5<s4 



Professor Sir James Dewar 



[Marcli 25^ 



latter is than nitrogen, we may safely predict that if a current of 

 helium were similarly directed through liquid hydrogen, the latter 

 would be reduced in temperature until it would freeze, seeing that 

 the melting point of hydrogen is just about half its critical tem- 

 perature. 



An evaporation calorimeter, where the calorimetric substance is 

 one of the liquefied gases, would be a convenient instrument provided 

 it could be easily constructed and was reliable in its working. The 

 efficiency of such an instrument depends (1) on the relatively large 

 quantity of gas given by evaporation, and (2) on the great range of 

 temperature easily available when liquid air, oxygen, nitrogen, or 

 hydrogen is the calorimetric substance. 



The following table gives the special physical constants of the 

 various liquid gases that are of importance in calorimetry. That 

 calorimetric substance will be the more sensitive which gives off 



the larger volume of gas for a given quantity of heat. Thus oxygen 

 gives 13 • 2 c.c. per calorie, while ethylene gives 7, hydrogen 88 * 9 ; 

 hence oxygen is twice as sensitive as ethylene, and hydrogen six times 

 as sensitive as oxygen. It is easy to detect a -^j^ gramme calorie 

 when lic[uid air is used, and as small a quantity as ^^ can be 

 observed with liquid hydrogen. 



In selecting the calorimetric substance of a liquid gas calorimeter, 

 hydrogen, as giving the greatest range of temperature and sensibility, 

 would be the best ; next to it would come nitrogen, then air, and 

 lastly oxygen. But we must remember that we are enveloped in an 

 atmosphere of air, and have to consider its effect. Passing over 

 hydrogen for the present, let us examine the advantages and dis- 

 advantages of the other three gases. As the boiling point of air is 

 below that of oxygen, even if there were no layer of cool oxygen gas 

 on the surface of the liquid oxygen, the air coming in contact with it 

 through the neck of the calorimeter would still remain gaseous. But 

 if we were to take liquid nitrogen as the calorimetric substance, air, 

 being heavier than nitrogen but having a higher boiling point, would, 

 in falling down the neck of the calorimeter, come in contact with the 



