PROCEEDINGS OP THE POLYTECHNIC ASSOCIATION. 593 



single drop of a dilute solution of the bi-chromate of potash, 

 instantly produces* the well known change of color. 



Metallic Spectacles. 

 M. Foucault recently communicated to the French Academy of 

 Sciences the fact that the sun may be viewed through a lens cov- 

 ered with a silver film; the sun's disk, shorn of its beams, can 

 thus be clearly seen. Subsequently M. Melscius made a useful 

 application of Foucault's discovery. Having been injured while 

 making an experiment in the laboratory, his eyes were painfully 

 aifected by light. In this condition he had recourse to specta- 

 cles with black glasses, such as are used by engine drivers; over 

 these he put green glasses, which answered pretty well; but on 

 further experiment he found the best method was to use pale 

 blue scog-irles covered with silver or gold film, and these he rccora- 

 mends to all persons troubled with weak eyes. 



The Dead Sea. 

 M. Terrell, who visited Palestine in 1865, has addressed a note 

 to the French Academy of Sciences on the chemical composition 

 of the waters of this inland salt lake. It has beeil generally 

 believed that there were no living creatures in it, but the author 

 says he savr, in one spot, near Sodom, a number of small fisli that 

 seemed to thrive well. The following is a brief of his observa- 

 tions : 1. The density of the waters of the Dead sea increases with 

 their depth. 2. Their-composition and concentration are likewise 

 variable ; thus samples taken five miles east of Wady-Mrabba con- 

 tain four times more calcium than those five miles east of lias 

 Teshkah, which contain twice as much soda as the former. 3. 

 Samples of water from north of Sodom, in that part which forms 

 a lagoon, contain more chloride of sodium (common salt) than 

 chloride of magnesium, which explains why fish may live there. 

 4. The bromides alone seem to be concentrated much more in 

 depths exceeding 300 meters. 5. This lake contains no iodine or 

 traces of phosphoric acid, and but small portions of the sulphates. 

 6. The residue, after evaporation, examined with the spectroscope, 

 does not show the presence of the rarer alkaline metals, lithium, 

 ccesium or rubidium. 



The November Meteors. 

 The long expected display of shooting stars, invisible in this 

 country, was seen by man3^ European observers on the morning of 

 [Am. Inst.] LL 



