method, the quantity of a substance in solution is determined from the volume of a solution of another body, which produces with the first a definite reaction, and the strength of which per unit of volume of its solution is known (called a standard solution or normal solution). The result is then found by calculation from the quantity of normal solution employed. The final reaction, which can sometimes be recognized only by a change occurring in another substance, especially added to the fluid (indicator), may be known

a. In saturating a base or an acid with the normal solution (analysis by saturation), by a change of color, or by decolorization of a colored solution (assay of copper with potassium cyanide), or by an indicator such as litmus, which is added for the purpose, as in the estimation of acids or alkaline carbonates.

b. In precipitating the body to be determined, with a standard solution, when precipitation ceases (Gay Lussac's silver assay), or by some change in an added indicator (Schaffner's zinc assay; Pelouze's copper assay); and frequently also by the drop-test, that is, a drop of the assay fluid and of the indicating fluid are brought in contact upon a porcelain plate by means of a glass rod, or alongside of each other upon filtering paper, in such a manner that the edges of the drops run together, or by allowing a drop of the assay fluid to flow down over paper saturated with the indicating substance, etc.

c. In oxidizing or reducing the substance to be determined by means of standard solution without adding an indicator, the final reaction will be recognized by the appearance or disappearance of certain colors (chameleon

1871. Rieth, Volumetrie, Bonn, 1871. Mohr, Titrirmethode, Braunschweig, 1874. Muspratt's Chemie, vii. 167.

assay), or by adding an indicator (starch in the assay of copper, assays of manganese, etc.).

The operations which may occur are as follows:

a. Solution, that is to say, bringing the substance to be tested into a state of solution as in 1, a (p. 38).

b. Preparation of the standard solution, namely— a. By dissolving a weighed quantity of a chemically pure solid substance, and diluting the solution to a definite volume, so that the chemical power of a unit of volume of the solution is known. These liquids are called normal solutions when as many grammes of the substance have been dissolved and diluted to 1 liter (1.76 pints) as are equal to the atomic weight of the substance, and decinormal solutions when a quantity of substance corresponding to of the atomic weight has been used for the solution.

3. By dissolving an unweighed quantity of the solid substance and making an empirical solution by diluting it in a corresponding manner, that is to 1 liter (1.76 pint). The titer of this is determined by allowing it to act upon a measured volume of a solution containing a known quantity of the body to be determined, until the reaction takes place. The titer is then found from the volume of the empirical solution consumed.

The standard (titer) of normal solutions subject to chemical alteration must be verified from time to time. c. Measuring and titration of the assay liquid.-For this are required

a. For measuring, stoppered measuring flasks (Fig. 18) divided up to a mark on the neck into 1,, liter (1.76, 0.88, 0.44 pints) and into small divisions (200, 100 cubic centimeters, etc., 12.2, 6.1, etc., cubic inches); a stoppered mixing cylinder (commonly called a test-mixer) (Fig. 19), having a capacity of from one to two liters (1.76

to 3.52 pints), also divided into cubic centimeters. By means of this, fluids can be measured off, diluted, and mixed in definite proportions. proportions. Pipettes (measuring

cubic

pipettes) (Fig. 20), divided into whole and centimeters; and whole pipettes, capable of holding a certain number of cubic centimeters up to a mark. The latter are used for transferring a certain quantity of assay fluid to a beaker glass, flask, etc. In doing this the lower end of the pipette is either held against the side of the vessel and the fluid allowed to run down

on it, or it is held free. Stohmann's siphon-pipette is

used for removing the clear supernatant liquid from precipitates, or poisonous, bad-smelling liquids, etc.

B. For titrating.-Burettes' for measuring the number

' Stender's glass manufactory in Lampspringe furnishes graduated glass vessels with graduation in red burned in with enamel. König's Ventilburette in Dingler, ccxvii. 134. Kleinert's Chameleon-burette in Fresenius' Ztschr. 1878, p. 183. Bürettenstative in Dingler, ccxxii. 465; ccxxix. 366. Fresenius' Ztschr. 1877, p. 82, 228.

Fig. 23.

It is

standard solution which have been allowed to run into the assay fluid until the final reaction is reached. For measuring assay and normal solutions, it is a very good plan to place two burettes in the same stand side by side. The burette represented by Fig. 22 (p. 47) is well adapted for all uses. provided with a glass cock a; b is a glass cap to protect the liquid from dust; c c are openings in it for the admission of air. Mohr's burette is the simplest form of the apparatus, and has the preference over all others for general purposes. It is, however, not to be recommended in cases where the rubber of the pinch-cock will be liable to act chemically on the liquid employed. (Fig. 23.)

3. Assays by colorimetric analysis.-This method is based upon the principle that equal volumes of solutions of an equally intense color contain also equal quantities