178 



On the Mysterious Appearance op a Diatom. 



By F. Kitton, Hon. F.R.M.S. 



{Read July 2ith, 1885.) 



In my list of Norfolk Diatoms I note two species occurring in 

 the water supply of the city of Norwich. These were Asterionella gra- 

 cillima, Heib, and Diatoma elongata. Previous to the covering in of 

 the reservoirs these forms were nearly always present in the deposit 

 left in a precipitating glass, or on a piece of filtering paper after 

 allowing the water to ran slowly through it for several hours. 

 Since the reservoirs have been covered I have not been able to find 

 these or any other forms in the water, but I have noticed that 

 occasionally during the spring and summer months rust-coloured 

 stains appeared on the carafes if the water was allowed to remain 

 unchanged for a few days. This I thought might be attributed to 

 the presence of lime and iron, particularly as they apparently dis- 

 appeared with effervescence when acid was introduced. A few 

 weeks ago I scraped off a little of the film and examined it with a 

 ^-inch objective, when I found it was composed entirely of frustules 

 of Achnanthes (Achnanthidium, Breb.) linearis ; this somewhat sur- 

 prised me, as I have never found it on the filter papers. I there- 

 fore (on June 25) filtered 8 ozs. of the water into a glass-stoppered 

 bottle, using a filter paper 1 inch in diameter and a very small 

 •^lass funnel. When the bottle was filled I removed the paper and 

 boiled it in sulphuric acid, which I afterwards decarbonized with chlo- 

 rate of potash ; the residuum, after the usual washing and concentra- 

 tion, I examined, but could find no indication of diatomaceous re- 

 mains. I now turned my attention to the bottle of filtered water. In 

 the course of a few days the stains began to appear, and on July 7th 

 were much increased in size and depth of colour. I poured off the 

 water and removed the film with a camel's-hair pencil ; after clean- 

 ing with nitric acid I found that this also consisted of the above- 

 named Diatom unmixed with any other form. As this is a very 

 minute species ('000 1" in length and less than '0002" in breadth), 

 I thought it just possible that some of the frustules might have 



