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SCIENCE. 



[N. S. Vol. XVI. No. 406. 



the value elicited by subsequent experi- 

 ments. 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 met- 

 als; 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 hy- 

 drogen, and the only way to arrive at the 

 truth was to prosecute low-temperature re- 

 search until success attended the efforts to 

 produce its liquefaction. This result I 

 definitely obtained in 1898. 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 

 colorless 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. Com- 

 pared with an equal volume of liquid air, 

 it reciuires only one fifth the quantity of 

 heat for vaporization ; on the other hand, 

 its specific heat is ten times that of liquid 

 air or five times that of water. The coeffi- 

 cient of expansion of the fluid is remark- 

 able, being about ten times that of 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 atmos- 

 pheric pressure it boUs at minus 252.5 de- 

 grees or 20.5 degrees absolute. The critical 

 point of the liquid is about 29 degrees 

 absolute, and the critical pressure not more 

 than fifteen atmospheres. The vapor 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 tem- 

 perature. Reduction of the pressure by an 

 air-pump brings down the temperature to 

 mimis 258 degrees, when the liquid becomes 

 a solid resembling frozen foam, and this 

 by further exhaustion is cooled to minus 

 260 degrees, or 13 degrees absolute, which 

 is the lowest steady temperature that has 

 been reached. The solid may also be got in 

 the form of a clear transparent ice, melt- 

 ing at about 15 degrees absolute, under a 

 pressure of 55 mm., possessing the unique 

 density of one eleventh 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 in- 

 vestigator to a world of solid bodies. The 

 contrast between this refrigerating sub- 

 stance and liquid air is most remarkable. 

 On the removal of the loose plug of cotton- 

 wool used to cover the mouth of the vacu- 

 um vessel in which it is stored, the action is 

 followed by a miniature snowstorm of solid 

 air, formed by the freezing of the atmos- 

 phere at the point where it comes into 

 contact with the cold vapor rising from 

 the liquid. This solid air falls into the 

 vessel and accumulates as a white snow at 

 the bottom of the liquid hydrogen. When 

 the outside of an ordinary test-tube is 

 cooled by immersion in the liquid, it is 

 soon observed to fill up with solid air, and 

 if the tiibe be now lifted out a double 

 effect is visible, for liquid air is produced 

 both in the inside and on the outside of 

 the tube— in the one case by the melting 



