REPORT ON SOME CARBONIC ACID TESTS ON THE 

 WEATHERING OF MARBLES AND LIMESTONES. 



By George P. Merrill, 



Head Curator, Department of Geology, United States National Museum. 



The tests registered below were made with a view of determining 

 not merely the relative solubility of certam calcareous rocks used for 

 building and ornamental work, but as well, the manner in which the 

 solvent acted. The ultimate aim of the experiments, as is obvious, 

 was to ascertam how the stones would withstand the effects of an 

 atmosphere and its rainfall made acid through absorbed carbonic 

 acid. To make the results appreciable within a reasonable time, it 

 was of course necessary to exaggerate the conditions. The process 

 was as follows : Two samples of each stone selected were cut into the 

 form of cubes approximately an inch in diameter, though without 

 any attempt at exact correspondence in weight. How close the 

 approximation is shown in the accompanying table of results. 



The surfaces of each cube were rubbed with flour of emery on a glass 

 plate as smooth as the nature of the material permitted, but no 

 attempt was made to polish. They were then thoroughly washed 

 and dried at 100° C. The cubes were then suspended by threads, in 

 each case passed but once around the cube, in a large jar of water 

 kept acid by a stream of carbonic acid from a charged cylinder. The 

 water was changed once each week. No attempt was made to have 

 the stream of bubbles constant and continuous, but the direction 

 was changed occasionally to make certain that aU were subjected to 

 like conditions. Twice during the trial the cubes were withdrawn 

 and while still suspended dried out by artificial heat and again 

 immersed. At the end of 3 months they were all withdrawn, dried 

 at a temperature of 100 degrees, and brushed off ^vith a soft fitch brush 

 to remove any loosened granules or dust. The appearance of each 

 cube was carefully noted as to color changes as weU as to the manner 

 in which the solvent acted. The tables below give the weight of the 

 cubes before and after and the loss of material both in weight and 

 in percentage amounts. The first table gives the results of some pre- 

 liminary tests which were not carried to completion, owing to imper- 

 fection of apparatus. They are, however, included here, since so 

 far as they go they are confirmatory of those in the second. The 

 results in both cases agree surprismgly weU. It will be noted that 

 while the amount of material lost in the first series is less than in the 

 second, owing to the shorter period of trial, the two are always in 

 accord. The amount of material lost by solution is not, however, 



Proceedings U. S. National Museum, Vol. 49— No. 2108. 



347 



