Lowest Temperature Produced

ON THE PROGRESS OF SCIENCE. 215 masses. We might keep always on hand a given mass and meas- ure successively the attraction between it and unknown masses, and thus we would be able to compare with each other the un- known masses. If one of these unknown masses, or the mass with which they are compared, is taken as a standard, we have a system of mass measurement no more arbitrary than those in com- mon use. As a matter of fact, this is exactly what is done by everyone. In other words, we weigh bodies. In this case the mass we have always on hand is the earth. The unknown masses whose attrac- tion for the earth is measured are compared with the mass of a cubic centimeter of water, or with some other arbitrary standard, as the pound or the ton. But we can do more than this, since the recent researches in physics referred to above. We can take a standard mass such as a cubic centimeter of water, measure its attraction for the earth in dynes, and hence can calculate the total mass of the earth. For this last calculation it is necessary to remember (what can be proved mathematically) that in the case of an attracting sphere the attraction due to it is the same as if the mass of the sphere were concentrated at its center. Considering the earth a sphere of four thousand miles radius, we merely have to calculate the total mass of the earth on the assumption that it is all concen- trated at a point four thousand miles from our cubic centimeter of water or standard gram. This calculation shows that the earth is composed of matter which on the average is about five and a half times as dense as water. Chester Snow, A. B.

Lowest Temperature Produced. The lowest temperature yet recorded is that reached recently by K. Olszewski in an attempt to liquefy helium. By the aid of solid hydrogen he cooled the gas to 259 degrees below freezing point under 180 atmospheres of pressure. Then he suddenly re- leased the pressure to that of the atmosphere. The expansion of the gas absorbed heat, and cooled itself to a degree estimated to be 271 degrees below zero. Yet the helium was not liquefied, and there appears to be little prospect of reducing it to a liquid. We naturally ask the question, how can such a low tempera-