304 TESTING BUILDING MATERIALS. 



removed at each time may not be indicated by the nicest balance. 

 An examination of the basin which formerly received the water from 

 the fountain at the western entrance of the Capitol, now deposited in 

 the Patent Office, will convince any one of the great amount of action 

 produced principally by water charged with carbonic acid. Again, 

 every flash of lightning not only generates nitric acid — which, in 

 solution in the rain, acts on the marble — but also by its inductive 

 efiects at a distance produces chemical changes along the moist wall, 

 which are at the present time beyond our means of estimating. Also, 

 the constant variations of temperature from day to day, and even from 

 hour to hour, give rise to molecular motions which must affect the 

 durability of the material of a building. Recent observations on the 

 pendulum have shown that the Bunker Hill monument is scarcely for 

 a moment in a state of rest, but is constantly warping and bending 

 under the influence of the ever varying temperature of its different sides. 



Moreover, as soon as the polished surface of a building is made rough 

 from any of the causes aforementioned, the seeds of minute lichens and 

 mosses, which are constantly floating in the atmosphere, make it a 

 place of repose, and from the growth and decay of the microscopic 

 plants which spring from these, discoloration is produced, and disin- 

 tegration assisted. But perhaps the greatest source of dilapidation in 

 a climate like ours is that of the alternations of freezing and thawing 

 which take place during the winter season ; but though the effect of 

 this must be comparatively large, yet, in good marble, it requires the 

 accumulated results of a number of years in order definitely to estimate 

 its amount. 



From a due consideration of all the facts, the commission are con- 

 vinced that the only entirely reliable means of ascertaining the com- 

 parative capability of marble to resist the weather is to study the actual 

 effects of the atmosphere upon it, as exhibited in buildings which for 

 years have been exposed to these influences. Unfortunately^ however, 

 in this country, but few opportunities for applying this test are to be 

 found. It is true some analogous information may be derived from 

 the examination of the exposed surfaces of marble in their out crops 

 at the quarry ; but in this case the length of time they have been ex- 

 posed, and the changes of actions to which they may have been sub- 

 jected during, perhaps, long geological periods, are unknown ; and 

 since different quarries may not have been exposed to the same action, 

 they do not always afford definite data for reliable comparative 

 estimates of durability, except where different specimens occur in the 

 same quarry. 



As we have said before, the art of testing the quality of stone for 

 building purposes is at present in a very imperfect state ; the object is 

 to imitate the operations of nature, and at the same time to hasten 

 the effect by increasing the energy of the action, and, after all, the 

 result may be deemed but as approximative, or, to a considerable de- 

 gree, merely probable. 



Al3out twenty years ago an ingenious process was devised by M. 

 Brard, which consists in saturating the stone to be tested with a solu- 

 tion of the sulphate of soda. In drying, this salt crystallizes and ex- 

 pands, thus producing an exfoliation of surface which is supposed to 



