340 Dr. LeConte o« a remarkable Exudation of Ice 



in the one case, the compactness of the superficial stratum not 

 only diminishes the porosity, but renders the resistance to 

 lateral expansion greater at the surface than it is below, and 

 consequently interferes with the protrusion of the column of 

 ice in the right direction ; while in the other case, the open- 

 ness of the soil prevents the formation of tubes possessing un- 

 yielding sides, a condition which is equally essential to the 

 process. When the intensity of the cold is sufficiently great 

 to freeze the soil, the process is arrested, because the capillary 

 tubes are closed, and a resistance opposed to further protru- 

 sion. The porous appearance presented by the subjacent 

 clay when the icy columns are removed is doubtless referable 

 to the enlargement of the orifices of the minute capillaries, 

 caused by the sudden expansion of the successive portions of 

 fluid as they were frozen at the surface. 



If the above is the true explanation of the phaenomenon, 

 we should expect, from a priori considerations, that in higher 

 latitudes, where the cold is more intense and persistent, the 

 conditions of its manifestation would exist only during the 

 early part of winter, before the ground became deeply and 

 permanently frozen ; or else only in certain favourable situa- 

 tions, — as in the neighbourhood of warm springs, and perhaps 

 along the margins of unfrozen streams, — where local causes 

 preserved the soil in a proper condition. Are not facts in 

 accordance with this view? 



The foregoing explanation appears to afford a satisfactory 

 interpretation of a very remarkable experiment recorded by 

 Sir John Leslie, which is so nearly the counterpart of what 

 takes place in nature, that we cannot forbear citing it on this 

 occasion. He says, in treating of artificial congelation, 

 *' When very feeble powers of refrigeration are employed, a 

 most singular and beautiful appearance is, in course of time, 

 slowly produced. If a pan of porous earthenware, from four 

 to six inches wide, be filled to the utmost with common water 

 till it rise above the lips, and planted above a dish of ten or 

 twelve inches diameter, containing a body of sulphuric acid, 

 and then a broad round receiver placed over it ; on reducing 

 the included air to some limit between the twentieth and the 

 fifth part of its usual density, according to the coldness of the 

 apartment, the liquid mass will in the space of an hour or two 

 become entwined with icy shoots, which gradually enlarge and 

 acquire more solidity, but always leave the fabric loose and 

 unfrozen below. The icy crust which covers the rim, now 

 receiving continual accessions from beneath, rises perpendi- 

 cularly by insensible degrees. From each point on the rough 

 surface of the vessel, filaments of ice, like bundles of spun- 



