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cases with blinds which the visitor has to remove in order to see the con- 
tents has also been tried, but is not satisfactory as the visitor either ignores 
such a case and turns to others more easily examined or leaves the blind 
pulled to one side. The best solution is the use of artificial light only, in 
the exhibition halls, as its actinic strength is less than that of daylight, 
but this is possible, as a rule, only in museums which have been built with 
that idea in mind. Much may be done, especially in summer, by covering 
all windows with heavy blinds which are lowered when the museum closes 
and not raised till it opens again the next morning. In this way, unneces- 
sary light is kept out of the exhibition halls for several hours a day and the 
specimens spared to that extent. 
Specimens on exhibition should be inspected just as regularly and as 
carefully as those in storage. The presence of insects can generally be 
detected by observing minute pellets on the deck of the case, or the cast 
skins of larvae. Sometimes adult insects are seen, but as most of them are 
negatively phototropic and shun light, the fact that they are not to be 
seen does not imply that larvae are not busily at work between the specimens 
and the diaphragm. 
FUMIGATION 
During recent years much experimental work has been done in search 
of a fumigant effective in the extermination of insect pests, but not harmful 
to the material to be fumigated or dangerous to the men using it. Nearly 
all the numerous chemicals suggested were found to have some defect 
and it was not until 1927 that a wholly satisfactory fumigant was evolved. 
This is a mixture of three parts by volume of ethylene dichloride with one 
part of carbon tetrachloride. Carbon disulphide, hydrocyanic acid gas, 
chloropicrin, and many others are still in use but all have their disad- 
vantages. 
Ethylene Dichloride-Carbon Tetrachloride Mixture 
This mixture was discovered by Messrs. Cotton and Roark, of the 
United States Department of Agriculture, who have performed numerous 
tests with it and find it highly effective. It consists of three parts by 
volume of ethylene dichloride and one part by volume of carbon tetra- 
chloride. As the boiling points of these liquids are within a few degrees of 
each other, once they are mixed they form a homogenous fluid and may 
be treated as a simple chemical. The liquid in evaporating gives off a 
gas which is about three times as heavy as air and which is highly toxic 
to insect life. 
Ethylene dichloride alone will burn, though with difficulty. It is for 
this reason that the carbon tetrachloride is added to it, making it absolutely 
non-inflammable. It may, in fact, be used as a fire extinguisher. The 
vapour, if inhaled, has an effect very like that of chloroform, but not so 
rapid. If breathed in high concentrations for a long time it would be 
dangerous, but under ordinary conditions it is perfectly harmless. It has 
no injurious effect on animal, vegetable, or mineral specimens. As the 
gas is heavier than air, the liquid should be placed in shallow pans sup- 
ported above the specimens. 
