258 Mr Forbes's Physical Notices of the Bay of Naples. 



ed in the number of this Journal for April 1827, an inter- 

 esting account of a similar fact, observed on the Lake of Morat 

 in Switzerland, in 1825, by Professor Decandolle. It appears 

 to occur there every spring, and to last from November to 

 March or April, which coincides very well with my account. 

 It is then subject to many variations, disappearing in the night, 

 and during high winds. M. Decandolle found this colouring 

 matter to be composed of a new species of animals of the genus 

 Oscillatoria, and imputes their origin to the decomposition of 

 organic matter in its sluggish waters. Such an explanation will 

 apply equally to the Lake Agnano. These animals are de- 

 scribed as less than $oVo "^ ^" ^^^^ ^^ diameter, and have re- 

 . ceived the name of Oscillatoria ruhescens. When kept in bottles 

 for twenty-four hours they exhaled a fetid odour ; but the spe- 

 cimens I took from Agnano, though dried merely in paper, 

 emitted none, and even now, when macerated, have no smell what- 

 ever. The appearance of the matter in a dry state is compact, 

 homogeneous, and brittle, of a reddish brown colour. I shall 

 be happy to furnish anyone interested in the subject with 

 part of the minute quantity I possess of this substance, for the 

 purpose of microscopic examination. 



Some authors have particularly described bubbles of air 

 which rise through the water of Lake Agnano. Hamilton 

 says this is so strong near the Grotto del Cane as to give the 

 appearance of ebullition, — a statement which is confirmed by 

 Ferrari, the Neapolitan topographer, who says it is observed 

 when the lake is full. Breislak denies it, and supposes the mis- 

 take to have risen from the motion of insects ; but there seems 

 no reason to doubt that so natural a phenomenon should oc- 

 cur, as it is nothing but aerial fluids which we know take their 

 rise under ground here, whether simply carbonic or sulphu- 

 reous, ascending through the fine felspathose and augitic sand 

 which composes the bottom of the Lake Agnano ; and from 

 the porous nature of the soil, nothing can be more easy to 

 imagine than that when the water assumes a higher level than 

 usual, a portion of it is imbibed, and gas developed. This 

 will explain the different relations of travellers on the subject. 



At the south-east edge of the lake occurs the small emissary 

 of carbonic acid gas, which has so long been vaguely or inac- 



