ADDRESS. 33 



hydrogen is magnesium, and that probably botli elements have the same 

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

 about 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 lowest member of the chlorine family ; but 

 Mendeleef in his later classiBcation placed hydrogen in the group of the 

 alkaline metals ; on the other hand. Dr. Johnstone Stoney classes hydrogen 

 with the alkaline earth metals and magnesium. From this speculative 

 divergency it is clear no definite conclusion could be reached regarding 

 the physical 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 efforts to produce its liquefaction. This result I 

 definitively obtained in 189S. The case of liquid hydrogen is, in fact, an 

 excellent illustration of the truth already referred to, that no theoretical 

 forecast, however apparently justified by analogy, can be finally accepted 

 as true until confirmed by actual experiment. Liquid hydrogen is a 

 colourless transparent body of extraordinary intrinsic interest. It has a 

 clearly defined surface, is easily seen, drops well, in spite of the fact that 

 its surface tension is only the thirty-fifth part of that of water, or about 

 one-fifth that of liquid air, and can be poured easily from vessel to vessel. 

 The liquid does not conduct electricity, and, if anything, is slightly 

 diamagnetic. Compared with an equal volume of liquid air, it requires 

 only one-fifth the quantity of heat for vaporisation ; on the other hand, 

 its specific heat is six times that of liquid air or three times that of 

 water. The coefficient of expansion of the fluid is remarkable, being about 

 ten times that of the gas ; it is by far the lightest liquid known to exist, 

 its density being only one-fourteenth that of water ; the lightest liquid 

 previously known was liquid marsh gas, which is six times heavier. 

 The only solid which has so small density as to float upon its surface is a 

 piece of pith wood. It is by far the coldest liquid known. At ordinary 

 atmospheric pressure it boils at minus 252 '5 degrees or 20'5 degrees 

 absolute. The critical point of the liquid is from 30 to 32 degrees 

 absolute, and the critical pressure not more, but probably less, than ] 5 

 atmospheres. The vapour of the hydrogen arising from the liquid has 

 nearly the density of air — that is, it is fourteen times that of the gas at 

 the ordinary temperature. Reduction of the pressure by an air-pump 

 brings down the temperature to minus 258 degrees, when the liquid becomes 

 a solid resembling frozen foam, and this by further exhaustion is cooled to 

 minus 259'5 degrees, or 13^ degrees absolute, which is the lowest steady tem- 

 perature that has been reached. The solid may also be got in the form of a 

 clear transparent ice, melting at about 1 5 degrees absolute, under a pressure 

 of 55 mm., possessing the unique density of one-tenth that of water. 

 Such cold involves the solidification of every gaseous substance but one 

 that is at present definitely known to the chemist, and so liquid hydrogen 

 introduces the investigator to a world of solid bodies. The contrast 

 between this refrigerating substance and liquid air is most remarkable. 

 1902. D 



