398 Professor Bewar [Jan. 19, 



The smaller one is filled with liquid air, and after the insertion of 

 an india-rubber stopper and glass tube, is completely immersed in 

 liquid air contained in the larger vacuum vessel. In the figure the 

 tube A connects with the inner vacuum tube and B with the outer. 

 As the latter receives all the radiant and conducted heat, air is con- 

 tinuously boiling off through the tube B, but as the supply of heat 

 is effectually cut off from the inner vacuum vessel, also containing 

 liquid air, no air distils through tube A. This is the most con- 

 venient arrangement to use for the production of solid air. For 

 this purpose B is connected with an air pump until the pressure 

 is reduced to about J inch, and therefore the temperature about 

 — 200° C. Then a good air pump is put on to the inner vessel of 

 liquid air (containing oxygen and nitrogen in the normal proportion 

 of oxygen and nitrogen), by means of the tube A, while maintaining 

 constantly the exhaustion in the outer vessel. In a short time the 

 air in the inner vessel solidifies to a transparent jelly-like mass. 



The same principle is used when the latent and specific heats 

 have to be determined. Fig. 5 shows the general plan of the ap- 

 paratus. Now a definite quantity of heat has to be conveyed into the 

 inner vacuum vessel containing liquid air, with the object of finding 

 the weight of liquid that distils off, on the one hand, or the elevation 

 of temperature in the liquid that takes place on the other. For the 

 purpose of adding a given quantity of heat it is convenient in some 

 cases to use mercury (as represented in the figure), or to lower a piece 

 of platinum or silver, or even glass, into the inner vessel : each unit of 

 heat supplied evaporates a definite amount of air, which is readily 

 ascertained by collecting the gas which comes off during the heat 

 conveyance. In Fig. 5, A is the mercury, C the inner vessel of liquid 

 air, D a three-way stop-cock, F a tube for collecting the air given 

 off; E is a barometric tube for observing the pressure when the 

 inner vessel is exhausted. In a latent heat determination all that 

 is necessary is to weigh the mercury added and to measure the amount 

 of air by volume which has distilled from the liquid state. If the 

 specific heat of the liquid is wanted, then the inner vessel is exhausted 

 (as well as the outer) through the tube F to about J inch pressure, 

 and the three-way stop-cock turned so as to shut off F and connect 

 the inner vessel with the manometer E. Mercury is now dropped 

 into the inner vessel until the manometer rises to the atmospheric 

 pressure or the liquid reaches its boiling point under atmospheric 

 pressure. Care must be taken to prevent the drops of mercury 

 falling exactly in the same place, otherwise a mercury stalagmite 

 grows up rapidly through the liquid, vitiating the results. Another 

 objection to the use of mercury arises from the drops causing the 

 rebound of small liquid air drops, which strike the cork and get 

 evaporated away from the main body of liquid. The amount of 

 mercury added conveys the necessary amount of heat needed to 

 raise the given amount of liquid from its boiling point under J inch 

 pressure to its boiling point under 30 inches. The relative pressures 

 give the temperature range, and the weight of liquid air or other 



