346 WRITINGS OF JOSEPH HENRY. [1855 



But perhaps the greatest source of dilapidation in a clim- 

 ate like ours, is that of the alternations of freezing and 

 thawing which take place during the winter season; but 

 though this effect must be comparatively large, j^et, 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 

 is convinced that the only entirely reliable means of ascer- 

 taining 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 ex- 

 posed 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 de- 

 rived 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 exposed, and the changes of actions 

 to which they may have been subjected 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 accurate 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 degree — merely probable. 



About twenty years ago an ingenious process was devised 

 by M. Brard, which consists in saturating the stone to be 

 tested with a solution of the sulphate of soda. In drying 

 this salt crystallizes and expands, thus producing an exfolia- 

 tion of surface which is supposed to imitate the effect of frost. 

 Though this process has been much relied on, and generally 

 employed, recent investigations made by Dr. Owen lead us 

 to doubt its perfect analogy with that of the operations of 

 nature. He found that the results produced by the actual 



