I have been told their is a technique of adding lime to a latrine (out house).
What is the purpose, interval and amout to be added?

The main purpose of adding lime appears to be odor control [
http://varsoon.wireboard.com/~jleitess/countryside/hv.greenspun.com/bboard/Wc3347b97148e2.htm
] and [ http://www.zetatalk.com/shelter/tshlt07d.htm ].

One website in particular [
http://www.mindspring.com/~deblair/outhouse.htm ] states that
"Sometimes a bucket of lime would be found sitting in one corner. The
lime bucket would have some form of scoop, such as an old bean can or
discarded dipper, to scoop up the lime. The lime was used to hold down
the smell and speed decomposition of the deposits made into the hole.
It also kept the flies from eating you alive while you were properly
exposed to do the business at hand."

Several places had stated that burned ashes could be use in place of
lime. This was explained here: [
http://www.zetatalk.com/shelter/tshlt07c.htm ] as saying that "Ashes
from the wood stove is a good source of lime to sprinkle into the hole
of the outhouse." In a side note, they also said "Also to keep up the
bacterial action, add the occasional left over vegetable wastes,
especially cooked cabbage but no meats. Treat it like a compost pile.
With a household of six, I never had a smelly outhouse."

According to this environews article [
http://www.gsenet.org/library/11gsn/1999/gs90417-.htm ], if you are
making fertilizer out from your outhouse, it is recommended to add
lime to keep the mixture from becoming acidic, which would leach out
toxic chemicals used to process the fertilizer.

The "leave no trace" page of a rock climbing website [
http://www.lnt.org/RockClimbing/RockDispose.html ] noted that lime can
be added in order to both reduc odor and absorb moisture.

The "handbook for the summer of 2002 at the rocky mountain biological
lab" [ http://www.rmbl.org/handbook.html ] had a section on
"MAINTENANCE AND REPAIR" which stated: "Add a scoop of lime to the
vault after each use. Do not get lime on the toilet seat; it burns
skin dreadfully." It is no surprise that it burns skin, as it is
highly basic (the opposite of acidic but just as dangerous).

One usenet post i found that dealt with the construction of an
outhouse [ http://varsoon.wireboard.com/~jleitess/countryside/hv.greenspun.com/bboard/Wc3374f573dbbb.htm
] had at least one participant recommending regular lime usage,
specifically "I put lime down it when ever it is used."

The general consensus amoung the websites I visited was either to put
lime in "occasionally" or "every time you use it".

So, purpose and frequency were easy enough to find... but why lime
works was a bit harder.

One paper that deals with agricultural waste [
http://www.dasc.vt.edu/knowlton/VDAreasofConcernOdor2.pdf ] and not
outhouses provided some insight. It is all manure though, so the
concept is the same.

They stated that "Dairy manure stored in tanks or in-ground pits
generally develops a floating crust, and with this crust, odors during
storage are not often a problem. When a crust does not form
spontaneously, adding chopped straw or peat moss (2.5-5 lbs/ft2)
encourages crust formation. If slurry pH is low, adding lye or
hydrated lime at 1 lb per 1000 cubic feet daily until pH is over
6.7will also encourage formation of a crust." I interpret this to mean
that average poo in outhouses is too acidic to form a crust, and thus
addition of lime will encourage this formation by reducing the
acidity.

Not content with this however, I look further.

Outhouse america tour faq page [ http://jldr.com/faqs.html ] states
from one reader sent in a report of the use of lime, specifically that
"It tended to slow down the "digestion" by sweetening or decreasing
the acidity and smell of the soil. This was more important in the
summer (for obvious reasons). Lime was used similarly in potter's
fields - mass grave yards for the poor, or for a large die-off during
a plague. While the lime would burn the flesh, the stench was not as
severe as with the natural acidity of rotting protein, urea, and amino
acids."

So basically I interpret this to mean that bases reduce the speed at
which the "stinking process" of human poo proceeds. This is not
surprising from my recent attendance of chemistry class, which
explained that some reactions are highly ph dependant. When in a basic
solution as would be present with lime, certain chemical processes
would proceed orders of magnitude (10's or 100's of times) slower than
in the presence of a normal amount of acidity.

I hope this fully answers your question. If you would like
clarification of this answer, please do not hesitate to ask for it.