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Mr. J. Napier on Damp Walls. 



may be stated shortly, as stones, bricks, and mortar. Stones employed 

 for building purposes may be comprised under two sorts, sandstone and 

 limestone; trap and granite are not in general use for this purpose. 

 Wliere sandstone can be had plentifully it is most generally used ; never- 

 theless many erections are built by preference of limestone ; but the 

 choice obviously depends upon which is most easily and cheaply obtain- 

 able. In and around Glasgow sandstone is universally used. 



In my former paper I showed that sandstones differ materially both 

 in their physical and chemical structure, and in nothing more than in 

 their power of absorbing, retaining, and giving out moisture. As for 

 instance, stones containing much clay become sooner wet when ex- 

 posed to a damp atmosphere than those having little clay : that two 

 stones containing different quantities of clay being equally wet, that 

 having the least clay will be soonest dry when exposed to a dry atmo- 

 sphere : that sandstone brought into contact with water, absorbs it 

 rapidly, and in great quantity ; and that if only a small part of a stone 

 be in contact with water, it is soon diffused through the stone by capillary 

 attraction ; nay, if placed upon a wet substance, such as clay or wet earth, 

 it will absorb the water from these freely, but this capillary attraction 

 differs in rapidity with the structure of the stone. The rate at which 

 this takes place I have made the subject of a few laboratory experi- 

 ments, most of which have been borne out by observations made 

 upon houses during last winter. The first experiment was with three 

 pieces of stone of different structure, one fine grained, the other coarser, 

 and the third still coarser, all irom one quarry, and having about equal 

 quantities of clay. The stones were dried and laid in water to the depth 

 of half an inch, there being 4 inches above the water ; the water rose in 

 these at the following rate : — 



Here there is a vast difference in absorption. The next trial was with 

 two pieces of equal fineness from different quarries, the one had 14 per 

 cent, of clay the other 12 per cent. These were treated in the same 

 manner, but there were seven inches above the water. 



