Chemistry and Physics. 585 



SCIENTIFIC INTELLIGENCE. 

 I. Chemistry and Physics. 



1. Ultra-filtration in Chemical Analysis. — Zsigmondy, Wilke- 

 Doreurt and Galecki have devised a method of analytical fil- 

 tration which permits the collection of the most finely divided 

 precipitates, and even of many colloidal substances where the 

 particles are invisible, such as those of certain gold and ferric 

 oxide " solutions." The method appears to be suitable also for 

 removing turbidity in cases where ordinary filtration with paper 

 is unavailing. To prepare these filters a film of collodion is first 

 prepared by diluting 200 cc of 6 per cent collodion with 200 cc of 

 ether and 500 cc of absolute alcohol, carefully pouring and spread- 

 ing a film of this liquid upon plate-glass, waiting until most of 

 the ether has evaporated and the collodion is no longer sticky, 

 then plunging the whole into water. After 10 or 15 minutes the 

 film may be separated from the glass and it may then be pre- 

 served under water for a long time. The sieve-like bottom of a 

 porcelain filtering funnel is now provided with a piece of mois- 

 tened ordinary ashless filter paper, then a piece of the collodion 

 film is laid upon the paper in the funnel, suction is applied with a 

 w r ater-jet pump, and the filter is pressed a little against the sides 

 of the funnel, so that the collodion filtering surface may be con- 

 tinuous and practically air-tight. Filtrations can now be made 

 in the usual way with this filter, but the process is considerably 

 slower than with paper alone. The authors have tested the 

 method by collecting colloidal gold and ferric oxide and weighing 

 the substances after ignition, both on the analytical balance and 

 with Nernst's micro-balance. They have compared the weight 

 of silver chloride filtered in this way, and with ordinary paper, 

 with closely agreeing results. By collecting barium sulphates 

 immediately after precipitation they have re-discovered the long- 

 known fact that this substance forms supersaturated solutions 

 and is not at once completely precipitated — Berichte, xlv, 579. 



h. l. w. 



2. Magnesia Rods as a Substitute for Platinum Wire. — 

 E. Wadekind recommends the use of small magnesia rods, about 

 l mm in diameter, for use in various blow-pipe tests. The best 

 material for the rods is not pure magnesia, but the same mix- 

 ture that is used for the supports of incandescent gas mantles. 

 The rods serve well for producing flame colorations, as the mass 

 itself after short ignition in the Bunsen flame gives no coloration. 

 With the salts of the alkali-earth metals the flame-color is not 

 as persistent with the rods as with platinum wire, but it is quite 

 sufficient for ordinary qualitative tests. It is possible with the 

 magnesia rods to produce much larger borax and salt of phospho- 

 rus beads than with platinum wire, a circumstance which is advan- 

 tageous particularly in the test for silica. The colors of the 



