Royal Society, 385 



bottle. After | of the mercury is distilled off (which is accom- 

 plished in a very short time), the iieck is withdrawn ; and while the 

 mercury- vapours are still streaming out, an iron test-tube, previously 

 prepared with great care and charged with 4-5 grms. of potassium, 

 is dropped into the bottle, the neck reinserted, and, after the whole 

 of the bottle has been immersed in the zinc, the blast of the forge 

 is forcibly increased so as, in the shortest possible time, to bring 

 the zinc into the state of boiling, proper arrangements being made 

 for keeping the neck of the bottle red-hot. The potassium in a 

 short time begins to volatilize, issuing in jets into the air and 

 depositing caustic potash at the nozzle, which must be kept clear 

 by means of an iron wire. As soon as the distillation of the potas- 

 sium ceases, the nozzle is closed by means of a ground-in wire 

 plug, at once immersed into a mass of mercury contained in a 

 test-tube, and the bottle withdrawn to a proper support, on which 

 it is allowed to cool. 



After it has reached a manageable temperature, the bottle is 

 inserted into a mass of recently boiled water, the wire plug with- 

 drawn, and the hydrogen formed by the action of the water on the 

 potassium pumped out, by means of a " Sprengel," into a eudiometer, 

 to be measured. 



In the experiments we have hitherto carried out, we have satisfied 

 ourselves that the amount of mercury-vapour not swept out by the 

 potassium is quite inappreciable ; and as our object has been in the 

 mean time to merely arrive at approximate results and to perfect 

 our methods of manipulation, we have neglected the minute cor- 

 rection which, on account of that small remnant of mercury, ought, 

 strictly speaking, to have been applied to the volume of the vapour 

 as calculated from the capacity of the bottle in the cold, the coeffi- 

 cient of expansion of iron, and the temperature (1040° Deville) at 

 which the vapour was measured. 



The results of our observations conclusively show that the density 

 of potassium-vapour, as produced in the process described, cannot 

 exceed 45 times that of hydrogen, and that therefore the molecule 

 of potassium consists of two atoms (K 2 ). 



We intend to prosecute our research in other directions, propo- 

 sing to ascertain, if possible, the densities of the iodides of caesium, 

 rubidium, and potassium, these being, according to Bunsen's experi- 

 ments, the most volatile of the haloids of the alkali-metals. 



March 13. — "William Spottiswoode, M.A., Treasurer and Vice- 

 President, in the Chair. 



The following communication was read : — 



"Note on Supersaturated Saline Solutions." By Charles 

 Tomlinson, F.R.S. 



In the year 1866, M. Grernez and M. Yiollette published each a 

 memoir on supersaturated saline solutions*, in which the same 



* Annales Scientifiques de l'Ecole Normals Superieure, tome 3 e , Amiee 

 1866, pp. 167 and 205. I am indebted for this reference to the courtesy of 

 M. l'Abbe Moigno. 



Phil. Mag. S. 4. Vol. 45. No. 301. May 1873. 2 C 



