66 NOTE ON THE PRINCIPLE OF REGELATION, ETC. 
to that of glass in general at certain temperatures much below its 
fusing-point, whenit presents suchremarkable characters of plasticity, 
tenacity, and ductility.* 
Is it possible that a lowering of the melting-point of glass, or of the 
exposed interior portions of it, by pressure, is concerned in the union 
of the two plates? The effect of the mere pressure of the atmo- 
sphere, ensuing upon the exclusion of the air from between the closely 
apposed plates, wouldof course be insignificant in depressing the tem- 
perature of fusion of the glass ; but the pressure occasioned by the 
cohesive foree—exerted, it will be remembered, through a very small 
thickness only of the material_—which finally unites two or more 
plates into one block, would probably be adequate to any conceivable 
effect of this nature which can be required for the production of the 
phenomenon observed. 
It may appear at first sight, that the fact that glass belongs to 
that class of bodies which contract on passing from the liquid to the 
solid state, and the melting-point of which, therefore, would be ele- 
vated—not depressed—by pressure, is opposed to this possibility. 
The objection would be a valid one were we now concerned with 
glass in a crystalline state. But we are treating of that substance 
in its familiar and ordinary condition, into which it passes from liqui- 
dity by a continuous gradation of temperature, through equally 
continuous states of softness into the solid form, like melted phos- 
phorus and selenium. 
I am now tempted to ask, in conclusion of this part of the sub- 
ject, Are all cases of the union of two apparently solid surfaces of 
the same substance by cohesive attraction, cases of melting and 
regelation, an infinitesimally 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- 
glass ? 
The negative of the last question does not appear to be proyed by 
* We are reminded by these facts of the view taken by Person, and adopted by Prof. 
Forbes, of the similarity of the liquefaction of ice to that of fatty bodies or of the metals, 
‘all which in melting pass through intermediate stages of softness or viscosity ;” and Sir 
J. F. W. Herschel (Art. “ Meteorology,” par. 119, Enc. Brit. eighth edit.), when he terms 
regelation “a sort of welding,” appears to concur in this view. 
