326 THE PRINCIPLES OF SCIENCE. 



from what it was in others. It is only in contact with 

 metallic surfaces or sharply broken edges that the tem- 

 perature is at all fixed at 212 Fahr. The suspended 

 freezing of liquids is another case where the action of 

 a law of nature appears to be interrupted. Spheroidal 

 ebullition seemed at first sight a most anomalous phe- 

 nomenon ; it was almost incredible that water should not 

 boil in a red-hot vessel, or that ice could actually be 

 produced in a red-hot crucible. These paradoxical results 

 are now fully explained as due to the interposition of a 

 non-conducting film of vapour between the globule of 

 liquid and the sides of the vessel. The feats of con- 

 jurors who handle liquid metals are readily accounted 

 for in the same manner. At one time the passive state 

 of steel was regarded as entirely anomalous. It may 

 be assumed as a general law that when two pieces re- 

 spectively oi electro-negative and electro-positive metal 

 are placed in nitric acid, and made to touch each other, 

 the electro-negative metal will undergo rapid solution. 

 But when iron is the electro-negative and platinum the 

 electro-positive, the solution of the iron entirely and 

 abruptly ceases. Faraday ingeniously proved that this 

 effect was due to a thin film of oxide of iron, which forms 

 upon the surface of the iron and protects it k . 



The law of gravity is of so simple and general a cha- 

 racter, and is apparently so disconnected from the other 

 laws of nature, that it never suffers any disturbance, and 

 is in no way disguised, but by the complication of its own 

 effects. It is otherwise, however, with those entirely 

 secondary laws of the planetary system, which have only 

 an empirical basis. The fact that all the long known 

 planets and satellites have a similar motion from west to 

 east is not necessitated by any principles of science, but 

 points merely to some common condition existing in the 

 k 'Experimental Researches in Electricity/ vol. ii. pp. 240-245. 



