Mr. J. Napier on Damp Walls. 8-il 



where there is seldom a fire kindled and little ventilation may cause de- 

 composition of the paste and paper, and produce other evils. At all 

 events, paper upon damp walls always has a heavy disagreeable smell. 



Another and very annoying kind of damp occasionally seen upon 

 walls, is caused by certain ingredients in the stone, brick, or piaster, 

 undergoing decomposition. In dry weather the wall seems as if covered 

 by hoar-frost, owing to an efflorescence, and in wet or damp weather 

 this either disappears or seems to run into water, and the wall becomes 

 damp. In other words this efflorescence is very soluble, and when the 

 moisture becomes condensed on the walls, as we have just noticed, if the 

 wall be absorbent, the whole seems to disappear ; but if of limestone or 

 other slow absorbent substance, the wall becomes wet. This efflores- 

 cence upon walls depends, as I have already stated, upon the presence of 

 certain salts in or upon the materials forming the wall. Sandstones 

 occasionally contain sulphuret of iron, which by exposure to air and 

 moisture becomes oxidized, the sulphur being converted into sulphuric 

 acid, which readily combines with some ingredient in the stone, and 

 forms salts, soluble in wat€r, and soon affects the surface of the stone, as 

 described as a fine hoar frost, mixed with sand from the disintegration of 

 the siliceous particles of the stone. Bricks often effloresce from the same 

 cause from the clay or fuel containing iron pyrites, which, during burn- 

 ing, if the heat be not very high, becomes converted into sulphuric acid, 

 which readily combines with one of the ingredients in the clay, and 

 forms salts which are dissolved by the moisture absorbed by the brick, 

 and ultimately brought to the surface. The plaster also is subject to 

 effloresce from the lime, water, or sand containing matters that will form 

 soluble salts. As an illustration, I will suppose the plaster made from 

 the waters of the city wells. According to Dr. R. D. Thomson, the 

 acid iu these wells, taking the average, stands thus in each gallon : — 



13 grains sulphuric acid, 

 10 grains muriatic acid, 

 2-7 grains nitric acid, 



in all 25-7 grains, 



either in combination with bases, or subject to combine with bases, and 

 form salts, giving at least twice its weight, or 67 grains per gallon. Now 

 every 100 pounds of plaster, as put on the walls, contains 50 lbs. or 5 

 gallons water, having in it 255 grains soluble salts. This no doubt 

 seems, and really is, exceedingly little, when diffused through 100 lbs. 

 plaster, covering about one yard of wall ; but it does not remain diffused 

 through the plaster, but is brought to the surface by the water which on 

 being evaporated leaves the solid salt exposed. It is therefore of impor- 

 tance to pay attention to the quality of the water used in mixing lime, 

 Vol. TII.— No. 6. c 



