180 Headden — Silicic Acid in Mountain Streams. 



finely powdered felspar. The object of the experiment was to 

 discover the line of decomposition suffered by this mineral 

 under ordinary conditions and at or near the surface of the 

 rock masses. In passing I will state that my observation upon 

 the eifect of the presence of sodic carbonate, in so far as the 

 conditions of observation are applicable, is that it does not 

 increase the amount of silicic acid present. This fact may be 

 due to the removal of the silicic acid by precipitation ; be this 

 as it may, the alkaline waters of our foot hills are always poor 

 in silicic acid even though they percolate through a sand rich 

 in felspar. I notice too that the geysers in the Yellowstone 

 National Park richest in silicic acid have an acid reaction. 



The experiments with felspar were conducted as follows : In 

 the first experiment felspar was placed in a gallon aspirator, 

 and treated with distilled water in separate portions, each por- 

 tion remaining in contact with the felspar 48 hours. In 

 order to keep the powdered felspar agitated a current of air 

 was caused to pass through it continuously, and to still further 

 imitate the actual conditions, a small amount of carbonic acid 

 (CO2) was mixed with the air. This was continued until 

 nearly 35 gallons of water were collected, which was evapo- 

 rated to dryness in platinum dishes. I feared to heat the resi- 

 due above the temperature of boiling water before analyzing 

 it. 1 would have evaporated it at a lower temperature had it 

 been feasible for me to do so, for it can scarcely be doubted 

 but that some of the CO^ besides that which was probably 

 present as bicarbonates, was expelled during the continued 

 boiling. The residue, as I analyzed it, was as nearly repre- 

 sentative of the solution as I could make it. 



The amount that went into solution in this experiment was 

 equal to 1'6 grains per gallon. The maximum quantity that I 

 succeeded in getting into solution was 4*536 grains per impe- 

 rial gallon, in which case I treated the pulverized felspar with 

 a saturated solution of COj for 16|^ days. The solution of CO^ 

 was put on the felspar and the bottle sealed. The whole was 

 shaken very frequently, on an average three times hourly dur- 

 ing the day. This sample of felspar was pulverized in an agate 

 mortar to avoid bringing any iron into the solution, as water 

 charged with CO^ dissolves finely powdered iron quite readily. 

 The distilled water used in all of the experiments had been 

 freshly distilled and left no residue on evaporation. The action 

 of the water upon the glass of the containing vessels has been 

 assumed to have been so small as to be negligible. 



The felspar used was a flesh-colored orthoclase from a point 

 of the mountains locally known as the Horsetooth. It was 

 such as is characteristic of the granites of this range. The 

 sample analyzed was perfectly fresh and resulted as follows : 



