64 TESTING MISCELLANEOUS SUPPLIES. 



about two days). When the rubber is all dissolved (as indicated by the appear- 

 ance of the insoluble matter), filter through a counterpoised paper. (If the 

 papor proves unsatisfactory use a Gooch crucible with a thick layer of asbestos.) 

 Dilute the filtrate with benzol and whirl in a centrifuge. In this way very 

 finely divided mineral matter, which may pass through the filter, can frequently 

 be determined. Wash the paper and beaker containing the mineral matter with 

 hot benzol, gasoline, ether, and alcohol; then wash again with ether, dry at 

 105 C., and weigh. 



9. Gum. 



Add together the sulphur-free acetone extract, the alcoholic potash extract, 

 the chloroform extract, the total sulphur, and the fillers ; subtract the sum from 

 100 to obtain the percentage of gum present. 



SOAP. 



1. Sampling. 



It is somewhat difficult to secure a fair sample from a cake of soap, as the 

 moisture content of the outer layer may be very different from that of the in- 

 terior of the cake. Weigh the whole cake, cut it in two in a diagonal direction, 

 and cut from one of these fresh surfaces in thin layers a sufficient quantity for 

 all determinations, taking care to cut entirely across so as to get a fair propor- 

 tion of the outer and inner parts. When the cake is not too soft it is sometimes 

 a good plan to run the whole cake through a meat chopper and reduce it all to 

 fine particles. By running through the chopper several times a homogeneous 

 sample is obtained. Preserve the sample in a tightly stoppered bottle and weigh 

 the different portions required as soon as possible. 



2. Moisture. 



The determination of moisture is at best unsatisfactory and it is better to 

 include it In the " Undetermined." If a determination is required, heat 2 grams 

 of the finely shaved soap for two hours in an oven at 105 C. The loss is con- 

 sidered as water, though, of course, this is not accurate, as volatile oils may 

 constitute part of the loss, and on the other hand, all of the water may not be 

 driven off, and if the soap contains free alkali, carbon dioxid will be ab- 

 sorbed from the air. This last source of error may be avoided by drying in 

 vacuo. 



F.ihrion's method 6 is much quicker and probably as exact. The author 

 states that it gives results within 0.5 per cent of the truth. For this method 

 have a supply of oleic acid which has been heated to 120 with stirring until 

 oil of the water is driven off and preserved in a well-stoppered bottle. Weigh 

 from 2 to 4 grams of the soap in a platinum dish, add three times its weight of 

 oleic acid, and weigh agsiin. Heat the dish cautiously with a small flame until 

 all of the water is driven off and nil of the soap is dissolved, being careful not 

 to heat enough to decompose the acid. At the moment when all of the water 

 is 'hi \cn off the soap forms a clear solution with the acid if there are no fillers 

 present. Cool in a desiccator and weigh. This determination does not require 

 more than fifteen minutes. 



For further information on this subject consult the standard works on the 

 subject, such as Weber on the Chemistry of India Rubber, Allen's Commercial 

 Organic Analysis, and Lunge, Chemisch-technische Untersuchungsmethoden. 



6 ZtB. angew. Chem., 1906, 19:385. 



