CHARCOAL AS A DEODORISING AGENT. 239 



it does ammonia, it nevertheless does absorb air, and especially 

 the oxygen therein contained, sufficiently readily to enable it to 

 act somewhat after the fashion of the spongy platinum in Expts. 

 284 and 285 ; i.e., if also brought into contact with gases capable 

 of being chemically acted upon by oxygen under suitable condi- 

 tions, it will absorb both and cause them to act upon one another 

 in its pores. In this way deleterious gases from drains and 

 offensive vapours from decaying organic matter that otherwise 

 would escape into the atmosphere and render it injurious to health 

 may be deodorised and rendered innocuous by making them pass 

 through a layer a few inches thick of charcoal broken into small 

 fragments ; thus a deodorising charcoal trap to cover sinks and 

 the orifices of drains, &c., is often employed as a remedy for bad 

 smells and injurious emanations ; the evil-smelling gases and 

 vapours being absorbed by the charcoal and converted by the 

 oxygen also absorbed into other substances not offensive to the 

 nostril nor injurious to health. The activity of charcoal in this 

 respect is far inferior to that of platinum sponge, so that the action 

 never goes on sufficiently rapidly to cause the charcoal to heat 

 perceptibly. 



The carcase of a dead cat or dog may be kept in the open air 

 for weeks in summer without being at all a source of nuisance by 

 thickly covering it with lumps of good charcoal; all offensive 

 products of putrefaction being absorbed by the charcoal and 

 deodorised as rapidly as formed. Coal smoke and soot have the 

 power of exerting a similar action, though to a lesser extent ; 

 accordingly, the visible " blacks " in the air of smoky towns in 

 which coal is largely burnt, although most objectionable from 

 many points of view, are nevertheless not wholly without use, as 

 they tend to purify the air by facilitating the natural process of 

 purification by the action of oxygen on deleterious gases and 

 vapours contained in the atmosphere. 



Expt. 287. Use of Charcoal and Analogous Substances for 

 Filtration. Ordinary water (as occurring in rivers, streams, lakes, 

 ponds, &c.) is liable to several sorts of contamination which it is 

 usually desirable to remove as far as possible by various puri- 

 fication processes. One of these applicable to hard spring water 

 containing much bicarbonate of lime consists in adding just so 

 much lime to the water as will convert the soluble bicarbonate of 

 lime into insoluble carbonate (Expt. 156) ; muddy waters are 

 clarified by filtering them through vast beds of sand or similar 

 material forming the top layer of a " filter bed," the lower layers 

 of which are successively gravel and pebbles ; so that as the water 

 percolates through it is finally received in channels and culverts, 



