736 Proceedings of the Royal Society of Edinburgh. [Sess. 
barium hydroxide. The gas first passed over heated platinum sponge, 
where any oxygen would be burned. It was then passed through an 
18-inch tube, closely packed with fused potassium hydroxide, then through 
a tube filled with pure phosphorous pentoxide to the combustion tube. 
As the apparatus was of glass throughout, the hydrogen did not come in 
contact anywhere with rubber. The hydrochloric acid formed upon reduc- 
ing the salt passes first into the tube A, fig. 3, which contains about J a 
c.c. of water. The purpose of the water here is to absorb any potassium 
chloride which might be volatilised during the reduction. From here the 
gas passes to two bottles placed in series, containing water, where the 
hydrochloric acid is completely absorbed. 
The temperature at which the reduction takes place in hydrogen is 
comparatively low for the more exposed portions of salt. It always 
required a much higher temperature, however, to carry the reaction to 
•-A 
Fig. 3. 
completion. This temperature was about 350 for the potassium salt. 
Even at this temperature a very little potassium chloride was usually 
volatilised and condensed on the upper parts of the tube. It was never 
necessary to continue the heating long enough to change any of the 
platinum black to spongy platinum. It was of course much more easily 
washed in the more porous form of platinum black than in the form of 
sponge. 
When the reduction of the salt appeared to be complete, the mass of 
platinum black and potassium chloride was allowed to cool with the stream 
of hydrogen still passing through the apparatus. While cooling, the 
platinum will of course absorb a great deal of the hydrogen. After it had 
cooled, it was reheated until a great deal of this hydrogen had been 
expelled. This treatment ensures complete reduction of the salt, as it 
brings the hydrogen into intimate contact with the whole mass, and at the 
same time serves as it were to wash the platinum black free from any 
gaseous hydrogen chloride, that might otherwise be retained even at a 
high temperature. 
After this treatment has ensured the complete reduction of the chloro- 
