Bridgman Makes Ice Too Hot to Hold With Pressure of Over 500 Tons to the Square Inch

Percy W. Bridgman '04, professor of physics, can now make ice which will not melt until a temperature higher than the boiling point of water is attained.

The principle applied is that the melting point of ice rises proportionately to the increase in pressure.

Hitherto Professor Bridgman has had to confine his experiments to a pressure of 12,000 atmospheres. Some tests were made as high as 20,000, but the apparatus was continually breaking. The steel cylinders bulged like lead.

One atmosphere pressure is equivalent to 14 2/3 pounds per square inch; 20,000 therefore is about 143 tons to a square inch.

In his new apparatus, however, the containers have been changed in shape from cylindrical to conical. The cone is pressed more and more tightly into a heavy external block of steel as the force from behind increases. Thus the external pressure becomes greater as the internal pressure is increased, and the cone retains the same size, allowing accurate measurements. With this apparatus routine pressures have been obtained as great as 50,000 atmospheres, and 70,000 atmospheres has occasionally been reached. 70,000 atmospheres is about 514 tons to a square inch.

This new type of "hot ice" forms at about 25,000 atmospheres of pressure.

The new instrument has been used principally to examine the abrupt changes in crystalline form which many solid substances undergo when subjected to great pressure.

A new tool for studying matter under severe stresses has also been devised. The substance is squeezed to a pressure up to 50,000 atmospheres between steel pistons. As an added torture, the pistons are then rotated in opposite directions, so as to produce a shearing force.

Results of this experiment are extremely interesting to the observer. Rubber loses its elasticity and becomes a translucent, horny material; paper is similarly affected. Red phosphorus is turned permanently black. The electrical resistance of some metals was found to be abnormally high under this treatment.

Up to date, nearly 200 elements and inorganic compounds have been examined, and in about half the cases new forms such as those mentioned above were found. Judging from the immense pressures which are present below the surface of the earth, it is believed that through this experiment much may be learned about the composition of that which is beneath us