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effect in overcoming chemical forces, can we escape the con- 
clusion that in this case we directly place our finger upon the 
felspars as the source of weakness. 
Not only are the felspars of the granites weak chemically, 
but as regards their crystalline structure, they are so built up 
that they easily give way ; being in fact a network of meshes, or 
a lattice work, by no means a solid structure, held merely 
together by the force of cohesion. 
We would indicate the lines of weakness that in this 
instance of the felspars, mineralogy denotes. For in the 
presence of higher temperatures, that allied property of matter, 
the conductivity is too important to omit; and besides, there 
are the lines of least resistance, or the passages through which 
the heat courses and vibrations act most energetically. Such 
are :— 
1. The cleavage planes of the crystals of felspar, and 
especially easy are those of mica (Biotite). 
2. The twinning planes of the double crystals. (In Albite, 
single are infrequent.) 
3. The fine lamellar structure of some species. 
4. The rounded grains of the granulites favour heat. 
5. The thermic properties too of the crystals are to be 
taken into account. 
All these properties incidental to crystalline substance, are 
on the side of thermic energy, and facilitating its action 
enables it to subdue and overcome the opposing force of 
cohesion of the molecules, and their power of resistance, and 
rupture and disintegration ensue. Brittleness is a state that 
sets in when to a heated substance like granite at a higher 
temperature, either cold water or cold air is applied, when the 
friable state or brittleness intervenes, it does not break up the 
granite, it only so far diminishes the force of cohesion as to ren- 
der it less powerful to resist the violent vibrations of the heat. 
The diffusion of heat through the granite by conduction, can be 
computed by obtaining the measures of the conductivity. The 
problem is to find the dimension of k, the specific thermal 
conductivity. To find the dimension and the measures, whether 
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