50 MISC. PUBLICATION 679, U. S. DEPT. OF AGRICULTURE 



in methacrylate plastics. All the compounds listed are poisonous, and 

 some are flammable. Dry picric acid is highly explosive, especially 

 in contact with metals or metallic oxides. Since some insurance 

 policies contain a clause to the effect that the storage of high explosives 

 in any quantity voids the policy, the purchase of picric acid, except in 

 solution and in small quantity, is not recommended. 



1. Formalin. — This is an aqueous solution of approximately 40 per- 

 cent of formaldehyde gas in water to which a small amount of methyl 

 alcohol has been added. - It has good preserving action in most cases 

 where the tissues are readily permeable, and it serves to toughen tissues 

 and prevent shrinkage -and distortion. It has but slight tendency to 

 alter dark colors if removed completely from the specimen as soon 

 as preservation has been accomplished. For most applications, for- 

 malin should be diluted with water to about 5 per cent of its original 

 strength, although in some cases a concentration as low as 2 percent is 

 adequate, while in others one as high as 10 percent may prove to be 

 necessary. 



2. Alcohol. — Ethyl alcohol is generally used for preservation and 

 dehydration purposes. If tax-free laboratory alcohol is not available, 

 isopropyl alcohol will be found to be almost equally suitable. De- 

 natured alcohol may also be used if its formula does not include de- 

 naturants that are immiscible with water or otherwise unsuitable for 

 application to specimen material. When used for preserving nonrigid 

 material, alcohol should be diluted with water to 70 percent, or in the 

 case of very soft material to 50 percent , of its original strength , to avoid 

 unduly rapid dehydration and consequent shrinkage of the tissues. 

 As soon as preservation is complete, the specimen material should be 

 passed through the dehydration stages employing successively higher 

 concentrations as quickly as practicable, because alcohol has a pro- 

 nounced decolorizing action except in the water-free state. 



3. Acetic acid. — Pure, or glacial, acetic acid tends to dissolve or alter 

 the pigmentation and otherwise alter most types of biological material. 

 It should be used therefore only in concentrations of 30 percent or 

 lower, and preferably in combination with other preservatives. It 

 serves to soften most tissues and in some cases acts as a clearing agent 

 to render specimens translucent. Care should be exercised in handling 

 acetic acid, because in concentrated form it can cause persistent burns. 



4. Chromic acid. — Chromic acid (chromium trioxide) is an effective 

 hardening agent for soft or gelatinous specimen material. Because of 

 its tendency to oxidize and discolor specimens, its use is limited to 

 special cases, and it should be applied for as short a time as possible 

 for obtaining the desired result. It is used only in concentrations of 

 1 percent or less, and preferably in combination with other preserving 

 substances. After being treated with fluids containing chromic acid, 

 specimens should be washed with extra care to remove all traces of it. 



5. Mercuric chloride. — Since mercuric chloride (corrosive sublimate) 

 is violently poisonous, great care should be exercised when using it. 

 It penetrates tissues rapidly and in many cases does not affect the color 

 undesirably if removed quickly and thoroughly by washing as soon as 

 proper preservation and hardening have been attained. It must always 

 be completely removed before the specimen is dehydrated because it 

 tends to be reduced to metallic mercury by alcohol with consequent 

 blackening of the tissues. Mercuric chloride is generally used as a 



