392 Royal Society :— 
and so rapidly repeated as to melt almost into a musical tone. He 
referred to slight irregular variations in the bending motion of the 
line marked by a row of pins on a glacier by Prof. Forbes, as being 
an indication of the absence of any quality that could properly be 
called viscosity, and of the occurrence of successive fractures and 
sudden motions in a material not truly viscous or plastic. I can only 
understand his statements on this subject by supposing that he con- 
ceived the material between the cracks to be rigid, or permanent in 
form, when existing under strains within the limit of its strength, or 
when strained less than to the point of fracture, 
*. This theory appeared to me to be wrong*; and I then published, 
in a paper communicated to the Royal Society, a theory which had 
occurred to me mainly in or about the year 1848, or perhaps 1850; 
but which, up till the date of the paper referred to, had only been 
described to a few friends verbally. ‘That theory of mine may be 
sketched in outline as follows :—If to a mass of ice at its melting- 
point, pressures tending to change its form be applied, there will be 
a continual succession of pressures applied to particular parts— 
liquefaction occurring in those parts through the lowering of the 
melting-point by pressure—evolution of the cold by which the so 
melted portions had been held in the frozen state—dispersion of the 
water so produced in such directions as will afford relief to the 
pressure—and recongelation, by the cold previously evolved, of the 
water on its being relieved from this pressure: and the cycle of 
operations will then begin again; for the parts re-congealed, after 
having been melted, must in their turn, through the yielding of other 
parts, receive pressures from the applied forces, thereby to be again 
liquefied and to proceed through successive operations as before. 
Professor Tyndall, in papers and lectures subsequent to the publi- 
* While the offering of my own theory as a substitute for Professor Tyndall’s 
views seems the best argument I can adduce against them, still I would point to 
one special objection to his theory. No matter how fragile, and no matter how much 
fractured a material may be, yet if its separate fractured parts be not possessed of 
some property of internal mobility, 1 cannot see how a succession of fractures is 
to be perpetuated. A heap of sand or broken glass will either continue standing, 
or will go down with sudden falls or slips, after which a position of repose will be 
attained ; and I cannot see how the addition of a principle of reunion could tend 
to reiterate the fractures after such position of repose has been attained. When 
these ideas are considered in connexion with the fact that while ice is capable of 
standing, without immediate fall, as the side of a precipitous crevasse, or of lying 
without instantaneous slipping on a steeply sloping part of a valley, it can also 
glide along, with its surface nearly level, or very slightly inclined, I think the 
improbability of the motion arising from a succession of fractures of a substance 
having its separate parts devoid of internal mobility will become very apparent. 
If, on the other hand, any quality of internal mobility be allowed in the fragments 
between the cracks, a certain degree at least of plasticity or viscosity is assumed, 
in order to explain the observed plasticity or viscosity. That fractures—both 
large and exceedingly small—both large at rare intervals, and small, momentarily 
repeated—do, under various circumstances, arise in the plastic yielding of masses 
of ice, is, of course, an undoubted fact: but it is one which I regard not as the 
cause, but as a consequence, of the plastic yielding of the mass in the manner 
supposed in my own theory. It yields by its plasticity in some parts until other 
parts are overstrained and snap asunder, or perhaps also sometimes slide suddenly 
past one another. 
