306 Scientific Intelligence. 



erties 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 

 definitely obtained in 1898. The case of liquid hydrogen is, in 

 fact, an excellent illustration of the truth ah-eady 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. Compared with an equal volume of liquid 

 air, it requires only one-fifth the quantity of heat for vaporization; 

 on the other hand, its specific heat is ten times that of liquid air 

 or fiA^e times that of water. The coefficient of expansion of the 

 fluid is remarkable, 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 ordi- 

 nary atmospheric pressure it boils at minus 252-5 degrees 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 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 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, melting at about 15 degrees absolute, 

 under a pressure of 55™™, 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 

 investigator to a world of solid bodies. The contrast between 

 this refrigerating substance and liquid air is most remarkable. 

 On the removal of the loose plug of cotton-wool used to cover the 

 mouth of the vacuum vessel in which it is stored, the action is 

 followed by a miniature snow-storm of solid air, formed by the 

 freezing of the atmosphere at the point where it comes into con- 

 tact with the cold vapor rising from the liquid. This solid air 

 falls into the vessel and accumulates as a white snow at the bot- 

 tom 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 tube be now lifted out a 



