~Percentage Formula~ for addition of salts, etc., to completed media.

~Formula for preparing any desired percentage~ of a given salt, etc., in
tubed media; e. g., to make 4 per cent. solution of KNO_{3} in a
series of tubes of broth each containing 10 c.c. of medium, when there
is already available a 25 per cent. stock aqueous solution of potassium
nitrate.

(_N_ + ~X~) _Y_ _A_ (~X~)
--------------- = ----------
100 100

_N_ = number of cubic centimetres contained in each tube.

~X~ = amount of stock solution to be added to each tube.

_Y_ = percentage required in the medium.

_A_ = percentage of stock solution.

Then

(10 + ~X~) 4 25 ~X~
------------ = ------
100 100

Therefore, 40 + 4~X~ = 25~X~.

Therefore, 21~X~ = 40.

~X~ = 1.9 c.c.

This allows for solution added to the original bulk of medium.

Therefore, 10 c.c. broth + 1.9 c.c. of a 25 per cent. aqueous solution
KNO_{3} makes 11.9 c.c. medium containing 4 per cent. KNO_{3}.

~TABLES FOR PREPARING DILUTIONS~

(of Serum, Disinfectants or other substances.)

In estimating the agglutinin content or _titre_ of a serum, testing
disinfectants and for many other purposes, it becomes necessary to
prepare a series of dilutions of the material under examination, and in
order to avoid unnecessary expenditure of labour it is convenient to
adhere to some definite scale of increment, such for example as the
following:

From dilutions of 1:10 to 1:80 rise by increments of 5.

From dilutions of 1:80 to 1:200 rise by increments of 10.

From dilutions of 1:200 to 1:400 rise by increments of 25.

From dilutions of 1:400 to 1:500 rise by increments of 50.

From dilutions of 1:500 to 1:1000 rise by increments of 100.

From dilutions of 1: 1000 to 1:5000 rise by increments of 250.

From dilutions of 1: 5000 to 1:10,000 rise by increments of 1000.

From dilutions of 1:10,000 to 1:100,000 rise by increments of 5000.

From dilutions of 1:100,000 to 1:1,000,000 rise by increments of 100,000.

When dealing with a substance of unknown powers--and this is especially
true with regard to agglutinating sera--it is customary to run a
preliminary test, using a few widely separated dilutions such as may be
obtained in the following manner:

FIRST DILUTION--I.

1 c.c. serum + 9 c.c. normal saline solution = 10 per cent. solution or
1: 10 dilution (of which 1 c.c. c