28 Building Materials. 
mechanical force thus exerted very gradually effected the 
disintegration of the surface. 
A good building stone would long resist such experiments, for 
it itis manifest that if the amount of water absorbed was incon- 
siderable, the mere crystallisation of the salt upon the surface 
would produce little or nochange. In estimating the durability 
of a building-stone it is particularly necessary to note what 
quantity of water it will absorb, and whether it long retains 
moisture. ‘This is a matter of primary importance where the 
frosts are severe and of long continuance, and it ought not 
the less to enter into our calculations, for we have climatic 
variations in this country which tend almost as rapidly to 
disintegration. 
Water requires its greatest density between the temperatures 
of 39° and 40° Fahrenheit, from which point both heat and 
cold cause expansion. Here, where we have sometimes a 
deluge of rain succeeded by extreme heat, it follows that 
the mere expansion of the fluid rapidly exerted, must cause 
the destruction of an absorbent body. 
If we observe the rocky bed of a river or creek where the 
water and the sun have alternately acted upon the stone it 
will be seen that the surface is, in every case, in a state of 
decay, sometimes to the depth of several inches. This 
’ illustration is drawn from the highly indurated Trappean , 
rocks which abound in our neighbourhood. How much 
more liable are the ordinary Sandstones to such influences. 
We must also take into account the dew which falls very 
heavily throughout some months of the year. Moisture 
absorbed in such a state will tend to lower the temperature of 
the walls, and that again is as quickly raised during the day. 
It is the continual repetition of these influences which 
ultimately destroys the cohesion of bodies so constituted. 
An accurate analysis of the Boroondara stone would give 
those constituents which are found in some of the best 
Sandstones, but that does not in itself afford us criteria 
whereby we may judge of its value. It is more dependent 
upon the mechanical structure, due to compression, and the 
unretarded progress of those chemical changes which tend to 
consolidate the mass. 
My meaning may be better understood by examining its 
fracture. We do not find the particles of quartz shivered and 
split, but they uniformly separate from the cement, leaving 
perfect casts of the imbedded grains. 
The freedom with which this stone may be wrought into 
