336 Mr._J. Napier on Damp IVulLi. 



slate embedded in Roman cement above the surface of the ground. 

 Were one layer put below, and another above the base course, no damp 

 would penetrate them. 



LlilESTOITE 



differs entirely from sandstone in its constitution : the latter is a rock 

 composed of siHceous particles cemented together by lime under pres- 

 sm-e. Limestone is a mineral chemically combined ; nevertheless its 

 physical structure is exceedingly varied, including from the marl to the 

 marble. It in general absorbs water much more slowly than sandstone, 

 and the capillary attraction is very slow, requiring, according to several 

 experiments I made with compact limestone, several hours to ascend one 

 inch ; nevertheless, although requiring more time to become wet, it in 

 many instances absorbs more water than some sandstones, and is much 

 longer in drying. I have not had the same opportunity of experimenting 

 or making observations upou limestones used for building purposes as 

 upon sandstones, and must therefore refer to the experiments and obser- 

 vations of others. The report of the commissioners upon building stones 

 for the New Houses of Parliament supplies a good deal of information 

 upon this question ; and from that report it appears, that limestone, when 

 thoroughly dry, and then put into water, absorbs a great quantity of it, 

 varying from half a gallon to two gallons per cubic foot of stone, equal 

 to that which I obtained in the sandstone. It may be stated in general 

 terms, that the oolite and magnesian limestones are most absorbent, 

 averaging 1^ gallon per cubic foot of stone, and the common compact 

 limestone least absorbent, averaging not more than f of a gallon per 

 cubic foot of stone. Analyzing the same class of limestones that differ in 

 this absorbing property, I found nothing to suggest a chemical cause for 

 the difference, and think it depends upon the pliysical structure. 



The slowness with which lime absorbs water is a feature I wish to be 

 remembered, as it has a peculiar bearing on damp walls to be after- 

 wards noticed. 



Bkicks. 



The next building materials to be considered are bricks. I need hardly 

 say that they vary much in their character, as the most casual observer 

 cannot fail to notice ; both their physical and chemical constitution, their 

 property of absorbing moisture, and the rapidity with which they do so 

 varying very greatly. This, however, depends much upon their baking. 

 When the brick has been so situated in the kiln or oven, that a partial 

 glaze has been, formed upon the surface, such bricks absorb little 

 water, and slowly and almost wholly by parts not so glazed According to 

 a number of experiments made upon different qualities of bricks, I found 

 them to absorb from 5 to 13 per cent, of their weight of water, being 



