-86 PETTERSSON, ON WATER AND I C E. 



in some degree afford a scale for our appreciation of the in- 

 fluence of the foreign substances in the ice. If we consider: 



that an amount of chlorine of O.273 per cent eauses the ice 

 [see table V and curve V on plate 21] to contract already at 

 — 14° C 



that a sample of ice, which contains O.015 p. c. of chlorine 

 begins to contract at — 4° C (see table IV and curve IV) 



and that ice from ordinary distilled water, which contains 

 scarcely perceptible traces of chlorine, begins to shrink in 

 volume at about — 0°.25 C, we will hardly feel inclined to 

 underrate the influence of foreig'-n substances on the entire 

 physical behavior of the ice and especially on the thawing 

 process. Many facts, familiar to most of my readers, will find 

 their interpretation from this circumstance. 



Walking upon a snowy plain on a winter da}^ when the 

 thermometer shows about — 7° or — 8° C in the air, we will 

 perceive, that every footstep eauses a keen, crunching sound 

 from the ice-particles crushed under our feet. Suppose the 

 temperature of the atmosphere to rise a few centigrades f. ex. 

 to — 5° or — 4° C, then we will tread quite noiselessly upon 

 the snow. The ice particles still retain their solid form, but 

 the thawing has already begun; instead of a network of härd 

 and brittle crystal needles, we tread upon a soft mäss, which 

 shrinks beneath our feet to a plastic mould and retains the 

 impression of the footsteps. 



Our pleasure of skating is very much dependent upon 

 the temperature, but also in a certain degree upon the pu- 

 rity of the ice. If the temperature of the air, and conse- 

 quently also of the upper layer of the ice, is next to zero, the 

 steel of the skates will draw deeper furrows in the ice, and 

 the friction will be considerably increased. The same will be 

 the case, if we practise upon a frozen fiord, where the ice 

 has formed from salt water, instead of on the ice of an in- 

 land lake. 



Ice, which arises by the freezing of salt water, retains a 

 part of its saltness, the greater, the more suddenly the freezing 

 has begun. We are told by eye witnesses, such as Wey- 

 precht, Nordenskiöld a. O. that the new ice, which arises 

 by sudden freezing of the calm surface of the arctic sea, is a 

 tough substance, which can be wrinkled and folded by external 

 pressure without breaking. Although it may be thick enough 

 to bear the weight of a man, it is so plastic, that a footstep 

 makes a deep impression as in mouldable clay. »If you 



