be attributed to nascent hydrogen present in the solution, or possibly to soluble bydrides of impurities present in the aluminum. Procedure for Reducing Stannic Chloride.-The following procedure for reducing stannic chloride is suggested, based on the foregoing results: The solution is concentrated to a small volume (say, 10 cc.) and half its volume of dilute KC1 (GM) added, giving the acid a concentration of about 2-molar. A piece of aluminum wire (about 10 mg. is sufficient) is added and the solution is heated just below its boiling point for three or four minutes. If amouse-colored coating (metallic tin) forms on the aluminum, it is dissolved by adding concentrated HC1 (12M) and boiling gently. Filter into mercuric chloride solution. A white precipitate of HgtC12 indicates the presence of Snf +++. The mercuric chloride test for stannous chloride is exceedingly sensitive. A trial showed that 0.00001 gram of tin in a volume of 10 cc. could be. detected in this way. ~ o n s e ~ u e n t leven ? , with a low efficiency in reduction, the test should be positive if stannic chloride is present in appreciable quantities. This points to the conclusion that the omission of tin by beginners is probably due to its loss in some operation prior to the reduction of the stannic chloride. Summary I . The reduction of stannic chloride solution by a metal under ordinary conditions of stirring is not a simple reaction, but consists of four concurrent reactions. 2. Acidity reduces the efficiency of the reduction. 3. The efficiency of the reduction becomes less with increasing temperatures, but the gain in speed compensates for the loss. 4. Better reduction is obtained by using the metal in the massive, rather than the powdered form. .5. Amalgamation reduces the speed of reduction. C,. Aluminum is the best metal for the reduction on account of the high speed of its action. 7. Concentrated ferrous chloride solution may give a pseudo test for Sn++ with mercuric chloride solution. S. These results have been embodied in a procedure for testing for tin in stannic chloride solutions.
Automobile Finish Used to Preserve Specimens. A small but important new use for the new cellulose lacquers, t o preserve from evaporation the delicate specimens mounted on microscope slides, is described by Prof. A. V: Mitchener of Manitoha Agricultural College. He seals the edges of the cover glass to the slide v,ith a ring of lacquer, which quickly dries and makes a permanent mount. Shellac and other substances hithcrto used for this purpose have not been very satisfactory.-Science Service
