-1859] WRITINGS OF JOSEPH HENRY. 243 



from the wall by a stratum of air. The surface of this 

 material readily assumes the temperature of the air, and 

 therefore does not allow of the deposition of much moisture. 

 This internal lining, known by the name of furring, is 

 usually composed of lath and plaster, but in some large 

 buildings it is formed of a single thickness of brick, which 

 prevents transmission of moisture from without, but does not 

 fully obviate the tendency to deposition within, since a large 

 amount of vapor is absorbed through the pores of the coat- 

 ing of plaster into the substance of the brick and again 

 given out with a change of temperature. 



The dampness of newly-plastered walls is in part due to 

 a chemical action, which (paradoxical as it may appear) is 

 not obviated by heating the wall. After a newly plastered 

 room has been dried by an excess of artificial heat, it con- 

 tinues for a long time to give off vapor, and this is due to 

 the chemical change going on while the lime in the plaster 

 is in process of being converted from what is called a hydrate 

 to a carbonate of lime. Perfectly dry slacked lime contains 

 in chemical combination a portion of water, and when it is 

 exposed to the atmosphere it absorbs carbonic acid from the 

 air and expels the water in the form of vapor ; hence, after 

 a plastered wall has been thoroughly dried it ought to be 

 exposed freely to currents of air, which ma}^ furnish the car- 

 bonic acid necessary to expel what may be called the solid 

 water or that of chemical combination. 



The water which is absorbed into the pores of stone by 

 capillary attraction does not change its dimension. Mr. 

 Saxton, of the Coast Survey, has shown that a rod of marble 

 of 3 feet in length is not increased the ten-thousandth part 

 of an inch by soaking it in water from a state of perfect dry- 

 ness produced by heating it in an oven. The experiment 

 was made on the marble of the Capitol, at the request of 

 Captain Meigs, the superintendent of the extension of that 

 national edifice. The absorption of moisture by organic 

 substances however produces a change in their dimensions, 

 which takes place with the exhibition of great force. The 

 water is absorbed in great quantities at the ends of the 



