LETHAL GASES 



181 



injury to the roses in untreated benches in another way. He beheved that 

 the HgCl2 passed as a vapor through the air and injured the roses at a 



distance. 



There is considerable evidence in favor of the Zimmerman-Crocker inter- 

 pretation. Organic matter reduces HgCls to mercurous chloride,20- p-833-834 

 and vegetable and animal substances -"■ p-^"^ reduce HgCl to metallic 

 mercury. The vapor pressure of metallic mercury is considerably higher 

 than the vapor pressure of HgCl2 throughout a considerable range of tem- 

 perature, and especially at temperatures suitable for the growth of plants. 

 Fig. 68 shows the vapor pressure of mercury, HgCl2, and HgCl at various 



2 



2 



q: 



^ 



(A 

 i/) 

 UI 

 CC 



a. 



tr 

 o 



Q. 



10 



20 



30 



40 



50 



60 70 80 



TEMPERATURE 



90 



100 



110 



120 



130 140 



150 



FiGtJKE 68. Curves showing the relationships of the vapor pressures of metallic mer- 

 cury, mercuric chloride, and mercurous chloride at different temperatures. All data 

 taken from International Critical Tables. 



temperatures. For instance, the vapor pressure of mercury at 40° C 

 (104° F), a temperature often attained in a greenhouse, is 66 per cent 

 higher than the vapor pressure of HgClo at 60° C (140° F), practically a 

 pasteurizing temperature. The vapor pressure of mercury at 60° C (140° F) 

 is five and one-half times that of HgClo. About one-fourth the HgCl2 

 molecule is chlorine. At saturation at 60° C (140° F), the air will contain 

 more than seven times as much mercury in the form of mercury vapor as 

 in the form of HgCl 2 vapor. WTien a current of air is drawn through a 

 column of HgCU crystals and then over a gold foil, no mercury is deposited 

 on the foil; but if air is drawn through a rich soil treated with HgCU 

 solution, mercury is deposited on the foil, showing that the soil reduces the 

 HgCl2 to the metal. If air was drawn through a bell jar containing a glass 



