better known, and is
the substance usually called white arsenic, or merely arsenic. It is
found as a mineral, but is usually obtained as a by-product in burning
pyrite in the sulphuric-acid industry. The pyrite has a small amount of
arsenopyrite in it, and when this is burned arsenious oxide is formed as
a vapor together with sulphur dioxide:

2FeAsS + 10O = Fe_{2}O_{3} + As_{2}O_{3} + 2SO_{2}.

The arsenious oxide is condensed in appropriate chambers. It is a rather
heavy substance, obtained either as a crystalline powder or as large,
vitreous lumps, resembling lumps of porcelain in appearance. It is very
poisonous, from 0.2 to 0.3 g. being a fatal dose. It is frequently given
as a poison, since it is nearly tasteless and does not act very rapidly.
This slow action is due to the fact that it is not very soluble, and
hence is absorbed slowly by the system. Arsenious oxide is also used as
a chemical reagent in glass making and in the dye industry.

~Acids of arsenic.~ Like the corresponding oxides of phosphorus, the
oxides of arsenic are acid anhydrides. In solution they combine with
bases to form salts, corresponding to the salts of the acids of
phosphorus. Thus we have salts of the following acids:

H_{3}AsO_{3} arsenious acid.

H_{3}AsO_{4} orthoarsenic acid.

H_{4}As_{2}O_{3} pyroarsenic acid.

HAsO_{3} metarsenic acid.

Several other acids of arsenic are also known. Not all of these can be
obtained as free acids, since they tend to lose water and form the
oxides. Thus, instead of obtaining arsenious acid (H_{3}AsO_{3}), the
oxide As_{2}O_{3} is obtained:

2H_{3}AsO_{3} = As_{2}O_{3} + 3H_{2}O.

Salts of all the acids are known, however, and some of them have
commercial value. Most of them are insoluble, and some of the copper
salts, which are green, are used as pigments. Paris green, which has a
complicated formula, is a well-known insecticide.

~Antidote for arsenical poisoning.~ The most efficient antidote for
arsenic poisoning is ferric hydroxide. It is prepared as needed,
according to the equation

Fe_{2}(SO_{4})_{3} + 3Mg(OH)_{2} = 2Fe(OH)_{3} + 3MgSO_{4}.

~Sulphides of arsenic.~ When hydrogen sulphide is passed into an acidified
solution containing an arsenic compound the arsenic is precipitated as a
bright yellow sulphide, thus:

2H_{3}AsO_{3} + 3H_{2}S = As_{2}S_{3} + 6H_{2}O,

2H_{3}AsO_{4} + 5H_{2}S = As_{2}S_{5} + 8H_{2}O.

In this respect arsenic