1855] WRITINGS OF JOSEPH HENRY. 347 



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 use 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, has 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 inclosed 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 satisfactorily comparable results 

 can be obtained. In prosecuting this part of the inquiries, un- 

 foreseen difficulties have occured 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 foreseen at the commencement. 



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

 it undertook the investigation held the opinion,) that but 

 little difficulty would be found in ascertaining the strength 

 of the various specimens of marbles. In this however it was 

 in error. The first difficulty which occurred was to procure 

 the proper instrument for the purpose. On examining the 

 account of that used by Rennie, and described in the Trans- 

 actions of the Royal Society of London, the commission found 

 that its construction involved too much friction to allow of 

 definite comparative results. Friction itself has to be over- 

 come as well as the resistance to compression, and since it 

 increases in proportion to the pressure, the stronger stones 

 would appear relatively to withstand too great a compress- 

 ing force. 



The commission first examined a hydraulic press which 

 had previously been employed in experiments of this kind, 

 for the use of the Government, but found that it was liable 

 to the same objection as that of the machine of Rennie. The 

 commission was however extremely fortunate in obtaining 

 subsequently, through the politeness of Commodore Ballard, 

 commandant of the Nav}'^ Yard, the use of an admirable in- 



