• HISTORY OF COLD AND THE ABSOLUTE ZERO. 223 



surface tension is only tho thirty-tiftli part of that of water, or about 

 one-tifth that of li([ui(l air, and can be poured easily fi-oni vessel to 

 vessel. The lii^uid does not conduct electricity, and. if anything, is 

 slightly diamagnetic. Compared with an equal volume of liquid air, 

 it requires only one-tifth the quantity of heat for vaporization; on the 

 other hand, its specilic h(>at is six times that of liquid air or three times 

 that of water. The coefficient of expansion of the fluid is remarkable, 

 l)eing al)()ut ten times that of the gas. It is 1)y far the lightest liquid 

 known to exist, its density being only one-fourteenth that of water. 

 The lightest licfuid pre\iously known was rujuid marsh gas, which is 

 six times heavier. The only solid wliich has so sn)all density as to 

 float upon its surface is a picM-e of pith wood. It is by far the coldest 

 liquid known. At ordinary atmospheric pressure it boils at minus 

 252..5- or 20. o absolute. The critical point of the liquid is from 

 30* to 82 absolute, and the critical pressure not more but probably 

 less 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 temperatui-e. Reduction of 

 the pressure by an air pump brings down the t(Mnp(>rature to minus 

 258 , when the liquid ])ecomes a solid reseml)ling frozen foam, and 

 this l»y further exhaustion is cooled to minus 259.. 5" or 13.5 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 alisolute, under a pressure of 55 mm., possessing the 

 unique densitv of one-tenth that of water. Such cold involves the 

 soliditication 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 l)odies. The contrast between this 

 refrigerating sul)stance and liquid air is most remarka])le. On the 

 removal of the loose plug of cotton wool used to cover the mouth of 

 the vacuum A'essel in which it is stored, the action is followed by a 

 miniature snowstorm of solid air. formed by tho freezing of the atmos- 

 phei-e 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 1)ottom of the li([uid hydrogen. 

 When the outside of an ordinary t(?st tube is cooled by innnersion in 

 the liquid, it is soon observed to fill up with solid air, and if the tube 

 be now lifted out a doulde effect is visible, for liquid air is produced 

 both in the inside and on the outside of the tube — in the one case b}^ 

 the melting of the solid and in the other by condensation from the 

 atmosphere. A tuft of cotton wool soaked in the liquid and then held 

 near the pole of a strong magn(>t is attract(>d, and it might be inferred 

 therefrom that Tuiuid hydrogen is a magnetic body. This, however, 

 is not the case. The atti'action is due neithin- to tlu; cotton wool nor 

 to the hydrogen — which indeed cvaporat(\s almost as soon as the tuft 



