolation and of the 



precarious nature of scientific assumptions is afforded by the various 

 estimates of the temperature of the sun. Pouillet placed this tempera- 

 ture between 1461° C. and 1761° C; Secchi at 5,000,000° ; Ericsson at 

 2,500,000°. The newer determinations, \ of the temperature of the 

 surface are, to be sure, in better agreement. Le Chatelier finds it to be 

 7600° ; Paschen 5400° ; Warburg 6000°. Wilson and Gray publish as 

 their corrected result, 8000°. The estimate of the internal tempera- 

 ture is of a more speculative character. Schuster's computation gives 



* Rucker, Philos. Mag., 27, p. 104, 1889. 



t See Arrhenius, Kosmische Physik, p. 131. 



