li»12] 



on Heat Problems. 



5S' 



oF liquid air, and more especially of liquid hydroi^'eu. When any 

 solid is dropped into such caloiinieters violent el)ullition ensues, and 

 either can be carried over with the evaporating gas, in the form of 

 fine drops or spray, into the collecting jar, and so falsify the results. 

 The possibility of this can be proved by showing that drops of liquid 

 air will retain their state through a fall of from 30 ft. to 100 ft. in air. 

 To illustrate this, special dropping pipettes were supported from the 

 ceiling of the Lecture Room. The construction of the nozzle is shown 



in Fig. 7 ; it is filled with a wad of tightly packed cotton-wool, pierced 

 with a coarse needle. By exercising a slight pressure on the liquid 

 air above — a screw clip on a short length of rubber tube serves for 

 this — the rate of dropping is easily regulated. Larger drops are 

 obtained when a pin with a round glass head replaces the needle. 

 The drops fall on a wet blackened board, and the impact is plainly 

 audible and made visible by the surface of the board l^eing obscured 

 by a white mist as the drops rapidly evaporate. Such drops pass 

 through a soap-bubble without bursting it. Hydrogen is remarkable 

 in this respect, as the surface tension of liquid hydrogen is only 



