210 Prof. J. D. Dana on some Results of 



resistance is proportional to the intensity : as we perceive from 

 what has been before adduced, in that case the division must 

 take place in the way experiment teaches that it does. 



But if galvanic resistance actually is, as we have endeavoured 

 to demonstrate, proportional to the intensity, it may possibly be 

 asked how this circumstance could so long have escaped obser- 

 vation in the determination of the resistances of conductors. 

 The reason for it can readily be discerned. When the resistance 

 of a conductor is to be investigated, either the conductor is in- 

 serted in the undivided circuit of a galvanic series, and the dimi- 

 nution of intensity thereby produced is compared with the 

 diminution of the same current-intensity occasioned by another 

 conductor of known resistance, or else a division of the current 

 is made use of by employing a differential galvanometer or a 

 Wheatstone's bridge. In the former case Ohm's formula is used 

 for the calculation, and in the latter the formulae which give the 

 division of the current among several conductors. But in all 

 these formulae no other resistances occur than those belonging 

 to the unit of intensity. Therefore, in the methods employ ed, 

 only resistances with equal intensity, namely intensity 1, have 

 to be compared one with another ; and from such a comparison 

 it is impossible to draw the conclusion that the resistance in- 

 creases with the intensity. Consequently the question of the 

 dependence of the resistance on the intensity can only be solved 

 in a theoretical way. 



XXII. On some Results of the Earth's Contraction from Cooling, 

 including a discussion of the Origin of Mountains. By James 

 D. Dana. — Part II. The Condition of the Earth's Interior, 

 and the connexion of the facts with Mountain-making.— 

 Part III. Metamorphism*. 



[Continued from p. 140.] 

 II. The Condition of the Earth's Interior. 



THE condition of the earth's interior is not among the geo- 

 logical results of contraction from cooling; but these 

 results offer an argument of great weight respecting the earth's 

 interior coudition, and make it desirable that the subject should 

 be discussed in this connexion. Moreover the facts throw ad- 

 ditional light on the preceding topic — the -origin of mountains. 

 It seems now to be demonstrated by astronomical and physical 

 arguments (arguments that are independent, it should be noted, 

 of direct geological observation), that the interior of our globe is 

 essentially solid. But the great oscillations of the earth's sur- 



* For Part I., on the Orig'n of Mountains, see p. 41. 



