16 Proceedings of the Royal Society of Edinburgh. [Sess. 
“reversed filter/’ and shows that reduction with zinc dust is extremely 
rapid. 
As our modus operandi differs considerably from that of Carnegie, a 
brief description may be given. The ferric solution should contain as little 
free acid as possible, the best condition being when there is just enough 
acid present to prevent precipitation of hydroxide or basic salts. Reduction 
is complete if this ferric solution is heated to boiling and poured through 
a narrow calcium chloride tube, packed to a depth of about four inches 
with fine zinc turnings, which are kept in the tube by a plug of asbestos. 
The solution is drawn through by suction and the filings and tube washed 
three or four times with a little, very dilute, hot sulphuric acid solution. 
Fine zinc turnings are not always obtainable, and the following method 
of reduction with zinc dust was found to be equally satisfactory if the 
unused zinc dust was efficiently washed. 
To the ferric solution in a boiling tube about 1 gramme of zinc dust was 
added, the mixture heated quickly to boiling, and at once poured through 
an ordinary filter paper, on which about 1 gramme of zinc dust had been 
placed. Reduction under these conditions was complete, but thorough 
( qyi\ 
about sulphuric acid was necessary to remove „ 
the last of the iron from the zinc dust. If the ferric solution contained so 
much acid that vigorous effervescence occurred in the cold, reduction was 
usually incomplete ; any excess of free acid must therefore be neutralised 
with alkali before reduction. It is convenient to use suction for the filtra- 
tion, but care must be taken that the end of the filter funnel projects at 
least 4 to 5 cms. below the side tube of the filter-flask, as otherwise part of 
the solution is carried into the pump. 
Zinc dust is usually almost free from iron, and the amount used in a 
reduction, as above described, is so small that the error from this source 
will usually be negligible. 
Chemistry Department, 
University op Edinburgh. 
{Issued separately January 10 , 1912 .) 
