430 COSMOS. 



singault), Eucu-Pichmclia, Antisana, Cotopaxi, Chimborazo,* 



enough that near the surface of the earth the decrease of caloric is 

 slower than at greater elevations, and in order to ascertain with pre- 

 cision the mean density of the whole column of air it would be neces- 

 sary to ascend in a balloon so as to examine the temperature of each 

 successive stratum or layer of the superimposed air" (Humboldt, Ilc- 

 citeil d' Observ. Astron., vol. i., p. 138 ; see, also, 371, in the appendix 

 on refraction and barometrical measurements). While the baromet- 

 rical measurement of MM. Truqui and Craveri gives only 17,159 feet 

 to the summit of Popocatepetl, whereas Glennie gives 17,889 feet, I 

 find that the lately-published measurement of Professor Carl Heller, 

 of Olmutz, who has thoroughly investigated the district surrounding 

 Mexico, as well as the provinces of Yucatan and Chiapa, corresponds 

 to within 32 feet of my own. (Compare my l^ssay on the Height of the 

 J\fexlcan Volcano Popocatep&l, in Dr. Petermann's Mittheilungen aus 

 Justus Perthcs Gcographischer Anstalt, 1856, s. 479-481.) 



* In the Chimborazo rock it is not possible, as in the ^Etna-rock, to 

 separate mechanically the feldspathic crystals from the ground mass 

 in which they lie, but the large proportion of silicic acid which it con- 

 tains, along with the fact connected therewith of the small specific 

 gravity of the rock, make it apparent that the feldspathic constituent 

 is oligoclase. The quantity of silicic acid which a mineral contains 

 and its specific gravity are generally in an inverse ratio; in oligoclase 

 and Labrador! te the former is C4 and 53 per cent., while the latter is 

 2*66 and 2*71. Anorthite, with only 44 per cent, of silicic acid, has 

 the great specific gravity of 2*76. This inverse proportion between 

 the quantity of silicic acid and the specific gravity does not occur, as 

 Gustav Rose remarks, in the feldspathic minerals, which are also iso- 

 morphous, but with a different crystalline form. Thus feldspar and 

 leucite, for instance, have the same component parts potash, alumina, 

 and silicic acid. The feldspar, however, contains 65, and the leucite 

 only 56 per cent, of silicic acid, yet the former has a higher specific 

 gravity, namely, 2'5G, than the latter, whose specific gravity is only 2*48. 



Being desirous, in the spring of 1854, to obtain a fresh analysis of 

 the trachyte of Chimborazo, Professor Rarnmelsberg kindly undertook 

 the task/ and performed it with his usual accuracy. I here give the 

 results of this analysis, as they were communicated to me by Gustav 

 Rose, in a letter in the month of June, 1854. He says : " The Chim- 

 borazo rock, submitted to a careful analysis by Professor Rammels- 

 berg, was broken from a specimen belonging to your collection, which 

 you -had brought home from the narrow rocky ridge at a height of 

 more than lO^OOO feet above the sea," 



ItanunclsLerg' 's Analgsu, 

 (Height, 19,194 English feet; specific gravity, 2'SOG.) 



Ojcrpen. 



Silicic acid .................. 59-12 ... 80-70 2-33 



Alumina ...................... 13-48 ... 6'30 



V 



Protoxyd of iron 7*27 



Lime 6-50 1- 



Magnesia 5-41 2-13 V G'93 



Soda 3-46 0-89 



Potash _2/G4 0'15, 



97-88 



