62 TESTING MISCELLANEOUS SUPPLIES. 



the absence of all other metals has been proved the magnesium can be rapidly 

 determined by adding sulphuric acid, evaporating to dryness, carefully heating 

 to drive off ammonia salts and excess of sulphuric acid, and weighing as 

 magnesium sulphate. 



Report the magnesium as magnesia (MgO) and calculate the equivalent of 

 magnesium carbonate (MgCO 3 ), and also of hydrated basic magnesium car- 

 bonate [(MgCO 3 )4Mg(OH)25H 2 O]. The last is not a definite compound, the 

 formula being only an approximation, but it approaches in composition the 

 magnesium compound used in manufacturing these articles. 



BUBBEB. 

 1. Preparation of Sample. 



No general rule can be laid down for sampling all kinds of rubber goods, but 

 pieces should be taken from as many parts of the article to be. analyzed as 

 possible, and shredded by passing first through a meat chopper and then through 

 a coffee mill. Hard rubber is best subdivided by rasping. 



2. Ash. 



Determine ash on a 1 or 2 gram sample in a porcelain crucible, distilling off 

 as much of the material as possible at the lowest possible temperature and 

 completing the ashing at a very low red heat. To completely burn out all the 

 carbon it will be necessary to break up the ash. In case the only mineral 

 fillers are silica or barium sulphate, the percentage of ash will give a fairly good 

 estimate of the amount of mineral filler used ; but with lead and zinc compounds 

 there will always be more or less loss of metal and with carbonates there will 

 also be loss. A qualitative analysis should be made of the ash. 



3. Acetone Extract. 



Extract from 1 to 2 grams of the sample with acetone, using an extractor 

 having no rubber or cork connections and one in which the hot vapors pass 

 around the extraction tube so that the solvent in contact with the rubber is 

 near its boiling point. With such an apparatus a five-hour extraction is usually 

 Bufflcient. Evaporate the solvent and weigh the extract. As a check the ex- 

 tracted rubber may be dried in a current of hydrogen or coal gas at 100 O., 

 and weighed ; but it is better to determine the extract by direct weighing. The 

 acetone extract contains the free sulphur in the rubber, the resins, and the 

 fatty and mineral oils. 



4. Free Sulphur. 



iMermine the free sulphur by adding about 25 cc of fuming nitric acid to 

 the acetone extract, heating on the stoara bath ovor night, boiling to expel 

 nitrons fumes, diluting t<> about 500 cc and determining sulphur in the ordi- 

 nary way as barium sulphate. Deduct the free sulphur from the total acetone 

 extract nnd report " sulphur-free acetone extract." 



5. Alcoholic Potash Extract (Fatty Substitutes). 



Transfer the residue from the acetone extract from the extraction tube or 

 eartridKe to a small beaker, add 20 cc of approximately normal alcoholic potash, 

 and heat on the steam bath for thirty minutes. Dilute with 20 cc of water, 



