454 



the same substance by cohesive attraction, cases of melting and 

 regelation, an infmitesimally thin film of liquid being momentarily 

 produced and as instantly solidified ? Will two surfaces of perfectly 

 dry ice, at temperatures much below 32, but under favourable me- 

 chanical circumstances, unite by mere apposition and pressure (which 

 ought to follow from Prof. James Thomson's theory), and thus prove 

 the identity of the acting principle in the two cases of ice and plate- 



The negative of the last question does not appear to be proved by 

 the fact cited by Faraday and Tyndall, that dry, hard-frozen snow 

 has not the property of becoming compacted into a snow-ball. The 

 cases seem not to be comparable, because the brittleness of the con- 

 stituent crystals of snow when in this state, its porous nature as a 

 whole, and its being consequently pervaded by air, will prevent the 

 required apposition of surfaces. Nor, as I conceive, is it proved by 

 Prof. TyndalPs most instructive experiment of crushing a ball of ice, 

 cooled by carbonic acid and ether, into white and opake hard frag- 

 ments ; for in this also the required apposition of surfaces would 

 be wanting. Further, it may be asked, whether this very experi- 

 ment does not demonstrate the limitation of the lowering of the 

 melting- or freezing-point by pressure ? and if so, there can be no 

 tendency to union at 100 below freezing. 



In discussing the philosophy of the union of two surfaces of glass, 

 I have alluded to the theory of regelation enunciated by Prof. J. 

 Thomson ; but I wish to be understood as not adopting, exclusively, 

 in these notes, any existing theory on the subject. Admitting the 

 operation of cohesive attraction and consequent pressure in the first 

 instance, the phenomenon, with respect to glass, readily admits of 

 explanation by the original view of Mr. Faraday, which is, " that 

 a film of water must possess the property of freezing when placed 

 between two sets of icy particles, though it will not be affected by a 

 single set of particles." If we regard the two apposed surfaces of 

 glass, each consisting of a thin stratum of particles, taken together, 

 as representing the film of water, then the other strata of particles 

 in contact with them respectively, and making up the entire thick- 

 ness of the plate on each side, will correspond to the two sets of 

 icy particles, the action of which by freezing the film of water effects 

 the union of the two portions of ice, and the phenomenon may be 



