of vegetation in the salt plain, the other into the larger section. In brilliant sunshine 

 (on May 3rci-4th, 1950), the main body of water reflected a vivid milky blue, though a 

 bottle sample was virtually colourless with a slight haze. The blue of the smaller 

 section was deeper and more vivid than that of the larger, probably owing to a greater 

 concentration of suspended particles of colloidal sulphur. Bordering the blue was an 

 uneven band of red gelatinous material, stretching in some places several yards from 

 the banks in shallow water. Bulbous formations (4" x 2") could be seen in this red 

 material and a few red masses were floating in the water. There was a pronounced 

 smell of hydrogen sulphide and some wind- blown sulphur was visible near the banks. 

 Salts were crystallising out round the edges of the lake. 



Other observers have given descriptions differing from ours in important details. 

 Pinfold and Gee, who visited Ain-ez-Zauia on October 7 th, 1949, made no mention of 

 the red material bordering the pools, but reported numerous floating masses of a red 

 jelly-like material. It is remarkable, too, that Mancuso, who saw Ain - el - Braghi 

 several times in 1937, makes no mention of the red material, and describes the water 

 as milky- white; when visited by us on 6th May, 1950, it showed the same colour 

 characteristics as Ain-ez-Zauia. Clearly the appearance varied considerably with 

 the season and with the observer. 



Detailed Examination of Ain-ez- Zauia. We carried a specially designed 'des- 

 ert laboratory' which contained sample bottles, apparatus for sampling water from dif- 

 ferent depths and bottom mud, solartions and apparatus for sulphide and pH determina- 

 tions and an alcohol lamp. Samples from different parrs of the lake were taken from 

 an inflated rubber dinghy. Most of the samples were stored for future examination at 

 Teddington ; others were examined microscopically beside the lake. The principal 

 results of the examination are given below. 



(1) General. The water was saline and had a pH of 7.4. Its probable com- 

 position is given in Table 1. Noteworthy points are the presence of about 2% NaCl 

 and of a saturated solution of calcium sulphate, the latter derived from crystalline 

 gypsum, lumps of which could be detached from the bed of the lake. The presence of 

 boron was also mentioned by Mancuso. The organic content is low. 



(2) Sulphide formation. The evolution of hydrogen sulphide was strong evi- 

 dence for the activities of sulphate- reducing bacteria. The concentration of sulphide 

 in bottom water samples was 108mg. HjS/l., and at the surface 15-20mg. HjS/l. 

 Microscopical examination showed that many vibrios resembling sulphate reducers 

 were present. Enrichment cultures of these organisms were subsequently obtained 

 from practically all water and mud samples, from which several pure strains were iso- 

 lated. Two of these strains were found to be unusual, in our experience, in being un- 

 able to utilise gaseous hydrogen for sulphate reduction, i.e. they contained no hydro - 

 genase enzyme (Adams et al.. 1951). It seems reasonably certain on this evidence 

 that sulphate reduction by sulphate- reducing organisms was taking place in the lake. 

 It is possible, however, that some HjS entered with the warm spring supplying the lake. 



(3) The Coloured Gelatinous Material. This carpet-like material developed 

 prolifically in the shallow waters, in places stretching several yards from the banks 

 and sometimes appearing above the water level. It was composed of gelatinous mater - 



114 



