"Dreft," or to even more concentrated fonns of high molecular weight half-esters, such as that used in "Dreft." F. B. WADE
useful and economical when used to dry laboratory glassware, avoiding the expense and hazard of dryingsolvents, in addition to the fact that moisture does not condense upon the surface of glassware dried in this
SHORTRIDGE HIGXSCHOOL INDIANAPOLIS, INDIANA
CONSERVATION OF MANGANESE DIOXIDE IN OUANTITATIVE
EXPERIMENTS
To the Editor DEARSIR: Quantitative experiments with oxygen are frequently ~erformedin the general chemistry laboratory by heating potassium chlorate with manganese dioxide. Since the manganese dioxide is easily carried out by the escaping oxygen, the correctness of the results depends very much on the care with which the reactants are heated. It is difficult to make beginning students understand the meaning of "gentle heating," and in this particular experiment even great care will not always prevent spasmodic action. If a glass wool plug about three-quarters of an inch long is inserted in the mouth of the test-tube the escaping manganese dioxide dust will collect on it, and being kept in the tube, the positive error usually resulting from its loss will he eliminated. The results obtained in an experiment conducted in this way are given. Weight of test-tube.. ................... 35.21 g. Weight of test-tube and potassium chlorate 42.71 g. Weight of potassium chlorate.. . . . . . . . . . . 7.00 g. Weight of test-tube, potassium chlorate, manganese dioxide, and glass wool... . . . 44.84 g. Weight of above after heating. . . . . . . . . . . 42.17 g. Weight of oxygen evolved.. ............. 2.67 g. Volume of oxygen reduced to standard conditions. ............................. 1910 cc. Density of oxygen as above.. . . . . . . . . . . . . 1.40 Density of oxygen, actual.. . . . . . . . . . . . . . 1.43 Per cent. error.. ....................... - 2 . 1 per cent. Per cent. oxygen in potassium chlorate as above.. ............................. 38.2 per cent. Per cent. oxygen in potassium chlorate, actual .............................. 39.2Percent. Per cent, error.. . . . . . . . . . . . . . . . . . . . . . . . - 2 . 5 per cent.
manner. It is only necessary to hold the water-washed glassware object over the exit tube or to support it on the ring, R, as illustrated. The coil can easily be formed from drawn copper tubing. If tubing no larger than quarter-inch is used, it is not necessary to flthe tube with sand to avoid collapsing the tube while forming, as is generally supposed. This size tubing is ample for laboratory needs, and indeed larger tubing does not improve matters as the Bunsen or Fisher burner cannot heat larger tubing profitably. It has been my experience that the use of one of these simple laboratory driers will completely eliminate the need of taking up the laboratory drying oven space with glassware. CHARLES G. MILLER POSTOFFICEBox 985 CoRCORAN, CALIFORNIA
OBJECTIVE TESTS IN ORGANIC CHEMISTRY
To Ihe Editor DEARSIR: During the past school year about 25,000 copies of the Cooperative Objective Tests in organic chemistry were distributed to approximately one hundred colleges and universities. Tom I. MAHLER The 193940 series, now available, covers twenty-six topic examinations, together with a final examination for each of the two semesters. The complete sets are available to instructors of organic chemistry, industrial chemists, or graduate students a t twenty cents per set DRYING LABORATORY GLASSWARE in orders of five or more or a t twenty-five cents for single sets. To the Editor These tests represent, we believe, an improvement DEARSIR: over the 1938-39 series. They are available from the Apropos of the item by H. V. Allison U. CAEM. chairman of thecommittee, Ed. F. Degering, Chemistry E ~ u c . 15, , 379 (1938jl i t may he of interest that an ap- Department, Purdue University, Lafayette, Indiana. paratus as sketched has been in use by the author for ED. F. DEGERWG many years and seems to be a more generally useful PURDUEUNIVERSITY LnmYEmE, INDIANA application than that mentioned by Allison. It is most
