The Air of Sewers. 



515 



It will be seen that the micro-organisms were diminished by nearly 

 one half in passing along the tube. This confirms our conclusions as 

 to the settling of micro-organisms in sewer gas. The micro- organisms 

 would settle out of a drain-pipe especially with great rapidity. 

 Judging from the rate at which they settle in a Hesse's tube, air 

 standing in, or passing along, a 4-inch drain-pipe would become 

 entirely free of micro-organisms within three or four minutes. Hence 

 it seems improbable that micro-organisms can penetrate into a house 

 from a sewer unless with a pretty rapid current towards the house. 



It will be seen from the table that the moulds are more numerous 

 in proportion to bacteria after the air has passed through the tube 

 than before. This is due to the fact, first observed by Hesse, that 

 moulds fall through air less rapidly than bacteria. We should expect 

 to find a similar alteration in the proportion in badly ventilated 

 sewers, but our observations in such sewers were not sufficiently 

 numerous to enable us to say whether this is actually the case. 



Although, as has been seen, most of the micro-organisms present 

 in the air of the sewers we examined seem to have come from the 

 outside air, yet in some cases we had distinct evidence of the 

 dissemination of micro-organisms from sewage itself. In Dundee a 

 few, and at Westminster a large proportion of the drains were found 

 to enter the sewers through the roof. This gave rise to a considerable 

 amount of splashing, the effect of which on the dissemination of 

 micro-organisms in the air it seemed of great importance to investi- 

 gate. The following observations in the sewers bear upon this point. 

 An analysis was made within about 2 feet of a shower of water pro- 

 ceeding from the roof of the Dock Street sewer, the draught being 

 very slight. The number of micro-organisms present was 103 (all 

 bacteria). An analysis made shortly afterwards a few feet to wind- 

 ward of the shower of water gave only twelve micro-organisms. 

 During one of the analyses made at Westminster, a sudden and very 

 violent shower of sewage occurred about 10 feet to windward of the 

 tripod carrying the Hesse's tube. In this case the number found 

 was 25 (all bacteria), whereas an analysis made at the same point 

 a few minutes later, after the dripping had ceased, gave only eight 

 micro-organisms. One of the analyses in the Murraygate sewer was 

 made within about 30 feet of the point where the Hill Town sewer 

 enters the Murraygate sewer, there being a draught of about 2 feet 

 per second from this point to the spot where the analysis was made. 

 The Hill Town sewer has a steep incline, and the water contained in 

 it rushes down with great force, forming a sort of water-fall, the roar 

 of which sounded most impressive as it echoed along the sewer. The 

 analysis only gave three micro-organisms per litre.* 



* The low number thus obtained was possibly owing to the fact that the waste 



