HI EXPEDITION OF THE " CHALLENGER " 73 



attention of the voyagers been particularly directed 

 to the importance of noting the occurrence of the 

 minutest forms of animal and vegetable life in the 

 ocean. 



Among the scientific instructions for the voyage 

 drawn up by a committee of the Royal Society, 

 however, there is a remarkable letter from Von 

 Humboldt to Lord Minto, then First Lord of the 

 Admiralty, in which, among other things, he 

 dwells upon the significance of the researches into 

 the microscopic composition of rocks, and the dis- 

 covery of the great share which microscopic organ- 

 isms take in the formation of the crust of the earth 

 at the present day, made by Ehrenberg in the years 

 1836-39. Ehrenberg, in fact, had shown that the 

 extensive beds of " rotten-stone " or " Tripoli " 

 which occur in various parts of the world, and 

 notably at Bilin in Bohemia, consisted of accumu- 

 lations of the silicious cases and skeletons of Diato- 

 macece, sponges, and Radiolaria; he had proved 

 that similar deposits were being formed by 

 Diatomacece, in the pools of the Thiergarten in 

 Berlin and elsewhere, and had pointed out that, if 

 it were commercially worth while, rotten-stone 

 might be manufactured by a process of diatom- 

 culture. Observations conducted at Cuxhaven in 

 1839, had revealed the existence, at the surface of 

 the waters of the Baltic, of living Diatoms and 

 Radiolaria of the same species as those which, in 



