J". Henry on testing Building Materials. 



31 



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 pro- 

 duced 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 in- 

 ductive effects 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 mole- 

 cular motions which must affect the durability of the material 

 of a building. Kecent 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 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 mi- 

 nute lichens and mosses, which are constantly floating in the at- 

 mosphere, make it a place of repose, and by the growth and 

 decay of the microscopic plants which spring from these, discol- 

 oration is produced, and disintegration is assisted. 



But perhaps the greatest source of the wearing away in a cli- 

 mate like ours, is that of the alternations of freezing and thawing 

 which take place during the winter season; and though this 

 effect must be comparatively powerful, yet, in good marble, it 

 requires the accumulated effect of a number of years in order 

 definitely to estimate its amount. From all these causes, the 

 commission are convinced that the only entirely reliable means 

 of ascertaining the comparative 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 exam- 

 ination of the exposed surfaces of marble in their out-crops at 

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

 exposed, and the changes of actions to which they may have 

 been subjected, during, perhaps, long geological periods, are un- 

 known ; 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 differ- 

 ent 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 degree, merely probable. 



