232 Proceedings of the Royal Society 
28. The Marriage with a Deceased Wife’s Sister Bill would make 
the following among other equations possible : — 
29. As the analysis of relationship is important not only in itself, 
hut also as throwing light upon the nature of operators in Mathe- 
matics, I propose to continue the investigation, and to bring the 
results before the Society at a future meeting. . 
Monday , 2 d June 1879. 
Sir 0. WYYILLE THOMSON, Vice-President, in the Chair. 
The following Communications were read : — 
1. Oh the Carboniferous Volcanic Rocks of the Basin of the 
Firtli of Forth : their Structure in the Field and under 
the Microscope. Second Paper. By Professor Geikie. 
2. Additional Observations on the Fungus Disease affecting 
Salmon and other Fish. By A. B. Stirling, Assistant 
Curator of the Anatomical Museum of the University of 
Edinburgh. 
In my former paper, read before the Society in June 1878,* I 
gave an account of observations which I had made on the fungus 
disease affecting salmon, and described the character of the fungus, 
which I referred to So/prolegnia ferax. 
In the present communication I propose to relate additional 
observations, and to discuss the theories which have been advanced 
by different writers in explanation of the cause of the disease. 
Four theories have been advocated, namely — pollution of rivers, 
overcrowding, absence of frost, diseased kelts and addled ova. 
In reference to the theory that pollution is the cause of the 
* See Proceedings of that date. 
