588 Professor Sir James Deivar [Jan. 19, 



one-seventh that of Hquid air, and its density one-fifteenth, while 

 its temperature is only 20^ Abs., i.e. one-fourth that of liquid air ; 

 still, small drops of the liquid are easily traceable throuo;h a fall of 

 8 feet, by the trail of frozen air caused by their passage. The forma- 

 tion of the liquid hydrogen drops requires a special form of silvered 

 glass siphon (Fig, 8, A). The siphon is connected to a large 

 silvered vacuum vessel containing the liquid hydrogen, fitted with a 

 screw-cock for regulating the pressure, which controls the rate of 

 dropping. The nozzle contains a loosely packed wad of wool, and 

 to prevent fouling by condensation of air, it is surrounded by a 

 piece of glass tube 4 inches long and Ih inch in diameter, drawn out 

 below to h inch, thus preserving an atmosphere of hydrogen about 

 the nozzle. When the drops of liquid hydrogen are started, a succes- 

 sion of tiny vortex rings of frozen air are visible from the nozzle to 

 the floor as they fall gently through the air. If the siphon be fitted 

 to a vacuum vessel about 18 inches long (Fig. 8, B), so that the drops 

 €an fall on some liquid air previously placed in the vessel, a cloud of 

 solid air quickly rises from the surface, rapidly settling again when 

 the drops are shut ofif. If a slow rate of dropping is maintained, the 

 liquid hydrogen drops are easily seen, producing little jets of con- 

 densed vapour as they dart rapidly about in the spheroidal state on 

 the surface of the liquid air. 



Liquid Air Calorimeter. 



A'acuum jacketed vessels have led to the construction of a simple 

 form of gas calorimeter for the measurement of specific heats. The 

 lecture form of the apparatus (Fig. 9) consists of a silvered vacuum 

 vessel A containing liquid oxygen, fitted with an air-tight cap ; a 

 ^small delivery-tube B leads to an inverted gas jar C. The cap has a 

 central vertical tube closed by a rubber cork D. A is supported in 

 a metal vacuum pot E containing liquid air. An aluminium cup F 

 rests on asbestos wool in the bottom of A. Normally there is 

 no evaporation from A, but if a fragment of the substance of 

 which it is desired to ascertain the specific heat, is dropped into F by 

 momentarily removing D, a quantity of liquid oxygen is evaporated 

 as gas, collected in C, and the amount measured, from which the 

 specific heat of the body can be calculated. A sphere of lead, of 

 weight proportional to its atomic weight, is taken as a standard, this 

 element maintaining a nearly constant atomic heat, following the 

 law of Dulong and Petit, for a considerable range of temperature. 

 Pieces of carbon, aluminium, copper and bismuth, in atomic propor- 

 tion to the lead sphere, all liberate smaller quantities of gas, especi- 

 . ally carbon, where the value is found to be much below the average. 



