10 



layers. Persist in this operation, if necessary, for a minute or more, using a piece of white 

 paper for a background and producing only a very gradual and partial mixing of the acid and 

 water. Nearly half of the acid should remain as a distinct unmixed layer at the end. If 

 meth\ 7 l alcohol be present, in the original sample, the shaking causes the separation of more 

 or less voluminous flocks of a very characteristic rose-red color. The appearance of colored 

 zones or flocks of other hues, even when tinged with red or of a rose-red solution without 

 flocks, should never be considered proof of the presence of methyl alcohol. However, if 

 the 'flocks are reddish brown, or if the upper layer has a pronounced red, it is often well 

 to repeat the test. By this method for the removal of acetaldehyde 10 per cent of methyl 

 alcohol may be readily detected, and an experienced operator may detect with certainty a 

 smaller amount. 



6. FLUORIDS. 



(a) MODIFIED METHOD OF BLAREZ.& 



Thoroughly mix the sample and heat 150 cc (in the case of solid foods the filtrate prepared 

 as directed under salicylic acid may be employed) to boiling. Add to the boiling liquor 5 cc 

 of a 10 per cent solution of potassium sulphate and 10 cc of a 10 per cent solution of barium 

 acetate. Collect the precipitate in a compact mass (a centrifuge may be used advan- 

 tageously) and wash upon a small filter. Transfer to a platinum crucible and ash. 



Prepare a glass plate (preferably of the thin variety commonly used for lantern slide 

 covers) as follows : First thoroughly clean and polish and coat on one side by carefully dip- 

 ping while hot in a mixture of equal parts of Canauba wax and paraffin. Near the middle of 

 the plate make a distinctive mark through the wax with a sharp instrument, such as a pointed 

 piece of wood or ivory, which will remove the wax and expose the glass without scratching 

 the latter. 



Add a few drops of concentrated sulphuric acid to the residue in the crucible and cover 

 the crucible with the waxed plate, having the mark nearly over the center and making sure 

 that the crucible is firmly embedded in the wax. 9 Place in close contact with the top or 

 unwaxed surface of the plate a cooling device, consisting of a glass tube considerably larger 

 in diameter than the crucible, the bottom of the tube being covered tightly with a thin sheet 

 of pure rubber. A constant stream of cold water is passed continually through the tube. 

 Heat the crucible for an hour at as high a temperature as practicable without melting the 

 wax (an electric stove gives the most satisfactory form of heat). 



Remove the glass plate and indicate the location of the distinguishing mark on the 

 unwaxed surface of the plate by means of gummed strips of paper, then melt off the wax by 

 heat or a jet of steam, and thoroughly clean the glass with a soft cloth. A distinct etching 

 will be apparent on the glass where it was exposed if fluorin be present. 



(b) SECOND METHOD. 



Dry 100 grams of sample thoroughly after making alkaline with lime water. In case the 

 sample be a solid it must first be taken up with a little water in order to be sure that every 

 portion of it is rendered alkaline. Incinerate the sample and volatilize the fluorin with sul- 

 phuric acid in a platinum crucible, detecting the presence of fluorin by means of an etched 

 glass plate, as described above. 



This method should only be used with substances low in ash and in which but a small 

 amount of carbonates results from incineration. Carbonates may be advantageously decom- 

 posed by treating the ash with acetic acid and evaporating to dryness. 



(c) THIRD METHOD. 



If it is desired, the preceding method may be varied by mixing a small amount of precipi- 

 tated silica with the precipitated calcium fluorid and applying the method given below for 

 the detection of fluosilicates. 



a In the examination of other alcoholic liquids the substances interfering with the resorcin test, 

 together with methods for their removal, may be found by consulting the original article Amer. 

 Chem. J., 1899, 21: 266. 



b Chem. News, 1905,-S/: 39; Ann. Kept. Mass. State Board of Health, 1905. 



