Mr. Thompson on separation of Lime from Magnesia, 8fc. 309 



Before the breaking up of ice by a thaw, the holes most 

 usually are to be observed at the sides of rivers. Indeed, I 

 have scarce ever seen them in the middle. After the break- 

 ing up of ice by a thaw, several circumstances serve to in- 

 crease the number of holes. On the sides of rivers, there is 

 generally a collection of small round stones. Upon these, 

 sheets of ice, when floated up by the rise of the tide, are left, 

 and, during the thaw, soon become perforated with holes up- 

 wards. If such holes should not be complete at the time, 

 they are rendered so by subsequent melting. As the stones, 

 in such situations, frequently rise an inch above the surround- 

 ing level, they may perforate holes in ice an inch thick. The 

 holes in ice of greater thickness, are commonly produced by 

 bodies lying above the ice, such as the small stones that are 

 washed upon the ice by the agitation of the water, or bits of 

 bark, or of brown wood. Such light bodies, when they float, 

 sometimes produce holes upwards, by being placed below the 

 ice. Holes break out, also, wherever light bodies have been 

 frozen into ice, as occurs in almost every situation. Collec- 

 tions of sand, earth, or clay, constitute another very usual 

 cause of holes. 



Standing waters afford peculiar occasions for foreign solids 

 to produce holes in its ice. In a pool, where small single 

 blades of grass had been frozen into ice of two or three inches 

 thickness, I have observed that, during a thaw, the ice imme- 

 diately surrounding the grass melted into a round hole, often 

 an inch or two wide; and, in ice upwards of three inches thick, 

 where a few withered rushes — one here, one there — were 

 frozen, in a horizontal position, in the middle of the ice, every 

 rush, after a few days' thaw, had a space around it, large 

 enough to hold ten rushes. The vital heat of the grass may 

 have contributed to the melting; but the dead rushes could 

 only have acted like any other opake inorganic solid. 



Holes of a similar origin to those in ice, accounted for in 

 this Essay, occur also in melting snow, where dust, or other 

 opake solid, has lain on the surface. 



Aberdeen, March 30, 1838. 



XLVI. On the Separation of Lime from Magnesia, and on 

 the Assay of Gold. By Lewis Thompson, Esq., M.R.C.S.* 



To separate Lime from Magnesia. 



he combined earths in dilt 

 and precipitate the filtered s 



• Communicated by the Author. 



"P\ISSOLVE the combined earths in dilute nitric or mu- 

 -*-^ riatic acid, and precipitate the filtered solution by means 



