1906.] on Studies on Charcoal and Liquid Air. 441 



would be seen on the sulphur side of the U-tube. But the contrary 

 is the case, as the sulphide is seen to form at first on the surface of the 

 mercury (Hg). The explanation that suggests itself is that the 

 ordinary molecule of sulphur containing 6 to 8 atoms, dissociates 

 and throws off single or double atoms which make their way rapidly 

 through the mercury vapour without wholly combining with it, until 

 they reach the surface of the liquid mercury, when the conditions for 

 chemical interaction are more favourable. If the tube be constricted 

 as at M (Fig. 15), the activity of the sulphur atoms on the mercury 

 surface is stopped, as they encounter the mercury atoms under 

 the most favourable circumstances for combination in the narrow 

 tube at M, and there form the sulphide which is quite visible after 

 long keeping. 



It is interesting to note the excessively small quantities of the 

 respective elements that are interacting in this case. The pressure of 

 mercury vapour at 15° C. is about Ij millionths of an atmosphere, 

 whereas that of sulphur is of the order of one thousandth part of the 

 millionth of an atmosphere. 



The same phenomenon was exhibited in an even more striking 

 majiner by the following experiment. Two equal bulbs A, B (Fig. 16) 

 were united by a tube containing a finely-ground stop-cock C. A small 

 piece of pure sulphur was put into A, and a small quantity of mercury 

 into B, and the whole exhausted through the side-tube D by either 

 a mercury pump or by the use of charcoal cooled in liquid air. While 

 the exhaustion was going on the sulphur in A was fused, and finally 

 the tube D was sealed off. On shutting the stop-cock C, and allowing 

 the apparatus to stand for some time, the application of a liquid air 

 sponge to any part of the bulb B at once caused the deposition of a 

 mercury mirror, which disappeared on allowing the cooled spot to 

 regain the ordinary temperature. If the same cooling is appUed to 

 any part of the bulb A, no deposit takes place ; but, if the sulphur 

 has been recently fused so as to get it deposited on a portion of the 

 glass surface in what is called the utricular state, then, in a short 

 time a fine film of highly refracting sulphur is seen to have distilled 

 to the cooled part, just hke what takes place with the mercury 

 in the other bulb. Now let the stop-cock be opened for a few 

 seconds and shut again, and the local liquid air cooling on parts of 

 both A and B repeated. On the cooled part of B nothing but 

 mercury will be deposited, as before ; but on the cooled part of A a 

 brilliant metallic deposit will be noticed, which is, however, not 

 wholly mercury, for, on heating it with the finger, it is only partially 

 volatilised, the portion which remains being sulphide of mercury. 

 On repeating the operation on another part of A in about a quarter 

 of an hour, all metallic deposition has disappeared. Thus, mercury 

 vapour requires time to react with sulphur, but ultimately all the 

 mercury vapour is removed by the sulphur. The apparatus enables 

 the experiment to be repeated at any time. 



