32 



/. Henry on testing Building Materials. 



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 crystal- 

 lizes and expands, thus producing an exfoliation of surface which 

 is supposed to imitate the effect of frost. Though this process 

 has been much relied on, and generally employed, recent investi- 

 gations 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 exposure to freezing and thawing in the 

 air, during a portion of winter, in the case of the more porous 

 stones, produced very different results from those obtained by 

 the drying of the salt. It appears from his experiments, that 

 the action of the latter is chemical as well as mechanical. 



The commission, in consideration of this, have attempted to 

 produce results on the stone by freezing and thawing by means 

 of artificial cold and heat. This process is, however, laborious ; 

 each specimen must be enclosed in a separate box fitted with a 

 cover, and the amount of exfoliation produced is so slight, that 

 in good marble the operation requires to be repeated many times 

 before reliable comparative results can be obtained. In prose- 

 cuting this part of the inquiries, unforeseen difficulties have oc- 

 curred in ascertaining precisely the amount of the disintegration, 

 and it has been found that the results are liable to be vitiated by 

 circumstances which were not in view at the commencement of 

 the inquiries. 



It would seem at first sight, and the commission when they 

 undertook the investigation were of the same opinion, that but 

 little difficulty would be found in ascertaining the strength of 

 the various specimens of marbles. In this, however, they were 

 in error. The first difficulty which occurred was to procure the 

 proper instrument for the purpose. On examining the account 

 of that used by Eennie, and described in the Transactions of the 

 Eoyal Society of London, the commission found that its construc- 

 tion involved too much friction to allow of definite comparative 

 results. Friction itself has to be overcome, as well as the resist- 

 ance to compression, and since it increases in proportion to the 

 pressure, the stronger stones would appear relatively to with- 

 stand too great a compressing force. 



The commission first examined an instrument — a hydraulic 

 press — which had previously been used for experiments of this 

 kind, but found that it was liable to the same objection as that 

 of the machine of Eennie. They were, however, extremely for- 

 tunate subsequently in obtaining, through the politeness of Com- 

 modore Ballard, commandant of the Navy Yard, the use of an 

 admirable imstrument devised by Major Wade, late of the Uni- 

 ted States Army, and constructed under his direction, for the 

 purpose of testing the strength of gun metals. This instrument 



