ANNIVERSARY ADDRESS. 21 



Sewer air is not always offensive to the sense of smell, and it 

 is a fact that sometimes when most evil smelling it is really least 

 poisonous, because when putrefactive decomposition is marked, 

 while the smell may be very bad, the chemical products of 

 putrefaction are so inimical to the lower forms of life, the 

 microbes, some of which may be the injurious or poisonous 

 elements in the sewer air, that these microbes are killed. It is 

 then important to note, that there is no necessary relationship 

 between the offensiveness and noxiousness of sewer air : the one 

 is not a measure of the other. 



The evil odour sometimes arises from highly odorous materials 

 of a chemical nature poured into the sewers, as, for instance, the 

 contents of tan pits. Here, clearly, however disagreeable it may 

 be, the odour is not particularly poisonous. But even where the 

 odours are due to the chemical bodies arising by decomposition 

 of the sewage matter themselves, Haldane found that they are 

 probably not necessarily poisonous, however offensive they may 

 be, for when considerable quantities of sewer air were drawn 

 through weak sulphuric acid no injurious results followed the 

 injection of the neutralised liquid under the skin of animals. 

 Again, sewer air, as already said, even the most offensive, is not 

 always injurious when inhaled. Haldane records that he and 

 many others, who were not " used to it," as might be said of 

 sewer-workmen, never suffered any evil consequences during all 

 their work. On the other hand, there are cases where the evidence 

 is very strong in support of its having been the cause of diseases, 

 although in most cases ascribed to sewer gas, an examination of 

 the evidence shows, that they were probably more coincidences 

 than examples of cause and effect. We have thus a set of 

 extremely variable conditions to deal with, and altogether a very 

 complex problem to solve. 



The absolute number per litre of organisms in the sewer air of 

 Sydney, as it would appear from the recently published results, 

 is greater than is commonly found in England, but less than was 

 found in Calcutta. Obviously, the warmer and moister air being 



