492 On Vapour Densities of Potassium and Sodium. [Dec. 11, 



III. 



•027 grm. gave 24'6 c.c. at 0° and 760 mm. Mol. wt 24*5 



•027 „ .... 23-2 „ „ „...-. 26-0 



•037 „ .... 31-7 „ „ „ ....26-1 



•030 „ .... 29-1 „ „ „ 23-0 



Mean molecular weight. . . . 24" 9 



The new determinations of the molecular weights of potassium and 

 sodium are just about half the former values, and would seem to 

 support the inference that the atom of each of these metals resembles 

 mercury and cadmium in the gaseous state, as regards molecular 

 volume. Such a remarkable result cannot be accepted without a very 

 thorough investigation of the secondary reactions which may be in- 

 duced at high temperatures. It is certain that in the platinum vessels 

 there is much less absorption of vapour than in the case of iron, and 

 that the action of the furnace gases is all but eliminated. The 

 following suggestions might be made to explain the results. In the 

 first place, the metals may contain a large amount of occluded 

 hydrogen. With reference to this assumption, we have examined 

 the sodium used in our experiments and find, on careful exhaustion 

 and boiling in a Sprengel vacuum, that one volume of sodium only 

 contains one-third of its volume of occluded hydrogen, so that 

 this will not account " for the double volume of the atom of the 

 metals." The alkali metals may form volatile compounds with 

 platinum, although, judging from analogy, such compounds are 

 not likely to exist. If so, the vapour blown out of the bottle ought 

 to contain platinum. The use of platinum tubes to blow out the 

 vapours prevented the test being applied. The metals may be 

 changed into oxides from occluded oxygen in the platinum. If so, no 

 free metal ought to remain in the bottle after this rapid evaporation. 

 This however is not the case ; so that, unless some other explanation can 

 be given, it will be necessary to admit some anomaly in the behaviour of 

 potassium and sodium, as regards their vapour densities at high tem- 

 peratures. In future the variation of the density with the tempera- 

 ture must be determined, but this is very difficult at temperatures 

 about 1,500° C. The chief drawback to the prosecution of the 

 experiments is the terrible waste of platinum vessels, which never 

 stand the combined action of potassium, sodium, and hydrogen at 

 high temperatures, beyond two or three operations. 



The following determinations of the molecular weight of potassium 

 iodide, made in the iron apparatus, are of importance in connexion 

 with the previous results. 



