xxix.] EXCEPTIONAL PHENOMENA. 659 



Some of the most invariable laws of nature are disguised 

 by interference of unlooked-for causes. While the baro- 

 meter was yet a new and curious subject of investigation, 

 its theory, as stated by Torricelli and Pascal, seemed to be 

 contradicted by the fact that in a well-constructed in- 

 strument the mercury would often stand far above 3 1 inches 

 in height. Boyle showed 1 that mercury could be made 

 to stand as high as 75 inches in a perfectly cleansed tube, 

 or about two and a half times as high as could be due to 

 the pressure of the atmosphere. Many theories about 

 the pressure of imaginary fluids were in consequence put 

 forth, 2 and the subject was involved in much confusion 

 until the adhesive or cohesive force between glass and 

 mercury, when brought into perfect contact, was pointed 

 out as the real interfering cause. It seems to me, how- 

 ever, that the phenomenon is not thoroughly understood 

 as yet. 



Gay-Lussac observed that the temperature of boiling 

 water was very different in some kinds of vessels from 

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

 metallic surfaces or sharply broken edges that the tem- 

 perature is fixed at 100 C. The suspended freezing of 

 liquids is another case where the action of a law of nature 

 appears to be interrupted. Spheroidal ebullition was at 

 first sight a most anomalous phenomenon ; 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 conjurors who handle liquid metals are 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 pieces of 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 



1 Discourse to the Royal Society, 28th May, 1684. 



2 Robert Hooke's Posthumous Works, p. 365. 



U U 2 



