200 i Prof. C. Olszewski on the 



I also showed that nitrogen, boiling in vacuo at 4 millim., 

 gives the lowest possible temperature, reaching —225°, but that 

 it can be used as a cooling agent only as far as —213°, for 

 at lower temperatures, under pressures below 60 millim. Hg, 

 it solidifies to a snowy opaque mass, which is a bad conductor of 

 heat. It follows that liquid oxygen and air are the best means 

 to obtain the lowest temperatures, for neither of them solidifies 

 at all, even at the lowest pressures, and neither possesses trans- 

 parency. I had already used these cooling agents in my 

 former attempts to liquefy hydrogen ; I then exhausted all 

 possible means of obtaining the lowest temperatures without 

 obtaining the desired results ; for the temperature of —220°, 

 i. e., the lowest which can be produced by means of liquefied 

 air, proved to be above the critical temperature of hydrogen. 



On repeating my former experiments I had no hope of ob- 

 taining a lower temperature by means of any cooling agent, 

 but I hoped that the expansion of hydrogen would be more 

 efficacious, on account of the larger scale on which the experi- 

 ment was made. 



The quantity of the frigorific medium, viz., of liquid air or 

 oxygen, did not exceed 2-3 cub. centim. under atmospheric 

 pressure, and became considerably less by the use of the 

 vacuum ; accordingly, the glass tube which contained hydro- 

 gen was only 2 millim. in internal diameter. The phenomenon 

 of liquefaction, or rather of sudden ebullition of hydrogen 

 which appeared in the tube during the expansion, lasted only 

 a fraction of a second, and required a relatively sudden but 

 not complete expansion. The hydrogen, cooled by its expan- 

 sion below its critical temperature, was at once heated in so 

 narrow a tube to the temperature of the surrounding frigorific 

 medium. 



In the subjoined diagram (fig. 2) a represents the lower 

 end of the steel cylinder, serving to liquefy oxygen or air ; 

 this cylinder is enclosed in a glass vessel (double, if oxygen 

 be employed ; triple, if liquefied air) which serves to receive 

 liquid ethylene. The cylinder a is a component part of the 

 apparatus represented in fig. 1, and is therefore marked with 

 the same letter ; but it was increased for these experiments to 

 200 cub. centim. in capacity. The glass tube /is destined for 

 the liquefaction of hydrogen ; the external diameter of this 

 lower and wider part is 11 millim., the internal diameter is 

 7 millim. Within it I place a short glass tube, with very 

 thin walls 6 millim. in diameter ; it serves to isolate from the 

 warmer walls of the larger tube the hydrogen which is con- 

 siderably cooled by its expansion. The tube f is placed in a 

 larger glass tube e, with thin walls, measuring 30 millim. in 



