SECTIONAL TRANSACTIONS.— G. 383 



The effect of stress concentration in the case of reversal of stress was studied by 

 using a fatigue-testing machine. By comparing the loads necessary to cause fracture 

 under reversal of stress of (1) standard specimens and (2) specimens with fillets, the 

 weakening effect of stress concentration can be obtained. This effect depends not only 

 on the fillet proportions but also on the property of the material. 



The experimental results contained in the copper show that the weakening effect 

 of stress concentration in the case of ductile material can only be estabUshed accurately 

 provided data on the fatigue tests of models of the particular machine-part are avail- 

 able. 



12. Mr. Edgar Morton. — Composition and Texture of Sandstones and 



Limestones in relation to Strength and Durability. 



Visit to Lewis Evans Collection of Surveying Instruments, Old 

 Ashmolean Building. 



13. Mr. Edward Hughes. — Influence of Voltage Harmonics on Power 



Factor Correction. 



Wednesday, August 1 1 . 



14. Prof. David Ellis. — The Use of Sulphur Bacteria as Indicators in the 

 Investigation of Polluted Water : The Blackening of the Sand in 

 the Clyde Estuary. 



In the routine bacteriological examination of water for evidence of sewage 

 jjollution it is customarj^ to make a count of the total number of bacteria per unit 

 volume, and then to ascertain the number of such that may be ascribed to Bacillus 

 colt commu7iis. Suoli a procedure involves a fair knowledge of bacteriological 

 technic, and takes time to accomplish. A clear water may or may not be polluted 

 by sewage : its appearance is no guide to its bacteriological content. The writer 

 has found the sulphur bacteria valuable agents in enabling him to detect .sewage 

 pollution in waters that superficially appear to be clean ; and he has been better 

 ■able in consequence of this aid to select samples of water for more detailed analysis. 

 The sulphur bacteria are readily identified, and in most cases the organism of this 

 group which is present in polluted waters is Beggiatoa alba. This microbe is 

 described, and methods of identification, both microscopically and macroscoiiically, 

 are given. It multiplies under circumstances where animal or vegetable remains 

 are undergoing decomposition under water, or where a water, otherwise pure, has 

 received sewage contamination. If it appears in a water, either flowing or still, in 

 which organic remains are not undergoing decomposition, an almost certain sewage 

 contamination is indicated. The presence of the organism may thus be used by the 

 water engineer for the detection of such pollution, enabling him to select the best 

 samples for more detailed analysis. 



Application to a specific rase. — The sand in the Clyde estuary at many parts is 

 ■coloured black under the surface. It has been assumed, on superficial investigation, 

 that the sewage poured into the Clyde was responsible. At various places in the 

 Clyde area where the sand is blackened those points were noted in which Beggiatoa 

 ■alba grew in abundance. Altogether some two dozen places of this kind came under 

 observation. In every case where the organism appeared, the cause for its appearance 

 •could be traced either to animal or vegetable remains undergoing decomposition or 

 to local sewage pollution unconnected with the hypothetically polluted matter in 

 the main body of the river. In no single case was Beggiatoa found in the neighbour- 

 hood of the black sand in which both of the above factors were absent. It was thus 

 possible to remove sewage-polhited water from the list of causative agents in the 

 formation of black sand. As a result the way was clear for the ascertainment of 

 the true reason, namely, the formation of ferrous sulphide produced by a chemical 

 reaction between the ferruginous constituents in the sand and the hydrogen sulphide 

 liberated by the bacteria which consume the animal and vegetable remains on the 

 shore. 



