26 Proceedings of Royal Society of Edinburgh. [sess. 
ash ” furnace at a very high temperature in the course of Leblanc’s 
process for the manufacture of soda ; and the transformations 
which take place in the “ alkali waste ” or refuse from this process,, 
under the influence of atmospheric agencies, are, perhaps, worth 
studying in this connection. 
Thus, in the case of calcium sulphate its reduction in this fashion 
produces calcium sulphide, a compound which is insoluble in water. 
Calcium sulphide is the principal constituent of the “black-ash” 
waste, after the carbonate of soda has been dissolved out ; and 
when this is left in contact with water it gradually decomposes, 
yielding calcium sulphydrate and calcium hydroxide thus : — 
(Krausliaar, Dingier' s Polytechnischcs Journal , vol. 226, p. 412.) 
2CaS + 2H 2 0 = CaH 2 S 2 + CaH 2 0 2 . 
The action of air upon the calcium sulphydrate has been shown 
by the same author to form calcium polysulphide and calcium, 
thiosulphate, CaS 2 0 3 . The action of carbon dioxide is well known. 
The sulphur in the mud when the sulphates have been reduced 
is supposed by Messrs Murray and Irvine in part to pass into solu- 
tion as sulphuretted hydrogen, and then to be oxidised back to sul- 
phuric acid, which in turn decomposes the carbonates and re-forms 
sulphates. “ A certain part of the sulphides, or it may be of the 
hydrosulphuric acid, reduces the ferric oxide of the deposit, forming 
sulphide of iron.” The sulphide of iron, it is pointed out, gives the 
characteristic blue-black colour to the great majority of blue muds. 
How, it is a well-known fact that living organisms reduce the 
sulphates and the mud becomes more or less black or dark in 
colour by reason of the formation of ferrous sulphide. Hence, 
those who are accustomed to examine rivers and streams for 
pollution first take note of the colour of the mud, and observe 
whether it undergoes any change, for when the pollution is inter- 
mittent the mud may be red or yellow one day and black the next, 
subsequently returning to its original colour. The chemistry of 
the changes which take place between the constituents of such 
muds has in a great measure been explained by Mr J. Y. Buchanan 
in a paper “ On the Occurrence of Sulphur in Marine Muds,” etc. 
( Proc . Royal Soc. Eclin. Read December 1, 1890). 
He points out the part which is played by manganese sulphide. 
