154 Scientific Intelligence. 



to avoid the great pressure required in handling the highly evapora- 

 tive substances of lowest boiling point which serve to produce ex- 

 treme cold, it is necessary to divide the difference of temperature 

 into several stages. Each stage is fitted with especial apparatus 

 consisting of an air pump worked by steam, which drains off the 

 vapors of the liquid from the refrigerator, and forces them into a 

 condensor where, reduced to the liquid state, they are again 

 offered for evaporation in the refrigerator. Thus the liquid, with- 

 out leakage, passes through a continuous circuit, and the opera- 

 tions can be carried on for any length of time. The liquid made 

 use of for the first stage is a mixture of sulphurous acid and a 

 small percentage of carbonic acid and is called " Liquide Pictet." 

 It is condensed- at a pressure of two atmospheres in a special tube 

 cooled by running water. Oxide of nitrogen is the liquid chosen 

 for the second stage. Its vapors are condensed in the same way, 

 at a pressure about five or six times as great in a tube maintained 

 at about —80° C. by the action of the first circuit. As a medium for 

 a third stage in which continuous circulation has not yet been 

 attempted, atmospheric air is employed, which passes into the 

 liquid state at a pressure of about 75 atmospheres, provided the 

 condenser is kept at —135° by the first two circuits. The evapo- 

 ration of the liquefied air causes the thermometer to fall below 

 — 200°. A remarkable difference has been noted in the radiation 

 of heat by Prof. Pictet. Material considered a non-conductor of 

 heat does not appear to affect much the passage of heat into a 

 body cooled down below —100°. In Professor Pictet's words the 

 slow oscillations of matter, which constitute the lowest degrees 

 of heat, pass more readily through the obstructions of a so-called 

 non-conductor, than those corresponding to a higher temperature, 

 just as the less intense undulations of red light are better able to 

 penetrate clouds of dust than those of the blue. The quantity of 

 heat which hourly floods a cylinder 1250 mm high by 210 mm wide 

 (the size of the refrigerator) at —80°, is no less than 600 calo- 

 ries, and no packing will keep it out. At a lower temperature, 

 the radiation being even greater, the power of the machinery 

 intended to draw off still more heat would have to be enormous. 

 The lowest temperature Prof. Pictet judges to be attainable is 

 about —255°. In order to measure the elasticity of mercury, 

 Prof. Paalzon had the metal cast and frozen in the shape of a 

 tuning fork. Professor Pictet by freezing chloroform has refined 

 it so that it becomes a stable substance. — Nature, Nov. 12, 1891. 



J. T. 



11. Vibrations of Strings. — Menzel and Raps have studied 

 this subject by the aid of photography. The light of an electric 

 lamp is thrown by means of a slit and a combination of lenses 

 upon the vibrating wire or string. The image of the slit is at 

 right angles to the string. A chronograph cylinder is placed 

 behind the vibrating string. This cylinder is covered with sen- 

 sitive paper. When the string fs at rest and the cylinder re- 

 volves, a dark band is photographed on the paper with a white 



