# 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 is manifest that if the amount of water absorbed was incon- 

 siderable, the mere crystallisation of the salt upon the surface 

 would produce little or no change. 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 caiise 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 Avill 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 



