168 



lluidity at li'iiiix-'raturcs iicai" thi' meltiiij;- jK)iiit. A study 

 of tliis property, namely, of tlie degree of fluidity of the 

 iee needles in formation, near 0°, would lielp in solving the 

 problem of the mcclianieal injury whieh ean be produced in 

 protoplasm by ice crystals. 



Some physicists discuss the viscosity of ice and, in gen- 

 eral, of crystalline substances. They assume that in a 

 crystalline body there are layers of molecules in the licpiid 

 state which give to the system the consistency of a '^viscous 

 solid." Hess (1902) observed that the presence of sand 

 and dust increases the viscosity of ice. This observation 

 might have some importance for the biologists since, in 

 biological material, a number of foreign substances 

 admixed with ice can modify its viscosity. 



Among the other mechanical properties of ice, let us 

 mention the tensile strength and the compressibility which, 

 according to Hess (1904), amount respectively to 7-8 kg. 

 and 25 kg. per sq. cm., and the crushing strength which 

 was found to vary from 23 kg. to 70 kg. per sq. cm. {cf. 

 Barnes, 1928). 



The strength of a layer of ice 18 inches thick would be 

 such that it could support a railroad train. 



2. Specific Gravity. Bunsen, in 1870, obtained 0.91676 

 as the specific gravity of ice at 0°. Barnes considers this 

 value as a **good average of all the latest and best measure- 

 ments." The specific gravity of water at 0° was found by 

 Chappuis (1897) to be 0.9998674. Due to the high pre- 

 cision of density determination, the method of utilizing the 

 difference in the specific gravity of ice and of water in the 

 study of such problems as that of the quantity of ice formed 

 in biological material at a given temperature, might prove 

 valuable. 



3. Thermal Coustants. Maass and Barnes (1927) give 

 0.4873 as the specific heat of ice at 0°. Callendar (1912) 

 found 1.00934 for water at 0°. 



The biologist working with low temperatures occasion- 

 ally needs to know the specific heat of ice from 0° C. to the 

 absolute zero. We represented in Figure 27 the values 



