280 On the Occurrence of Silver in Volcanic Dust. [Mar. 6 y 



On finding the carbonates of calcium and magnesium, I was at first 

 inclined to suspect that they were merely impurities, which had 

 become mingled with the ash in collecting it — fragments of plaster 

 from a wall, or something of that kind — but from their pretty uniform 

 distribution, which was ascertained by two or three separate ex- 

 periments, and from their fine state of division, no particles separately 

 detectable by the naked eye being found, this does not seem likely, 

 and it is rather to be supposed that these carbonates are present as 

 minute particles of a magnesian limestone, torn away by the larger 

 ejected masses from some portion of the inner surface of the crater of 

 the volcano. A large part of the silver of South America is said to 

 occur associated with limestones of Carboniferous age in the Bolivian 

 and Peruvian Andes, but whether such rocks have been observed to 

 extend into Ecuador I do not know. 



As for the distinct trace of organic matter observed, this may in 

 part represent ordinary dust swept down from the lower regions of 

 the atmosphere, but was no doubt partly at least derived from the 

 cloth on which the volcanic ash is said to have been collected, since 

 a few cotton fibres were easily identified under the microscope. 



It was proved, as in the case of the ash from Cotopaxi formerly 

 examined, that the silver present in minute quantity could be dissolved 

 out by boiling with an aqueous solution of ammonia, or of potassium 

 cyanide, or of sodium thiosulphate, but was not appreciably extracted 

 by nitric acid. Hence, as was remarked in the former paper, it seems 



Bischof, ' Elements of Chem. and Phys. Greology, translated for the Cavendish 

 Society,' vol. 3, pp. 392—393.) 

 B. 1, 2, and 3. Analyses of dust from the great eruption of Krakatoa in the Sunda 

 Strait, August 26 and 27, 1883. (' Report of the Committee of the Royal 

 Society on the Eruption of Krakatoa,' p. 40.) 1, referring to the dust which 

 fell at Krakatoa ; 2, to that which fell at points within 100 miles from the 

 volcano ; and 3, to a specimen which fell nearly 900 miles from the yolcano ; 

 volatile matters are omitted, and the totals calculated to 100 parts. 



B. 



—A- 



2 3 



66-77 .... 68 -99 



0*67 .... 0-39 



16-44 .... 15 24 



3- 41 0-28 



1-37 372 



0*38 trace 



1- 67 .... 0-83 



2- 90 .... 2-76 



4- 14 4 32 



2 25 .... 3-47 



1. 



Si0 2 65 09 .... 61 36 ... 



Ti0 2 — .... 1 12 ... 



A1 2 3 15-58 .... 17-77 .. . 



Fe 2 3 3-83 4-39 ... 



EeO 1-73 .... 1-71 ... 



MnO — 0-41 



MgO 4-10 2-32 



CaO 2-61 3-45 ... 



Na 2 4-46 .... 4 -98 ... 



K 2 1-99 .... 2-51 ... 



Loss by ignitioxi 0*41 



99-8 



100-02 



100-00 



100 -00 



