R. M. Deeley— -Mountain Building. 115 



"Now, if instead of removing and replacing the weight in the 

 manner supposed, we cause it to rest continuously on the cube, the 

 flattening, which above was intermittent, will be continuous ; no 

 matter how hard the cube may be, there will be a gradual yielding 

 of its mass under pressure." 



Since the above was written a great deal of additional information 

 has been obtained experimentally ; but verj^ few attempts have been 

 made to put this in a form which would be useful to the geologist, 

 or even to the engineer who has not made a special study of the 

 subject; and one may say with truth that although Tyndall's state- 

 ment now seems very far from being a complete or even correct one, 

 many modern Avriters seem to base their geological theories upon 

 some such theory of the solid and liquid states. 



From the scientific point of view there is every reason to believe 

 that the solid and liquid conditions of matter must be regarded as 

 quite distinct physically, and that when the one state passes into the 

 other tbe change is abrupt. 



In the case of a liquid, if it be placed in a hollow cup-shaped 

 vessel it will be found that the upper surface is a perfectly level one. 

 Water, for example, after a few oscillations, settles down quickly. 

 A thick lubricating oil does so more slowly ; but pitch may take 

 weeks or even years to reach the perfectly horizontal position. 

 There is no question but that all liquids, under such conditions, flow 

 until their upper surfaces are quite horizontal, and in this respect 

 they are all true liquids but differ in viscosity. In every portion of 

 the mass the stresses eventually become equal and opposite. 



If small boats be placed on these liquids, in the case of oil and 

 water they will quickly sink until they have displaced their weight 

 of oil or water. In the case of the pitch, a similar boat, when first 

 placed on the surface, will rest there, but it will slowly sink into the 

 pitch until it also floats and displaces exactly its own weight of pitch. 

 Oil, water, and pitch are all perfect liquids, but pitch is more 

 viscous than oil, and oil is more viscous than water. At the same 

 temperature and pressure the fluidity (viscosity) is always the same 

 in the case of liquids possessing definite chemical compositions, and 

 may be expressed in terms of some particular unit such as Poise, 

 which is a C.G.S. scale. 



A solid maybe either hard or soft, but however soft it may be it is 

 not a liquid. Thus, if we had a very large vessel partly full of soft 

 clay (a material much softer than pitch), its upper surface would 

 never become quite flat. If a heavy boat were placed upon the soft 

 clay the boat would sink some distance into it, but it would never 

 sink until it displaced an equal weight of clay. Indeed, it would 

 sink to a certain distance into the clay quite rapidly, and would then 

 come practically to a standstill. However, it might go on con- 

 tinuously sinking slowly and yet never displace its weight of clay. 

 This may be illustrated in the following way: Imagine that the rate 

 at which it sinks gets slower and slower. Take it that in the first hour 

 it sinks half a yard. In the next hour it sinks one-quarter of a yard, 

 in the next one-eighth of a yard, and so on. At this rate it will 

 always be sinking, but will never sink more than one yard. Tyndall's 



