222 HISTOEY OF COLD AND THE ABSOLUTE ZERO. 



tiuie be emplo^^ed for the liquefaction of yet more ^•olatile elements, 

 apart from the importance which its liquefaction must hold in the 

 process of the stead}^ advance toward the absolute zero. Hydrogen is 

 an element of especial interest, because the study of its properties and 

 chemical relations led great chemists like Farada}^ Dumas, Daniell, 

 Graham, and Andrews to entertain the view that if it could ever be 

 brought into the state of liquid or solid it would reveal metallic char- 

 acters. Looking to the special chemical relations of the comljined 

 hydrogen in water, alkaline oxides, acids, and salts, together with the 

 behavior of these substances on electrolysis, we are forced to conclude 

 that hydrogen liehaves as the analogue of a metal. After the beautiful 

 discovery of Graham that palladium can absorb some hundreds of times 

 its own volume of hydrogen, and still retain its luster and general 

 metallic character, the impression that hydrogen was probabh^ a mem- 

 ber of the metallic group became ver}^ general. The only chemist who 

 adopted another view was my distinguished predecessor, Professor 

 Odling. In his Manual of Chemistr3% published in 1861, he pointed 

 out that hydrogen has chlorous as well as basic relations, and that they 

 are as decided, important, and frequent as its other relations. From 

 such considerations he arrived at the conclusion that hydrogen is essen- 

 tially a neuti'al or intermediate body, and therefore we should not 

 expect to find liquid or solid hydrogen possess the appearance of a 

 metal. This extraordinary prevision, so characteristic of Odling", 

 was proved to ])c correct some thirty-seven years after it was 

 made. Another curious anticipation was made by Dumas, in a letter 

 addressed to Pictet, in which he says that the metal most analogous to 

 h3^drogen is magnesium, and that probably both elements have the 

 same atomic volume, so that the density of hydrogen, for this reason, 

 would be al)out the value elicited by subsequent experiments. Later 

 on, in 1872, when Newlands began to arrange the elements in periodic 

 groups, he regarded hydrogen as the low^est member of the chlorine 

 family; but Mendeleef, in his later classification, placed hydrogen in 

 the group of the alkaline metals; on the other hand, Dr. Johnstone 

 Stoney classes hydrogen with the alkaline earth metals and magne- 

 sium. From this speculative divergenc}^ it is clear no definite con- 

 clusion could be reached regarding the j^hysical properties of liquid 

 or solid hydrogen, and the only way to arrive at the truth was to 

 prosecute low^-temperature research until success attended the efiorts 

 to produce its liquefaction. This result I definitely obtained in 1898. 

 The case of liquid hj^drogen is, in fact, an excellent illustration of the 

 truth alread}' referred to, that no theoretical forecast, however 

 apparently justified by analogy, can ])c finally accepted as true until 

 confirmed by actual experiment. Licjuid hy<lrogen is a colorless trans- 

 parent })ody of extraordinary intrinsic interest. It has a clearly 

 defined surface, is easiW seen, di'o\)s well, in spite of the ftu't that its 



