PHYSICAL PROPERTIES OF ICE. 641 



appearance, identical with that of sal ammoniac, gradually dis- 

 appearing as the mass thickens and gets more compact ; still, the 

 lowest portion, or that most recently formed, always presents this 

 aspect. When this pellicle or covering becomes of the thickness 

 of a quarter of an inch, or more, small white crystals appear on its 

 surface, at first sparse and widely separated, but gradually forming 

 into tufts, and ultimately covering the whole surface. This may 

 be called " efflorescence." This also differs as to its appearance, 

 quantity, and quality, according to the temperature of the air at 

 the time ; its true nature and the purpose it fulfils will be seen 

 presently. For the first few hours or days the increase of the 

 thickness of the ice is rapid, on account of the comparative 

 exposure to the coldness of the atmosphere, but afterwards the rate 

 of growth is much slower and more uniform. 



In this way is formed the great body of ice known as " the floe," 

 or " great pack " extending approximately from 78° to 60° N. 

 latitude, about 900 miles in length and 200 miles broad, enclosing 

 an area of about 500,000 square miles. Dr. Kane states that the 

 average thickness of this ice in 1850-ol was 8 feet; during our 

 drift in 1857-58 it was 5 feet. This body of ice is not all level ; 

 its otherwise uniform level is disturbed by hummocks, these being 

 produced by the piling up of the tables of the floe by pressure ; 

 such accumulations not only appear above the surface of the floe, 

 but exist also underneath it to a far greater degree, so that a 

 larger quantity of ice is formed annually than the mere superficial 

 extent of the floe would lead one to suppose, and at the same time, 

 on account of their abundant presence, materially increasing its 

 average thickness. The ice thus formed varies in specific gravity 

 according to the temperature at which it was frozen, the density 

 decreasing in proportion to the degree of cold; such decrease 

 being more apparent from the freezing point of sea-water down to 

 zero, and from that becoming less evident, yet still present. Yet 

 in no case (and my observations extend from below the freezing 

 point to — 42^^) could I obtain fresh water, the purest being of 

 specific gravity 1 * 005, and affording abundant evidence of the 

 presence of salts, especially chloride of sodium, rendering it unfitted 

 for culinary purposes, much less for photographic use. Table No. 1 

 gives the observations on the changes of specific gravity of the ice 

 formed under different degrees of cold ; this is surface ice, but i s 

 this ice increases in thickness and becomes exposed to greater degrees 

 of cold, it gradually becomes more compact and close, and part of 

 the remaining suit is expressed, so that, by the termination of the 

 season, this same ice is somewhat fi-esher than when it was formed. 

 This is shown by the observation on the 15th of January and the 

 22nd of March 1858. All the floe which annually drifts out of 

 Davis' Straits is not formed at the same temperature, as wide 

 cracks and lanes appear abundantly at times throughout the 

 whole winter, so that parts of the surface are formed at -f 10° or 

 so, and others — 20° or — 40°. I say parts of the surface', 

 because that portion first formed acts as a covering to the 

 remainder from the cold, so that, proportionately, there is but 

 36122. S S 



