done on board H.M.S. e Challenger? 611 



thermometer used was one of Geissler's normal ones, divided into tenths 

 of a degree Centigrade, whose zero had been verified the day before in 

 melting snow. The melting-point of the ice-crystals was found to 

 be — 1°*3. The temperature of the melting mass was observed to re- 

 main constant for twenty minutes, after which no further observations 

 were made. 



In the same way the melting-point of the pack-ice was determined. 

 The fresh ice began to melt at — 1° ; after twenty minutes the thermo- 

 meter had risen to — 0°*9, and two hours and a half afterwards it stood at 

 — 0°*3, having remained constant for about an hour at — o, 4. Another 

 portion of the ice rose more rapidly ; and when three fourths of the ice 

 was melted, the thermometer stood at 0°. 



These determinations of the temperature of melting sea-water ice show 

 that the salt is not contained in it in the form of mechanically enclosed 

 brine only, but exists in the solid form, either as a single crystalline 

 substance, or as a mixture of ice and salt crystals. Common salt, when 

 separating from solutions at temperatures below 0°, crystallizes in hexa- 

 gonal planes ; sea-water ice, therefore, may possibly have some analogy 

 to the isomorphous mixtures occurring amougst minerals. 



A very important practical consequence follows from these observa- 

 tions, namely, that pack-ice, though unfit to drink when a lump of it is 

 melted as a whole, may serve as a source of fresh water if melted frac- 

 tionally. As the melting-point of the salt ice is lower than that of pure 

 ice, it melts first, and at the same time, by keeping down the temperature 

 of the mass to its own melting-point, it prevents any of the fresh ice 

 being wasted. When the salt ice has all been melted the brine may be 

 thrown away, and the remainder of the ice will supply fresh water. If 

 a thermometer be kept in the ice during the process of melting, it 

 will indicate by its reading when drinkable water is being formed. 



Observations made on Shore in some of the Islands visited. 

 Viewed from the sea, the island of St. Vincent, one of the Cape-Verd 

 group, presents an extremely rugged and desert appearance, which it pre- 

 serves even upon the closest inspection. The hills round the harbour 

 consist of interbedded igneous rocks, the individual beds being seldom 

 over three or four feet thick, and generally only about two feet, dipping 

 at a gentle angle away from the centre of the harbour. These beds are 

 much cut up by vertical dykes of basaltic rock running in two principal 

 directions, namely, north and south and east and west. They frequently 

 present an eminently columnar cleavage in the direction of their breadth. 

 Many of the beds which form the mass of the hill are of similar nature 

 and cleave in the same way. The rock on each side of these basaltic 

 masses, whether beds or dykes, is generally much altered, and has 

 suffered considerable disintegration and decomposition, being frequently 

 transformed into a mass of kaolin in the neighbourhood of the basalt. 



