Chemistry and Physics. 479 



this conduction and then the fourth cell, when illuminated, gave 

 a deflection of I4 mms as contrasted with 850 mms for the standard 

 cell. The fifth cell, containing potassium which had been distilled 

 four times, did not give the slightest photoelectric effect even 

 Avhen the slit between the lamp and the cell was opened so wide 

 that the standard cell could no longer be used for comparison. 

 The entire experiment was repeated with similar results. Conse- 

 quently, it has now been shown that the photoelectric effects of 

 potassium and zinc can be completely suppressed by removing all 

 traces of gases. — Verh. d. deutsch. physik. Gesellsch., No. 2, 

 p. 107, 1914. h. s. u. 



8. Heat of Formation of Hydrogen from Hydrogen Atoms. — 

 In an important theoretical paper on the constitution of atoms 

 and molecules N. Bohr has calculated the heat of formation of 

 hydrogen molecules from the atoms to be 60,000 calories per 

 gram-molecule. On the other hand, the experimental work of 

 Irving Langmtjir gave the much higher value 130,000. This 

 discrepancy incited the latter investigator to repeat his work with 

 greater care and under more favorable conditions. The method 

 adopted was similar to that previously used and it consisted in 

 the determination of the heatdoss from tungsten wires heated 

 electrically in hydrogen. The gas pressures ranged from l mm to 

 760 mms of mercury. By subtracting the heat-loss due to normal 

 heat-conduction from the total observed heat-loss, the heat carried 

 by diffusion of the hydrogen atoms was deduced. "When the 

 logarithm of this quantity was plotted against the reciprocal of 

 the absolute temperature, straight lines were obtained for each 

 experiment, and the slopes of all these lines were practically equal 

 and independent of the pressure of the hydrogen gas. Assuming 

 that the diffusion coefficient varies with the f power of the tem- 

 perature, it follows that the heat of formation of hydrogen is 

 about 76,000 calories per gram-molecule instead of 130,000 as 

 previously determined. The higher value was based on a calcula- 

 tion of the actual value of the diffusion coefficient of hydrogen 

 atoms through ordinary hydrogen. The more recent results show 

 that the degree of dissociation is much smaller than had been 

 found in the earlier work, and that at temperatures as high as 

 3500° K. hydrogen is probably not dissociated to a very great 

 extent. Langmuir concludes that the experimental evidence now 

 indicates a value between 75,000 and 80,000 calories and that it 

 seems very improbable that the heat of formation of hydrogen 

 molecules can be as low as 60,000, which is the figure derived 

 from the theory of Bohr. — Phil. Mag., xxvii, p. 188, Jan., 1914. 



h. s. u. 



9. Thermal Expansion of Quartz Glass. — The absolute expan- 

 sion of quartz glass, between +100° C. and —253° C, has been 

 investigated by Karl Scheel and Wilhelm Heuse. The speci- 

 men was made in the form of a ring of 9 - 942 mins altitude, and the 

 method followed was the well-known interference scheme due to 

 Fizeau. The expansions of the ring in the intervals — 253° C. to 



+ 16° C, —190° to +16°, —78° to +16°, and +16° to +100° 



Am. Jour. Sci.— Fourth Series, Vol. XXXVII, No. 221.— May, 1914. 

 33 



