59 ^ 
Sanitary Agency of Light. 
[October, 
becomes intelligible why the air carries along so many dead 
germs ; why many diseases, in spite of winds and atmo- 
spheric currents, remain confined in their places of ongin, 
and that, when coming from afar, aerial germs reach us in 
an attenuated condition. a , , • 
To retain their virulence they require to be conveyed in 
bales of merchandise, in clothing packed up, or in the dark 
and moist holds of ships. 
In fine, M. Duclaux pronounces solar light the most uni- 
versal, the most economical, and the most efficient agent 
which we can employ, either in private or public sanitation. 
We may be permitted to point out certain practical 
applications of the truths which experiment has thus prougnt 
t0 It S Vs ’ generally admitted by official, or would-be official, 
sanitarians that one of the greatest evils of polluted waters 
is that they serve as a nursery for the propagation of disease- 
germs, and as an organised service for their distribution. 
Surely, therefore, it is exceedingly important that the light 
of the sun should penetrate, without let or hindrance, to and 
into such waters. But so long as any water is rendered 
turbid by suspended matters, or deeply coloured by waste 
waters from tanneries, dye-works, &c., the germ-destroying 
adtion of the sun upon such waters is much enfeebled or 
altogether annulled. . , . r 
We have therefore to ask, what is the meaning ot the 
sneer that “ clarification is not purification ” ? Be it so ; 
but if clarification permits the sun’s rays to penetrate into 
waters where they were previously shut out by the opacity 
of suspended filth, clarification is a step— and no unim- 
portant step— towards purification. . , . 
Again we have heard municipal authorities contend that 
the removal of, or the exclusion of, colouring matters from 
a river is of no consequence. “ So long as we are free 
from stench,” they argue, “ the colour of the river is of no 
consequence.” We now see that it is of great consequence, 
not merely from an aesthetic or sentimental, but horn a 
practical and sanitary point of view. 
This is a consideration which tells heavily against preci- 
pitation processes in which lime plays an important pait. 
We know a town containing extensive tanneries and dye- 
works We know that its sewage can be readily decolourised 
by salts of alumina, whilst if treated with lime-water or 
cream of lime it takes an intense mahogany colour, ad- 
mirably adapted to hinder the sun s rays fiom penetrating. 
