50 
Transactions Texas Academy op Science. 
no crops grow, besides which the land may become frozen over and incap- 
able of having any purifying effect. The method that has so far been 
most used with success is known as intermitten downward filtration. 
This requires either a very porous natural soil or else the construction of 
artificial beds. These beds are usually from four to seven feet in depth, 
and should be underdrained, and if material of varying coarseness is 
used the larger particles should be below in order to prevent clogging. 
By applying the sewage in intermittent doses the beds become aerated, 
air being drawn down as the sewage flows out through the underdrains, 
and the oxidizing action of the bacteria in the beds converts the unstable 
organic matter into harmless stable compounds. With the proper 
amount of rest between doses the effluent may be made as clear and 
limpid as spring water, and in many cases just as fit for drinking pur- 
poses, for neither chemical nor bacteriological examinations show suffi- 
cient foreign matter present to endanger the consumer. The bacteria 
that produce the effect in this case are aerobic — that is, they require the 
presence of air before they become effective. Usually it requires some 
little time for the filter bed to attain its maximum purifying power, and 
examinations show that the sand grains — particularly those near the 
surface — are then covered with colonies of these oxidizing bacteria. 
Occasionally the surfaces of the filter beds — of which there should be 
several in order to allow for the period of rest above referred to — have 
to be raked to prevent clogging by larger particles in the sewage, but the 
body of the filter does not become clogged, for most of the insoluble mat- 
ter is converted into stable compounds that are. soluble in water and 
which pass out with the effluent. Many modifications of the sand filter 
have been tried, but the process of intermittent filtration has so far been 
found most effective. One of the modifications that still retains some of 
the characteristics of this method is to force large quantities of air 
through the filters, and thus accelerate their action. Cof. George E. 
Waring, Jr., who died of yellow fever contracted in Havana while investi- 
gating the sanitary conditions there last year, devised a method that is 
said to give no offence when used in close proximity to dwellings. Some 
months ago I wrote to the company in regard to the cost of a plant for 
purifying the sewage from a village of 1000 inhabitants and they said 
that the wastes could be easily purified in a space 80x80 feet square. 
Their method of cleansing the beds is somewhat similar to that used in 
the mechanical filtration of water. 
In the sand filtration method the action of the bacteria is aerobic, but 
within the last two or three years it has been discovered that another 
class of bacteria exist that are active in the absence of air and light, 
though I have recently seen it stated that it is not absolutely necessary 
to exclude either. These are the kind of bacteria that produce putre- 
