solvents and diluents	211
the more volatile among them are the more toxic. Acetone, alcohol, ether, and
chloroform are narcotics. They affect the nerve centers causing stupefaction,
intoxication, and ultimately anaesthesia. The coal-tar hydrocarbon, benzene
or benzol, has a direct physiological effect on blood cells, causing leucopenia and
anaemia. Excessive use of this material seems permanently to injure bone marrow
where the cells originate. This poisoning can reach an acute stage resulting in
death. The National Safety Council has set as a safety limit for the concentration
of benzene vapor 100 parts of benzene per I million parts of air. Toluene and
xylene are less toxic, but relatively impure grades of toluene may contain benzene.
The vapors of chlorinated aliphatic hydrocarbons—particularly trichlorethylene,
tetrachlorethane, and, to some extent, carbon tetrachloride and ethylene dichlo-
ride—affect the central nervous system and damage the liver and kidneys.
Petroleum hydrocarbons are far less injurious. Exposure to concentrated vapors
of them may produce dizziness and nausea. Carbon disulphide has an effect on
the central nervous system. The toxicity of methyl or wood alcohol when taken
as a fluid is well known. It injures the central nervous system, but moderate ex-
posure to the vapors has not been shown to be seriously harmful. Most operations
in the painting and restoration of pictures do not involve high concentrations of
any of these vapors. Spraying of varnishes, particularly on large areas, is an ex-
ception, as is the use of a solvent in the removal of such film materials, especially
where the operator is working with magnifying lenses and is close to the evaporat-
ing surface. In all cases, good ventilation should be provided and work should be
stopped immediately at any sign of physical irritation, headache, or drowsiness.
The National Safety Council supplies the following information with regard
to the toxicity of some of the common solvents. The figures after the solvent are
the maximum allowable concentrations of solvent vapor in parts per million of air.
SOLVENT      CONCENTRATION         SOLVENT         CONCENTRATION
(p.p.m.)	(p.p-m.)
Amyl acetate	400	Carbon disulphide	15
Butyl acetate	400	Carbon tetrachloride	100
Butyl alcohol	100	Ethylene dichloride	100
Methyl alcohol	100	Ethyl ether	400
Benzene	100	Turpentine (American)	200
Benzol	75	Xylol	200
Spike Oil (see Oil of Spike Lavender).
Spirits. Specifically, this term is used for volatile substances dissolved in
alcohol, but in older usage it was applied to any distilled volatile liquid. It is still
commonly given to ethereal essential oils and distillates—spirits of turpentine,
petroleum spirits, Cologne spirits, and wine spirits* Methyl alcohol is still occa-
sionally called ' wood spirits/