INLETS FOR INFECTION. 



75 



taminating air is not so much due to occasional and temporary failure 

 in the efficacy of the trap as to an almost constant absorption of sewer- 

 air by the water on the sewer-side of the trap, and its subsequent dis- 

 charge from the house-side. Dr. Fergus has made a series of experi- 

 ments in a glass tube so bent as to resemble the ordinary " siphon " 

 trap, and charged with water. Certain gases were evolved on what 

 we may call the sewer-side of the trap (J), and tests were applied to 

 ascertain whether the gases succeeded in passing through the water. 

 The results as tabulated by Dr. Fergus are as follows : 



It was, however, urged that the results would probably be different 

 if the trap were ventilated. A ventilating-shaft (c) was, therefore, 

 inserted in the upper part of the bend on the 

 sewer-side, and the experiments were repeated. 

 "The results," says Dr. Fergus, "were much 

 the same, except that the reaction was a little 

 longer in showing itself." 



Ordinary sewer-air may be taken to contain 

 in every hundred parts about seventy-nine parts 

 of nitrogen, nearly twenty of oxygen, not quite 

 half a part of carbonic acid, and traces of sul- 

 phuretted hydrogen, marsh-gas, and ammonia. 

 These gases, however, when inhaled in the pro- 

 portions indicated, can hardly be regarded as 

 materially affecting health. Sewer-air also con- 

 tains organic matter in the form of vapor, and 

 of definite particles ; but doubts have been ex- 

 pressed as to whether these organic particles 

 succeed in making their way through water- 

 traps, and some carefully executed experiments 



of Dr. Neil Carmichael, of Glasgow, have gone far to show that they 

 do not do so. 



There are other ways, however, in which danger comes about. 

 The water in traps is apt to be sucked out by siphon-action, as the 



lu 



Fid. 1. 



