442 Professor Sir James Dewar [June 8, 



Shortly after the discovery of oxygen by Priestley some Dutch 

 physicists began the study of the gases absorbed and given off by 

 living plants ; and for this purpose the plants were covered by bell- 

 jars whose lower edge was laid in a channel of mercury. But a 

 difficulty arose, as it was found that the plants in these circumstances 

 soon became sickly and died. Then the remarkable discovery was 

 made that if sulphur was sprinkled over the leaves and the surface 

 of the vessel the deleterious effect of the mercury was overcome and 

 the plants became healthy. Boussingault repeated and confirmed 

 these results in 1868, and made many experiments to elucidate the 

 remedial action of sulphur. He explained this action by pointing 

 out that the poisoning of the plants arose from the presence of 

 mercury vapour, and that as one volume of sulphur vapour was able 

 to combine with six volumes of mercury vapour, the sulphur 

 although less volatile was able effectually to neutralise the deleterious 

 effects of the mercury vapour. 



Now, the ratio of the pressure of mercury vapour to that of sulphur 

 at the temperature of 115° C, when both bodies are in the liquid condi- 

 tion, is as 10 to 1. Seeing the sulphur molecule is S^ to Sg, one volume 

 of sulphur vapour is sufficient to neutralise or combine with 6 to 8 

 volumes of mercury vapour, so that an excess of mercury pressure of 

 from 6 to 8 times that of the sulphur vapour can always be removed 

 by combination with the sulphur. If, on the other hand, we compare 

 the relative pressures of mercury at 100° 0. to that of solid prismatic 

 sulphur at the same temperature, this ratio is 87 to 1, so that the 

 removal of the mercury vapour at the ordinary temperature, when the 

 pressure ratio is still higher, cannot be fully explained. The effect of 

 air and water vapour complicates the action in the case of plants 

 as the molecules of sulphur and mercury have not the free play 

 they have when no inert gas is present. It would seem that in 

 high vacua the S2 molecule of sulphur may be considered as produced 

 by molecular dissociation, and that moving with great rapidity rela- 

 tively to the mercury gas molecules soon gets at the surface of the 

 liquid mercury and coats it with a thin layer of mercury sulphide, 

 which acts as a trap, stopping the exit of further mercury molecules. 

 The effect of such a coating in preventing the escape of mercury 

 vapour may be shown by the use of an inverted boiling point flask 

 like Fig. 17 containing a layer of fused sulphur at B, and mercury at 

 A. Before and during very complete exhaustion, the mercury at K is 

 kept in liquid air and hnally the flask is sealed off at C. On standing 

 at the ordinary temperature, the surface of the mercury gets acted upo^i 

 by the sulphur, and if the mercury is not shaken so as to crack the film 

 of mercury sulphide, a little sponge of liquid air placed upon the sur- 

 face at D shows no metallic deposit ; but the moment the mercury is 

 slightly shaken to break the surface film of the liquid mercury, metallic 

 , deposit is instantly formed by the local cooling. This deposit is not 

 all mercury, but partly sulphide, because on taking away the sponge 



