280 THE PRINCIPLES OF SCIENCE. [chap, 



never be exhaustively treated. The same kind of questions 

 lecur as regards the solution or absorption of gases in 

 liquids, the pressure as well as the temperature having 

 then a most decided effect, and Professor Roscoe's re- 

 searches on the sul)ject present an excellent example of 

 the successive determination of various complicated laws.^ 



Tliere is hardly a branch of physical science in which 

 similar complications are not ultimately encountered. 

 In the case of gravity, indeed, we arrive at the final 

 law, that the force is the same for all kinds of matter, 

 and varies only with the distance of action. But in 

 other subjects the laws, if simple in their ultimate nature, 

 are disguised and complicated in their apparent resuhs. 

 Thus the effect of heat in expanding solids, and the reverse 

 effect of forcible extension or compression upon the tem- 

 perature of a body, will vary from one substance to 

 another, will vary as the temperature is already higher or 

 lower, and- will probably follow a highly complex law, 

 which in some cases gives negative or exceptional results. 

 In crystalline substances the same researches have to be 

 repeated in each distinct axial direction. 



In the sciences of pure observation, such as those of 

 astronomy, meteorology, and terrestrial magnetism, we 

 meet with many interesting series of quantitative deter- 

 minations. The so-called fixed stars, as Giordano Bruno 

 divined, are not really fixed, and may be more truly 

 described as vast wandering orbs, each pursuing its own 

 path through space. We must then determine separately 

 for each star the following questions : — 



1. Does it move ? 



2. In what direction ? 



3. At what velocity ? 



4. Is this velocity variable or uniform ? 



5. ]f variable, according to what law ? 



6. Is the direction uniform ? 



7. If not, what is the form of the apparent path ? 



8. Does it approach or recede ? 



9. What is the form of the real path ? 



The successive answers to such questions in the case of 

 certain binary stars, have affbi'ded a proof that tlie 



^ Watts' Dictionary of Chemistry, voL ii. p. 790. 



