98 METHODS OF COLLECTING AND PEESERYINGr 
Commercial formalin now costs in London 15s. per gallon; accordingly 
the 10 per cent, solution costs only Is. 6d. per gallon. Formalin is 
excellent for preserving some specimens ; but caution and further 
experiment as to the use of it are necessary. A collector should 
not entrust all his specimens to this new preservative, but keep some 
in alcohol. 
Commercial formalin generally contains a considerable percentage 
of free acid, and consequently will injure calcined structures, such as 
molluscan shells, unless the acid has been previously neutralized. 
This may conveniently be effected by placing a little chalk or pounded 
coral into the strong solution. 
Before the final preservation of animals in formalin or alcohol, 
certain preliminary measures are necessary, or rather, desirable. 
Animals should be killed by means of a reagent which rapidly causes 
death, prevents contraction, and " fixes " the tissues as nearly as 
possible in the position they had during life. 
The killing and fixing reagents most commonly used are chromic 
acid, osmic acid, and corrosive sublimate ; after employing them, it 
is necessary to wash the animals in fresh or distilled water before 
placing them in formalin or alcohol. In the case of soft contractile 
organisms it is necessary to narcotize them first before attempting 
to kill, in order to prevent distortion. A solution of chloral hydrate, 
varying from 1 per cent, to 1 in 1,000 in sea-water or fresh-water, is 
often used for this purpose. Lo Bianco recommends immersion in 
weak alcohol. He generally uses a mixture of sea-water 100 ccm., 
and absolute alcohol 5 ccm. In other cases 70 per cent, alcohol 
may be carefully poured upon water in which the specimen lies, so 
that it forms a layer at the surface. It will gradually mix with 
the subjacent water, and in the course of a few hours will narcotize 
the animal, so that it may be treated with fixing reagents without 
fear of contraction. Tobacco smoke is employed for Actinice. Dr. 
Sorby has obtained beautiful results by poisoning with menthol. 
Corrosive Sublimate. — A saturated solution in fresh- or sea-water 
may be used either hot or cold. It acts quickly, and preserves 
admirably for histological purposes. It is especially good combined 
with copper sulphate, acetic acid, or chromic acid. Objects hardened 
in it should be subsequently well washed in distilled water and in 
