166 JOHN M. SMAIL. 
Smoke particles as is well known aggravate, sometimes actually 
cause fog, and the fog practically stops all efficient circulation of 
air, consequently sewer air from the sewer ventilators cannot. 
diffuse as it does under more favourable conditions, but is retained 
near the surface of the ground, and in the opinion of the writer 
in the Spectator, effects a poisonous action on suffering humanity. 
That under such conditions sewer air, in common with respired 
air, combustion gases, and all the other aerial ashes of civilized 
life, will be retained more or less in the neighbourhood of their 
inception we can hardly deny, but in view of the enormous exten- 
sion of London, annually, and considering that sewer air forms 
but a fractional part of the “aerial sewage ” arising from habita- 
tion, it is clearly absurd to charge the increasingly poisonous 
nature of the fogs of rapidly extending cities entirely to sewer 
ventilation. On the face of it, it must be infinitely safer so to 
dilute the sewer gases and constantly sweep them out into the 
air where they can be harmlessly and readily dissipated through- 
out the atmosphere all the year round excepting on a comparatively 
few foggy days, rather than store them up in increasingly com- 
pacted or concentrated form to be intermittently delivered into 
the houses under the constantly recurring conditions pointed out 
by Mr. Smail. Ventilation of sewers is clearly a necessity in 
some direction, but at the same time it has not been clearly shown 
that under certain conditions ventilation without disinfection is 
free from danger. 
It is now a pretty generally accepted dictum that organised 
poisons do not find their way into the air from water surfaces, 
but that the danger from these arises when the margins become 
dry and so enable these poisons to be borne by the air currents in 
the form of dust and so scattered throughout the atmosphere, as — 
for example the malaria so commonly following the partial drying 
up of swampy lands. If this is correct it is quite possible to con- 
ceive that the very drying up of our sewers caused by the ventila- 
tion currents as described by Mr. Smail, may furnish an element 
of danger by carrying freely into the open air organized poisons, 
