of Ammonia, Sulphur dioxide, and Carbon dioxide. 371 



free from iron and chlorides. The silica was dried on the 

 water-bath and afterwards strongly heated in a silica 

 crucible. The perfectly white mass was finely ground in an 

 agate mortar, and the finest powder obtained by shaking- 

 through satin. A little of this powder was heated strongly 

 in the silica crucible, and introduced while very hot into the 

 gas tube, which had previously been dried out as described 

 below. 



In order to obtain satisfactory dust figures, it is absolutely 

 essential that the moisture adhering to the surface of the 

 glass tube should be removed. Warming with a flame is 

 useless. The tube was packed with asbestos into an iron 

 tube which covered it completely, the part J alone projecting. 

 The iron tube was strongly heated by a combustion furnace, 

 whilst the air in the gas tube was pumped out through drying- 

 tubes containing calcium chloride and phosphorus pentoxide. 

 The temperature should be as high as it is safe to go without 

 softening the tube, say 350°-400° C. Dry air was then 

 allowed to pass slowly into the tube through the drying- 

 apparatus, and the process of exhaustion and filling with dry 

 air repeated several times. The tube was allowed to cool 

 still connected with the drying tubes, and when cool a small 

 quantity of the silica powder introduced as rapidly as possible. 

 The exact amount required can be ascertained by trial ; a 

 very small quantity is sufficient. The operation of drying 

 out was then repeated. The first drying cannot be carried 

 out with the silica in the tube, as this leads to adherence of 

 the powder to the glass. The tube was then connected with 

 the apparatus for filling with gas, shown in fig. 1. 



This apparatus was entirely of glass, no rubber or cork 

 joints coming in contact with the gas passing into the tube. 

 All taps and connexions were lubricated with vaseline in the 

 case of ammonia and carbon dioxide, and with syrupy 

 phosphorus, pentoxide in the case of sulphur dioxide. The 

 ammonia gas was obtained from a cylinder of anhydrous 

 liquid ammonia of 99*8 per cent, purity, and was re-liquefied 

 in a 50 c.c. condensing vessel B, of thin glass, contained in 

 a Dewar vessel A filled with solid carbon dioxide and ether. 

 The vessel B was then disconnected from the supply cylinder 

 and the ground cap C attached. The whole apparatus was 

 then brought into connexion with a Geryk pump through P. 

 Tube D contained broken sticks of caustic soda and E quick- 

 lime, in the case of ammonia; in the case of sulphur dioxide 

 both tubes contained phosphorus pentoxide. Tube G con- 

 tained dry glass wool. The pump was protected by the 

 second set of drying tubes. H contained quicklime and N 



2B 2 



