35 



perfect experimental treatment, since glass, when strongly 

 compressed, and so long only as it is compressed in one 

 direction, becomes capable of doubly-refracting light, and 

 as there is probably no alteration in the glass but change of 

 elasticity, \ve learn that the power of double refraction is 

 very probably due to a difference of elasticity in different 

 directions. 



Removal of Usual Conditions. 



One of the great objects of experiment is to enable us 

 to judge of the behaviour of substances under conditions 

 widely different from those which prevail upon the surface 

 of the earth. We live in an atmosphere which does not 

 vary beyond certain narrow limits in temperature or 

 pressure. Many of the powers of nature, such as gravity, 

 which constantly act upon us, are of almost fixed amount. 

 Now it will afterwards be shown that we cannot apply a 

 quantitative law to circumstances much different from those 

 in which it was observed, without considerable risk of error. 

 In the other planets, the sun, the stars, or remote parts 

 of the Universe, the conditions of existence must often be 

 widely different from what we commonly expeiience here. 

 Hence our knowledge of nature must remain very re- 

 stricted and hypothetical, unless we can subject substances 

 to very unusual conditions by suitable experiments. 



The electric arc is an invaluable means of exposing 

 metals or other conducting substances to the highest 

 known temperature. By its aid we learn not only that 

 all the metals can be vaporized, but that they all give off 

 distinctive rays of light. At the other extremity of the 

 scale, the intensely powerful freezing mixture devised 

 by Faraday, consisting of solid carbonic acid and ether 

 mixed in vacuo, enables us to observe the nature of sub- 

 stances at temperatures immensely below T any we meet 

 with naturally on the earth's surface. 



D 2 



