Illustrative of the Formation of Rocking Stones. 361 



edges had been removed, evaporation took place chiefly from 

 the top piece, which being more freely exposed diminished 

 far more rapidly than the lower piece, which it partially 

 sheltered and protected from evaporation. Hence there was 

 a gradual decline in the loss suffered from day to day, namely, 

 from 10 or 12 grains to 3 or 4. This decline was apparently 

 due to the diminution of surface exposed, rather than to the 

 state of the weather outside. The temperature was noted 

 when the weight was taken, both with the wet and dry bulb ; 

 but being in an inhabited room it did not greatly vary, and 

 the external atmospheric conditions seemed to have but little 

 influence. The experiment was terminated on the 9th Decem- 

 ber, when the two fragments of camphor were weighed 

 separately. The lower piece weighed 54 grains and the 

 upper 14. It will be seen from fig. 3 that the upper frag- 

 ment rests upon a kind of point due to the loss of matter 

 from the upper side of the lower piece, and the under side of 

 the upper piece around the point of contact, and this is exactly 

 what takes place in the logging or logan stones by the slower 

 action of weathering. It will be seen that the conditions are 

 now fulfilled to allow the upper piece to be rocked upon the 

 lower. 



An interesting result was obtained by inserting a square 

 piece of filtering paper between another pair of the 1J inch 

 camphor blocks. The two blocks were held between the 

 finger and thumb, and the paper between them was trimmed 

 along the four sides with a small pair of scissors, so that the 

 paper projected over the lower block only to the extent of 

 half the thickness of the scissor blades. The blocks, put on 

 a glass plate, were placed on a high shelf in an inhabited 

 room, and were left undisturbed for many weeks, when it 

 was found that the slight paper projection had been sufficient 

 to protect the lower block from evaporation, so as to preserve 

 the solid angles and edges. 



It was formerly supposed that light had a subtle but power- 

 ful influence in promoting crystallization and producing 

 deposits of camphor and other volatile bodies on the most 

 illuminated side of the closed vessels containing them. This 

 idea was favoured in our text-books by such recent authorities 

 as Daniell, Brande, and Miller. In the Philosophical Magazine 

 for November 1862, I showed by a number of experiments 

 that these deposits are made, not on the most illuminated side 

 of the vessel, but on the coldest side. With respect to crys- 

 tallization it was supposed that a saline solution would not 

 deposit its salt if the vessel containing it were screened from 

 the light. Thus Chaptal observed that by covering over 



