492 CONSIDERATION OF CARBON COMPOUNDS. 



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

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

 and with many other substances. In the absence of such other sub- 

 stance it unites often with itself, forming polymeric modifications, 

 such as paraldehyde 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. It is official as Paraldehydum and a hypnotic. 



Metaldehyde, (C 2 H 4 0) 3 , is stereo-isomeric with paraldehyde ; it is obtained by a 

 process similar to the one mentioned for paraldehyde, but at a lower tempera- 

 ture. It is a solid crystalline substance, insoluble in water, but slightly soluble 

 in alcohol, ether, and chloroform. 



Trichloraldehyde, Chloral, C^CLjO or CC1 3 .C^Q (Trichlorace- 

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 C 2 H 4 O, in which three atoms of hydrogen have been replaced by 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

 duct separates into two layers ; the lower is removed and shaken with 

 three times its volume of strong sulphuric acid and distilled, the dis- 

 tillate is mixed with calcium oxide and again distilled ; the portion 

 passing over between 94 and 99 C. (201 and 210 F.) is collected. 



The decomposition taking place between alcohol and chlorine may 

 be explained by the formation of aldehyde : 



C 2 H 6 O + 2C1 = C 2 H 4 O + 2HC1, 



and by the subsequent replacement of hydrogen by chlorine : 

 C 2 H 4 O + 6C1 = C 2 HC1 3 + 3HC1 



