Apkil 27, 1900.] 



SCIENCE, 



675 



Ludwig Mond, Professor S. P. Thompson, Pro- 

 fessor Hellmann, and Mr. Fletcher Moul- 

 ton, M.P. 



Professor Dewar began with some remarks 

 about the nature of hydrogen, pointing out that 

 many of the most advanced chemical thinkers 

 had regarded it as being metallic in character, 

 and that to Professor Odling belonged the 

 credit, so far as he knew, of being the first to 

 suggest the contrary, now recognized to be the 

 fact. Proceeding to show a long series of ex- 

 periments, he explained how it was a conse- 

 quence of the physical properties of liquid hy- 

 drogen that not much could be done with it 

 unless it was available in reasonable quantities. 

 After proving that its temperature was some 

 70 degrees below that of liquid air, and explain- 

 ing how reduction o'f temperature became dis- 

 proportionately more diflicult the lower the 

 starting-point on the scale, he illustrated the 

 difference in the behavior of liquid nitrogen 

 and liquid oxygen when made to boil under 

 diminished pressure. The temperature of both 

 was reduced, but, while the nitrogen became 

 viscid and ultimately solid, the oxygen abso- 

 lutely refused to solidify. In fact, it was im- 

 possible to get solid oxygen in this way, the 

 reason being that at the lowest temperatures it 

 had an inappreciable pressure of vapor, whereas 

 that of nitrogen was considerable. To give an 

 idea of the power of liquid hydrogen as a cool- 

 ing agent he performed an experiment depend- 

 ing on the same principle as WoUaston's cryop- 

 horus, with the difference that the fluid to be 

 solidified was not water but liquid air contain- 

 ing a large proportion of nitrogen, while the 

 material used to effect condensation was liquid 

 hydrogen. He next showed how hydrogen 

 could be liquefied in a closed tube, explaining 

 the importance of this fact with regard to the 

 determination of its density and other ques- 

 tions, and then exhibited the gas in the solid 

 form. This result was effected by putting a 

 portion of the liquid into a vacuum vessel iso- 

 lated from heat as perfectly as possible. When 

 the pressure in the vessel was slowly reduced, 

 the hydrogen was suddenly seen to appear like 

 a white mass of solidified foam, possessing the 

 lowest steady temperature it was possible to 

 obtain at present — vii;, 258° below zero Centi- 



grade, or 15° on the absolute scale. The fact 

 that hydrogen did solidify in this way was in a 

 sense a disappointment to any one who was 

 anxious to reach very low temperatures, for a 

 solid was a bad substance for cooling purposes. 

 Coming to the uses of liquid hydrogen for sci- 

 entific research, Professor Dewar first showed 

 how it afforded the only means of obtaining solid 

 oxygen. Another important application was to 

 the separation of the more volatile gases of a 

 mixture. The behavior of metals with regard to 

 electrical conductivity at very low temperatures 

 was a very interesting question. From experi- 

 ments with liquid air it was expected that at 

 the zero of absolute temperature pure metals 

 would have no electrical resistance at all. But 

 although the resistance curves appeared to be 

 going straight to zero at the temperature of 

 liquid air, he found that lower down, below the 

 temperature of solid air, they bent sharply 

 round, so that a finite resistance was indicated. 

 In conclusion, Professor Dewar acknowledged 

 the kindness of those who had contributed to 

 the cost of these investigations, and paid a trib- 

 ute to the skill and devotion of his assistants. 

 Such researches were necessarily costly, but he 

 could not share the view of those who suggested 

 that the results would not be worth the cost. 



X-BAYS AND PHOTOGEAPSIC PLATES. 



A DISCOVERY of very great practical interest 

 in X-ray work has been made by Professor 

 Nipher at Washington University. He has 

 discovered that when photographic plates are 

 exposed to the light of an ordinary room for a 

 few days, that they may still be used for taking 

 X-ray pictures. If while the Crookes tube is 

 acting on the plates they are still exposed to 

 the ordinary light of a room, they develop as 

 positives. The shadows are dark. If they are 

 in a plate holder when exposed to the X-ray, 

 the pictures are like those formed in the ordi- 

 nary way, and they are apparently as clearly 

 defined. 



The advantage of the method is that the 

 plates may be developed by the light of a lamp. 

 The developer (hydrokinone) being weak and 

 cool, the process may go on for an hour if de- 

 sired, and all the details may be studied as 



