ALDEHYDE 65 



specific gravity of the liquor is first determined, half its volume is next evaporated in 

 the open air, sufficient water is then added to the remainder to restore its original 

 volume, and the specific gravity again ascertained. By deducting the specific gravity 

 before the expulsion of the alcohol from that obtained afterwards, the difference gives 

 a specific gravity indicating the per-centage of alcohol, which may be found by 

 referring to Gay-Lussac's or one of the other Tables. Tabarie has constructed a pecu- 

 liar instrument for determining these specific gravities, which he calls an cenometer ; 

 but they may be performed either by a specific-gravity bottle or by a hydrometer in 

 the usual way. 



Of course this method cannot be absolutely accurate; nevertheless, Prof. Mulder's 

 experience with it has led him to prefer it to any of tie methods before described, 

 especially where a large number of samples have to be examined. He states that the 

 results are almost as accurate as those obtained by distillation. The evaporation of 

 the solution may be accelerated by conducting hot steam through it. 



Adulterations. Absolute alcohol should be entirely free from water. This may be 

 recognised by digesting the spirit with pure anhydrous sulphate of copper. If the 

 spirit contain any water, the white salt becomes tinged blue, from the formation of 

 the blue hydrated sulphate of copper. 



Kectified spirit, proof spirit, and the other mixtures of pure alcohol and water, 

 should be colourless, free from odour and taste. If containing methylic or amylic 

 alcohols, they are immediately recognised by one or other of these simple tests. 



Dr. Ure states that if wood-spirit be contained in alcohol, it may be detected to 

 the greatest minuteness by the test of caustic potash, a little of which, in powder, 

 causing wood-spirit to become speedily yellow and brown, while it gives no tint to 

 alcohol. Thus 1 per cent, of wood-spirit may be discovered in any sample of spirits 

 of wine. 



The admixture with a larger proportion than the due amount of water is of course 

 determined by estimating the per-centage of absolute alcohol by one or other of the 

 several methods just described in detail. 



The adulterations and sophistications to which the various spirits known as rum, 

 brandy, whisky, gin, &c. are subjected, will be best described under these respective 

 heads, since these liquors are themselves mixtures of alcohol and water with sugar, 

 colouring matters, flavouring ethers, &c. 



AliDEKYBi:. Aldehyde was first obtained by Dobereiner, who named it Light 

 Oxygen Ether. The name is an abbreviation of Akohol dehydrogenatum. It is the 

 fluid obtained from alcohol by the removal of two atoms of hydrogen. Thus, alcohol 

 being represented by the formula C 4 H 6 0- (C z H 6 O), aldehyde becomes C 4 H 4 0* 

 (C 2 H 4 O). See LAMPIC ACID. 



Preparation. Aldehyde is prepared by various processes of oxidation. Liebig 

 has published several methods, of which the following is perhaps the best. 

 Three parts of peroxide of manganese, three of sulphuric acid, two of water, 

 and two of alcohol of 80 per cent., are well mixed and carefully distilled in 

 a spacious retort. The extreme volatility of aldehyde renders good condensation 

 absolutely necessary. The contents of the retort are to be distilled over a gentle and 

 manageable fire until frothing commences, or the distillate becomes acid. This 

 generally takes place when about one-third has passed over. The fluid in the 

 receiver is to have about its own weight of chloride of calcium added, and, after slight 

 digestion, is to be carefully distilled on the water-bath. The distillate is again to be 

 treated in the same way. By these processes a fluid will be obtained entirely free 

 from water, but containing several impurities. To obtain the aldehyde in a state of 

 purity it is necessary, in the first place, to obtain aldehyde-ammonia ; this may be 

 accomplished in the following manner: The last distillate is to be mixed in a 

 flask with twice its volume of ether, and, the flask being placed in a vessel surrounded 

 by a freezing mixture, dry ammoniacal gas is passed in until the fluid is saturated. 

 In a short time crystals of the compound sought separate in considerable quantity. 

 The aldehyde-ammonia, being collected on a filter, or in the neck of a funnel, is to 

 be washed with ether, and dried by pressure between folds of filtering paper, followed 

 by exposure to the air. It now becomes necessary to obtain the pure aldehyde 

 from the compound with . ammonia. For this purpose two parts are to be 

 dissolved in an equal quantity of water, and three parts of sulphuric acid, mixed with 

 four of water, are to be added. The whole is to be distilled on the water-bath, the 

 temperature, at first, being very low, and the operation being stopped as soon as the 

 water boils. The distillate is to be placed in a retort connected with a good condens- 

 ing apparatus, and, as soon as all the joints are known to be tight, chloride of calcium, 

 in fragments, is to be added. The heat arising from the hydration of the chloride 

 causes the distillation to commence, but it is carried on by a water-bath. The dis- 

 tillate, after one more rectification over chloride of calcium, at a temperature not 



VOL. I. F 



