453 



hot climates*), in consequence of the soft nature of the substance 

 exposed by the polishing, or of its nature being such as readily to 

 soften by a temperature very much below that of the proper fusion, 

 or even softening, of the glass in its integrity. The state of the in- 

 terior portions of a plate of plate-glass appears, therefore, to be similar 

 to that of glass in general at certain temperatures much below its 

 fusing-point, when it presents such remarkable characters of plasticity, 

 tenacity, and ductility f . 



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, would of course be insignificant in depressing the tem- 

 perature of fusion of the glass ; but the pressure occasioned by the 

 cohesive force 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 phe- 

 nomenon 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 



* These particular facts were communicated to me by Mr. Tite, F.R.S., who had 

 himself observed them. 



t 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. 



