June 30, 1899.] 



SCIENCE. 



915 



solid. To prove that the liquid he was 

 manipulating with such freedom was really 

 liquid hydrogen Professor Dewar put a light to 

 a small quantity, a brilliant burst of flame being 

 the prompt result. Of its exceedingly small 

 density he gave an idea by showing that a light 

 material like cork would not float on its sur- 

 face, but sank to the bottom as if it were lead. 

 The lowness of its temperature he illustrated 

 by a number of experiments. Thus a solid body 

 immersed in it for a short time was shown to 

 become so cold that the air round it was lique- 

 fied and ran otT in drops, while when a tube 

 containing liquid air was plunged into it the 

 air immediately became solid. On this tube 

 being lifted out again a double effect was seen, 

 for the melting of the solid within it yielded 

 liquid air, which was also formed by condensa- 

 tion on its outside surface. An empty vessel 

 placed for a short time in the cold atmosphere 

 just above this liquid, filled with solid air in the 

 form of snow, soon melted into liquid. Oxygen 

 in a sealed tube when lowered into it quickly 

 became solid, and when lifted out it could be 

 seen, as heat was absorbed, to assume first the 

 liquid and then the gaseous form. A sponge of 

 porous material, soaked in liquid hydrogen and 

 brought into a magnetic field, apparently be- 

 haved as if it were magnetic. That, however- 

 was due to the condensation of the oxygen of 

 the air, which, of course, was magnetic, and, 

 though an observer might in this way be easily 

 deceived into thinking hydrogen magnetic, Pro- 

 fessor Dewar said he was satisfied that it was 

 nearly neutral or diamagnetic. 



Speaking of the real temperature of this 

 liquid, he said it had taken him nearly a 

 year to come to a definite conclusion on 

 that point because he could not get any 

 two thermometers to agree. Pure platinum 

 resistance thermometers gave 35° absolute (or 

 238° below zero Centigrade), one of the plati- 

 num-rhodium alloy 27°, while hydrogen itself 

 in a gas thermometer gave 21°, a reading nearly 

 identical with one obtained with a German-sil- 

 ver electrical thermometer. The last part of 

 the lecture was devoted to the extraordinarily 

 low vacua obtainable by the use of liquid hydro- 

 gen. Thus, by immersing one end of a closed 

 tube in it for a short time and then sealing it 



off in the middle, a vacuum was formed in the 

 upper part which was substantially perfect, as 

 was shown by the fact that the electrical charge 

 could not be made to pass. In conclusion, Pro- 

 fessor Dewar, after exhibiting several other 

 beautiful experiments, including one to illus- 

 trate the rapidity with which gases were dis- 

 charged into a vacuum, claimed that the lique- 

 faction of hydrogen was a triumph for theory 

 not less than for practice. 



Lord Kelvin, in moving a vote of thanks to 

 Professor Dewar for his brilliant, beautiful and 

 splendidly interesting lecture, said that if those 

 present wished to measure the importance of 

 the occasion, let them think what Count Rum- 

 ford, or Davy, or Faraday would have thought, 

 could they have been present. They could not 

 have hoped for their scientific dreams and 

 prophecies to be so splendidly verified within 

 the century. The end of experiment in re- 

 search at low temperatures had by no means 

 been reached, and perhaps in a few years sub- 

 stances yet unknown and more refractory than 

 hydrogen would have been found which would 

 bring the experimenter to within five degrees 

 of the absolute zero. 



AUTOMATIC SHIP-PBOPULSION. 



Automatic ship-propulsion is once more pro- 

 posed, this time by M. Linden, Secretary of the 

 Naples Zoological Station, according to Sr. Me- 

 nard in Cosmos of December 17, 1898. He at- 

 taches elastic plates to the bow and stern of the 

 boat, which act precisely as does the tail of a 

 fish. They are bent by the pitching of the boat 

 in a seaway, and the reaction of their forcible un- 

 bending, as well as that of their motion against 

 the water while being bent, produces forward 

 motion in the boat, in effect as the fish drives 

 himself forward bj' springing its tail in lateral 

 movements. Thus every motion of the boat on 

 the surface of the waves produces greater or 

 less acceleration. 



The boat employed is stated to be four meters 

 (13 feet) long, its driving plates 50 centimeters 

 long (20 inches) and one-half that width. They 

 are thicker at the point of support than toward 

 their extremities, giving a proper flexure when 

 pressed by the water into their impelling 



