APPARATUS 



metres (aver 2 fluid ounces) of liquid helium were obtained, so 

 that there could be no ambiguity about the result. And it is 

 fortunate that such very definite success was secured, for it is 

 not very likely that so costly and difficult an experiment will be 

 often repeated. The boiling-point of liquid helium is estimated 

 to be 268 to 269 below C. The density of the liquid is 0-15, 

 or less than J the density of water, and a little more than twice 

 the density of liquid hydrogen. 



By causing liquid helium to boil under reduced pressure a 

 temperature about 2 lower was reached, but the helium did not 

 solidify. This is the lowest temperature known, and is believed 

 to be only about 2-5 C. ab -ve the absolute zero. 



At these very low tern* '^tures the properties of many solid 

 bodies are considerably um ftie'd. Thus the conductivity of metals 

 diminishes with rise of 

 temperature and increases 

 with fall of temperature, 

 and in the case of all the 

 pure metals which have 

 been examined the resist- 

 ance at successively low 

 temperatures down to 

 about -200 is such that 

 at the absolute zero it 

 would disappear alto- 

 gether, and all metals 

 would be equally good 

 conductors. The specific 

 heat of solids is also 

 greatly reduced, but not 

 to zero ; the atomic heat, 

 that is, the product of 

 the specific heat into the 

 atomic weight, is now 

 found to be periodic, 



rising and falling at regular intervals with increase of atomic 

 weight. (See Periodic Law, p. 126.) 



Reduction of Pressure. Vacuum. It has already been men- 

 tioned in describing the interior of a laboratory (see Imperial 

 College, p. 9) that nearly all the tables or benches for work 

 are provided with water pumps, used chiefly for aiding filtration 



6. 



FIG. 43. FILTER-PUMPS. 



