[barnes] 



THE PHYSICAL CONSTANTS OF ICE 



19 



perature, and, by comparison witli the second, shows how suddenly the 

 ice yields. The last column gives the pressure from the melting-curve. 



Tensile Strength and Compressibility. 

 From the researches of Pf aff it was shown that an ice bar stretched 

 under a force of 1 Kg. per sq. cm, in seven days by .03-2 of its length 

 during a time when the temperature was near 0°C. 



Fabian, in 1877, showed from a study of the extensibility and 

 elasticity that under a load of . 5 Kg. per sq. cm. ice behaved very nearly 

 as a completely elastic body. Under a load of 30 Kgs., which was 1 . 5 

 Kgs. per sq. cm., the temporary extension was 0.0633, while the per- 

 manent extension amounted to .030 mm. A rupture of the bar took 

 place when it had extended by 1/5000 of its length under a load of 2 . 5 

 Kgs. per sq. cm. 



As a result of his experiments Hess placed the tensile strength of 

 ice as high as 7 to 8 Kgs. per sq. cm., and the compressibility as 25 

 Kgs. per sq. cm. 



The crushing strength of ice, as determined by U. S. testing ma- 

 chine, has been given as ranging from 327 to 1000 pounds per square 

 inch. 



The strength of surface ice has been lucidly expressed in the fol- 

 lowing way : 1 . 5 inches will support a man ; 4 to 5 inches will support 

 cavalry and guns; 6 inches, wagons drawn by horses; 18 inches, a rail- 

 road train. 



Colour and Refractive Index of Ice. 

 Pure ice is colourless to white and pale blue in large masses. It is 

 transparent, being optically uniaxial, crystallizing in the hexagonal sys- 

 tems and showing a positive character as regards its indices of refrac- 

 tion. Passing over the early work of Brewster, Wollaston and Fraun- 

 hofer we have that of Reusch, in 1864, who measured the indices of re- 

 fraction for the ordinary and extraordinary ray and found: 



TABLE X. 

 Index of Refraction. 



The effect of temperature is to decrease the refractive index 

 slightly. 



