BUILDING AND ORNAMENTAL STONES. 333 



freezing temperatures upon stones saturated with moisture. At a tem- 

 perature of 30° Fahr. the pressure exerted by water passing from a liquid 

 to a solid state amounts to not less than 138 tons to the square foot, or 

 as Professor Geikie has strikingly put it, is equal to the weight of col- 

 umn of ice a mile high. Is it, then, astonishing that a porous sandstone 

 exposed in a house-front to be saturated by a winter's rain and then 

 subjected to temperatures perhaps several degrees below the freezing 

 point shows signs of weakness and exfoliation after a single season's 

 exposure ? 



Since, then, as every quarryman knows, no stone, however strong, 

 can endure the enormous strain to which it would be subject if frozen 

 solid when' holding any considerable amount of water confined within 

 its pores, it is but natural to conclude, as a matter of course, that other 

 things being equal those stones are most durable which will absorb and 

 retain the least moisture.* 



This rule is not to be accepted, however, without a considerable grain 

 of allowance, since a coarsely porous stone, though capable of taking 

 up a large amount of moisture will also part with it readily, or if frozen 

 while saturated will permit a considerable proportion of the expansive 

 force of the solidifying w 7 ater to be expended otherwise than in push- 

 ing apart the grains composing it. Otherwise expressed, the w r ater will 

 freeze out of a coarsely i orous stone, while in one that is compact it 

 may create sad havoc. This is well illustrated by the common occur- 

 rence of water freezing in straight cylindrical or widely-expanding ves- 

 sels, and in narrow-necked pitchers and bottles. In the first instance 

 the open space above is sufficient to allow all the expansion to take 

 place vertically. The narrow-necked vessel, on the other hand, is almost 

 invariably broken. 



To ascertain, then, the porosity or ratio of absorption of any stone is 

 an important test.f 



Obviously the best method of ascertaining the power of a stone to 

 withstand the effects of frost is to actually expose prepared blocks to 

 such a temperature, when saturated with water, as to freeze them solid 

 and then note the amount of disintegration, or loss in strength. Uu- 



* '' Other things being equal, it may probably be said that the value of a stone for 

 building purposes is inversely as its porosity or absorbing power." (Hunt, Chein. 

 and Geol. Essays, p. 104.) 



tHunt in a series of tests obtained results as follows : 



Potsdam sandstone, Canada, absorbed from 0.50 to 3.26 per cent, in twenty-four 

 hours. 



Medina sandstone, Canada, absorbed from 3.31 to 4.04 per cent, in twenty-four 

 hours. 



Sub-Carboniferous sandstone, Ohio, absorbed from 9.59 to 10.22 percent, in twenty- 

 four hours. 



Lower Silurian limestones and dolomites, Canada, absorbed from 0.11 to 5.55 per 

 cent, in twenty-four hours. 



Tertiary limestones, Caen, France, absorbed from 15 to 1C.05 per cent, in twenty- 

 four hours. 



