202 THE POPULAR SCIENCE MONTHLY. 



agined tliat the respiration of the new inmates increases the amount 

 of carbonic acid in the air, and accelerates the process, setting free 

 the water, which renders ihe wall damp and chokes up its pores. 



This explanation is not based on any single direct observation 

 made on the wall itself; it is nothing but a specious conclusion. Al- 

 thouo-h hydrate of lime exposed to air which contains carbonic acid 

 chano-es into carbonate of lime, no one has ever found it becoming 

 moist. The liberation of the water, however much it may be acceler- 

 ated in the indicated way, is unable to refill the pores of the wall, 

 which were supposed to be already filled with air. To do this it would 

 be necessary that the water in the hydrate should not have occupied 

 any space, or that, when set free, it underwent such an expansion as 

 water becoming a gaseous substance. All scientific analogies and ob- 

 servations protest against this. Changes of solid into liquid bodies 

 take place without any considerable increase of volume ; it is different 

 with the transition of liquids into gases when the increase is very con- 

 siderable. 



It is only by the complete choking up of the pores by water, and 

 the complete expulsion of the air from the sui'face of the wall, that 

 the damp spots can be formed ; and the freed water of the hydrate, 

 whicli cannot fill a space which it had not filled while in its former 

 combination, cannot do this. So the absorption of the carbonic acid 

 is unable to produce the required increase of volume. 



The fresh spots in new buildings can only arise from the precipita- 

 tion of water from the air on the walls. 



The inhabitants of a house give rise to a great amount of watery 

 vapor, not only by the functions of their lungs and skin, but also by 

 the nuraei'ous manipulations of the household, such as cooking, wash- 

 ing, cleaning, etc. If the air in the house is already saturated with 

 water in proportion to its temperature, a small degree of cold in the 

 wall is sufficient to produce a dew, a precipitation of water from the 

 vapor, just as one sees it on window-panes. But the porous wall can 

 imbibe a good deal, and in old buildings we may see the windows 

 sweating profusely while the walls seem to remain dry. It may last 

 a long time before a well-constructed wall or partition gives any sign. 

 They go on condensing water till their pores are filled and all the air 

 expelled then, not slowly and gradually, but all at once, numerous 

 damp spots make their appearance. 



It is, therefore, clear why those youngsters of houses are so much 

 more subject to damp spots than their brethren of more mature age. 

 Their walls have lost just enough of the building-water to allow the 

 air to occupy part of the pores ; optically, they seem dry, but still 

 very little water is required to choke up the pores here and there 

 anew, and wherever this takes place the spots break out. The effect 

 of a fire is very instructive ; notliing produces damp spots so easily in 

 a fresh building as the first fire, when doors and windows are well 



