400 Prof. Tyndall on Ice and Glaciers, ' 



whose freezing-point, on account of a defect of pressure, was 

 0°0075 C. higher, deposited crystals of ice. The heat abstracted 

 from the water in this operation had, moreover, to pass through 

 the glass of the flask, which, together with the small dif- 

 ference of temperature, explains the slowness of the freezing- 

 process." 



A single additional condition in connexion with this beautiful 

 experiment I should like to have seen fulfilled — namely, that the 

 water in which the flask was immersed, as well as that within it, 

 should be purged of its air by boiling. It is just possible 

 that the point of congelation may not be entirely independent of 

 the presence of air in the water. 



The revival of this subject by Professor Helmholtz has caused 

 me to make a few experiments on the moulding and regelation 

 of ice. The following experiment illustrates both : — A quantity 

 of snowy powder was scraped from a block of clear ice and placed 

 in a boxwood mould having a shape like the foot of a claret- 

 glass. The ice-powder was compressed by an hydraulic press, 

 and a clear mass of ice of the shape shown in section at the bottom 

 of fig. 4 was the result. In another mould the same powder was 

 squeezed so as to form small cylinders, three of which are shown 

 separate in fig. 4. A third mould was then employed to form 

 a cup of ice, which is shown at the top of fig. 4. Bringing all 

 the parts into contact, they were cemented through regelation to 

 form the claret-glass sketched in fig. 5, from which several 



Fig. 4. Fig. 5. 



draughts of wine might be taken if the liquid were cooled suffi- 

 ciently before pouring it into the cup of ice. 



There are brass shapes used for the casting of flowers and 

 other objects which answer admirably for experiments on the 

 regelation of ice. One of them was purchased for me by Mr. 

 Becker. Ice-powder squeezed into it, regelated to a solid mass 



