M. TECHNOLOGY. 595 



rated to the proper consistency. The odor during evapora- 

 tion was not unpleasant ; the glue proved to be clear, elastic, 

 and adhesive, without odor or taste, and capable of being 

 used as gelatine. 14 (7, 1873, CCVIL, 500. 



PREPARATION OF CHROME ALUM. 



In the use of alcohol, sulphurous acid, or sulphureted hy- 

 drogen, as reducing agents, in the preparation of chrome 

 alum (now so much used in photography) from bichromate 

 of potash and sulphuric acid, the end of the reaction is ob- 

 scure, and, according to the concentration, a greater or less 

 elevation of temperature occurs, which requires careful atten- 

 tion, lest a green uncrystallizable solution be formed. By 

 the employment of oxalic acid as the reducing agent. Profess- 

 or Lielegg simplifies the process, avoids heating in the reduc- 

 tion, and all uncertainty as to the quantity of the reducing 

 agent necessary without requiring any sj^ecial apparatus. To 

 make. 100 parts of chrome alum, 29.5 parts of the bichromate 

 of potash, 38 of crystallized oxalic acid, and 39 of concen- 

 trated sulphuric acid are weighed out, and the last is diluted 

 with water enough to cause it to dissolve the bichromate, 

 Avhich is added to the acid, still warm from dilution. When 

 the solution is complete and the liquid cool, the oxalic acid 

 is stirred in, in small portions, the evolution of carbonic acid 

 indicating the commencement of the reaction. Crystals of 

 chrome alum separate from the filtered liquid left to spon- 

 taneous evaporation. 14 (7, 1873, CCYIL, 321. 



PRODUCTION OF COAL-TAR, ETC. 



Dr. Behrens gives, in detail, results of experiments Avith 

 reference to the eifect of temperature in gas manufacture 

 upon the tar formed. The tar from earthen retorts, requiring 

 a higher temperature, w^as much richer in benzole and toluol 

 than that from the Pauwel system in a coke furnace, and 

 also contained a large quantity of naphthaline and other sol- 

 id matters ; while that from the Pauwel arrangement, with 

 more volatile oils, also contained a considerable amount of 

 substances soluble in alkalies, the carbolic acid being little 

 more than a trace. By introducing a stream of heavy oils, as 

 used in the preservation of wood on a large scale, into a suit- 

 able gas manufacturing retort, two per cent, of benzole and 



