76 Scientific Intelligence. 



lard have examined these crystals with considerable care. They 

 were purified by recrystallization from hot glacial acetic acid and 

 were dried over sulphuric acid. When free from acetic acid the 

 substance is extraordinarily sensitive to the presence of water, 

 which at once converts it into lead peroxide and acetic acid. 

 This fact was made use of to analyze the substance, the lead per- 

 oxide being collected on a tared filter and the acetic acid titrated 

 with alkali. The results leave no doubt that the compound is 

 lead tetracetate Pb(C„H s O„) 4 . The crystals begin to melt at 175° 

 and decompose a few degrees above this. They dissolve in con- 

 centrated hydrogen chloride, producing lead tetrachloride ; as is 

 shown \>-y the fact that when a cold saturated solution of- ammo- 

 nium chloride is added, a characteristic yellow double ammonium 

 salt is precipitated. An analogous crystalline tetrapropionate 

 maybe similarly obtained. — J. Chem. Soc.,\x'u\, 1136, Sept. 1893. 



G. F. B. 



8. On the Thermochemistry of (Jhloracctic acid. — It has been 

 observed by Tanatat that the unstable modification of chlo- 

 racetic acid (fusing point 52°) changes into the stable modifica- 

 tion (fusing point 63°) with an evolution of heat of 0*65 calory 

 per gram-molecule. The heat of dissolution of the unstable acid 

 was found to be — 2 - 77 calories, that of the stable acid —3*47 cal- 

 ories ; the difference being 0*70 calory, nearly the same as the heat 

 of transformation directly observed. Both acids show a normal 

 molecule in aqueous solution and a double molecule in solution in 

 benzene. — J. Muss. Chem. Soc, xxiv, 694 ; J. Chan. Soc, lxiv, 

 i, 624, Nov. 1893. G. i\ b. 



9. A method of using the arc light for reading rooms and 

 libraries. — Mr. Benjamin A. Dobson, a manufacturer at Bolton, 

 England, has lately made experiments in regard to the best 

 method of lighting his workshops which are very suggestive in 

 regard to the disposition of lights in reading rooms and libraries. 

 It was found that incandescent lamps were not much superior to 

 gas while the arc lamp threw sharply defined shadows. It there- 

 fore occurred to Mr. Dobson to invert the ordinary arc lamp, 

 so that the positive carbon from which most of the light proceeds 

 was below the negative carbon. The light which was thus 

 thrown upward by the positive carbon was received on a well 

 whitewashed ceiling and was thus reflected downward and the 

 room was flooded with a well diffused light. One could stand in 

 any part of the rooms thus lighted and read a book or paper with- 

 out being troubled with shadows. When the strength of the 

 light is considered the system is cheaper than that of gas. — 

 Nature, Nov. 2, 1893, p. 18. j. t. 



10. Application of light waves to metrology. — Michelson in a 

 preliminary communication states that two complete and inde- 

 pendent determinations give for the numbers of waves of red 

 light in one meter of air at 15° C. and 76 mm pressure the follow- 

 ing: 



1st series 1553163.6 



2d series 1553164.6 



