362 



On the Formation of Rocking Stones. 



certain parts of the evaporating dish with black taffeta, so as 

 entirely to exclude the light, crystals would creep up only on 

 the side to which light had access. But the function of the 

 screen was mistaken ; its action being to prevent evaporation, 

 as may be seen in an experiment so contrived as to admit 

 light freely, but exclude or limit the evaporating force. Fig. 4 

 represents two vessels, on different levels, each containing a 



Fipf. 4. 



saline solution : they were placed on the ledge of a window 

 facing the west, so as to be in the full light of day, and often 

 in sunshine. The top vessel was half covered with a thin 

 glass plate, which projected above and over one half of the 

 lower vessel. The exposed half of each vessel had a crystalline 

 deposit running round it. The left-hand glass contained a 

 solution of acetate of lime. This is well adapted to the purpose 

 of this test experiment. The glass being replenished every few 

 days, large rounded cauliflower masses of great beauty are 

 formed on the uncovered half of the glass, gradually swelling 

 over and adhering to the outside. They are first white, but 

 by exposure to the sun become of a delicate yellow, touched 

 with a deep brown on the most projecting portions, while the 

 solid in the dish is striated after the manner of certain fibrous 

 minerals. The glasses may, however, contain a solution of 

 almost any salt, and a deposit will be found on the uncovered 

 half of either glass, and none on the covered half. 



Highgate, 14th December, 1891. 



