316 CONSIDERATION OF CARSON COMPOUNDS. 



aldehyde may be obtained by repeated distillation. Use the distillate for 

 silvering a test-tube by adding some ammoniated silver nitrate. How much 

 potassium dichromate is needed for the conversion of 5 grammes of pure 

 alcohol into aldehyde? 



Aldehyde is a neutral, colorless liquid, having a strong and charac- 

 teristic odor ; it mixes with water and alcohol in all proportions and 

 boils at 21 C. (69.8 F.). The most characteristic chemical property 

 of aldehyde is its tendency to combine directly with a great number 

 of substances; thus it combines with hydrogen to form alcohol, with 

 oxygen to form acetic acid, with ammonia to form aldehyde-ammonia, 

 C 2 H 4 O.NH 3 , a beautifully crystallizing substance, with hydrocyanic 

 acid to form aldehyde hydrocyanide, C 2 H 4 O.HCN, and with many 

 other substances. In the absence of such other substance it unites 

 often with itself, forming polymeric modifications, such as paralde- 

 hyde and metaldehyde. 



Aldehyde is a strong reducing agent, which property is used in the 

 silvering of glass, which is done by adding aldehyde to an ammoniacal 

 solution of silver nitrate, when metallic silver is deposited on the walls 

 of the vessel or upon substances immersed in the solution. 



Paraldehyde, C 6 H 12 O 3 . When a few drops of concentrated sul- 

 phuric acid are added to aldehyde, this becomes hot and solidifies on 

 cooling to C. (32 F.). This solid crystalline mass of paralde- 

 hyde, which liquefies at 10.5 C. (51 F.), has been formed by the 

 direct union of three molecules of common aldehyde. Paraldehyde 

 is soluble in 8.5 parts of water, boils at 124 C. (253 F.), and is 

 reconverted into common aldehyde by boiling it with dilute sulphuric 

 or hydrochloric acid. 



Metaldehyde, (C 2 H 4 O)z, is another polymeric modification of aldehyde, ob- 

 tained by a process similar to the one mentioned for paraldehyde, but at a 

 lower temperature. It is a solid crystalline substance, insoluble in water, but 

 slightly soluble in alcohol, ether, and chloroform. 



Trichloraldehyde, Chloral, C 2 HC1 3 O or CC1 3 .COH (Trichloracetyl 

 hydride). This substance may be looked upon as acetic aldehyde, 

 C 2 H 4 O, in which three atoms of hydrogen have been replaced by 

 chlorine. It is made by passing a rapid stream of dry chlorine into 

 pure alcohol to saturation, keeping the alcohol cool during the first 

 few hours, and warming it gradually until the boiling-point is 

 reached. According to the quantity of alcohol operated on, the con- 

 version requires several hours or even days. The crude liquid pro- 



