490 CONSIDERATION OF CARBON COMPOUNDS. 



acid may be represented by the formula CH 3 .CO.OH ; the radical of 

 acetic acid or acetyl is the group CH 3 .CO, and the hydride of acetyl 



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is acetic aldehyde, CH 3 .C^Q. It is the group C\o which is char- 

 acteristic of, and found in, all aldehydes. Only a few aldehydes are 

 of practical interest, as, for instance, formaldehyde, acetic aldehyde, 

 paraldehyde, and benzoic aldehyde. 



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Formic aldehyde, CH 2 O or H.C/Q (Formaldehyde, methyl alde- 

 hyde). This is obtained by the dry distillation of calcium formate, 

 or by gentle oxidation of methyl alcohol. The latter process is 

 carried out by passing vapors of methyl alcohol with air over a 

 heated spiral of platinum or copper. The condensed vapors are 

 formaldehyde dissolved in undecomposed methyl alcohol. Another 

 process is by heating paraformaldehyde, which yields formaldehyde 

 in a pure condition. 



Formaldehyde is a colorless gas, possessing a strong, penetrating 

 odor; it may be condensed to a liquid which boils at 20 C. 

 (-4 F.). 



Solution of formaldehyde, Liquor formaldehydi. Forma-lde- 

 hyde is readily soluble in water, and a solution containing 37 per cent, 

 by weight is official. It is a colorless liquid which has a pungent 

 odor and caustic taste ; its vapors act as an irritant upon the mucous 

 membrane. Sometimes on standing, always on slow evaporation, white, 

 solid paraformaldehyde separates. With ammonio-silver nitrate the 

 solution gives a precipitate of metallic silver. The solution is a 

 strong antiseptic, and when diluted to 4 or 5 per cent, it is one of the 

 best hardening and preserving agents for tissues. 



Formic aldehyde may be recognized by Schijf's reaction: A solution of 

 fuchsin (rosanilin chloride) decolorized or nearly so with sulphurous acid turns 

 pink or violet when brought in contact with any aldehyde solution. For the 

 examination of air, suspended filter-paper, moistened with the decolorized 

 fuchsin solution, may be used. 



Paraformaldehyde, C 3 H 6 O 3 or (CH 2 O) 3 (Formalin). On slow 

 evaporation of a solution of formaldehyde in methyl alcohol poly- 

 merization takes place, and paraformaldehyde separates in colorless 

 crystals which are insoluble in water. On heating the compound, 

 which is now found in the market in the form of tablets, it splits 

 up into three molecules of formaldehyde, which, escaping as a 

 gas, is used for disinfecting purposes. It acts powerfully on all 

 germs, and has the advantage over chlorine and sulphur dioxide that 

 it does not act injuriously on the fabric or color of household goods. 



