at the Royal Institution, 1900- 1907. 



357 



surprising that he failed even to confirm the observations of earher 

 workers. 



The subject was first dealt with in a thorouo^h manner by 

 Theodore de Saussure in 1812. An English translation of his 

 important memoirs is to be found in the "Annals of Philosophy," 

 vol. vi. pp. 241-255, 831-347. In his experiments red-hot box- 

 wood charcoal was plunged under mercury and introduced into the 

 gas to be absorbed after it was cool and without ever coming into 

 contact with atmospheric air. He gives the following table : — 



De Saussure showed that the property of absorbing gases was 

 common to all porous solids but found no substance possessing the 

 property in so high a degree as charcoal. He arrived at interesting 

 results with mixtures of gases and proved the incorrectness of the 

 statement made by Rouppe and Xorden that water was formed by 

 the interaction of hydrogen and oxygen when these two gases were 

 absorbed by charcoal. The volume of gas expelled from charcoal by 

 another gas was found to vary according to the proportion in which 

 both gases exist in the unabsorbed residue. In several cases, the 

 presence of one gas in the charcoal was found to facilitate the con- 

 densation of the other. 



An interesting series of communications was made to the 

 Chemical Society during the " sixties " by John Hunter. Besides 

 confirming De Saussure's observations, he described the results of 

 measurements of the absorption by charcoal at temperatures extending 

 up to 200"" C. not only of gases but also a large number of vapours. 

 Hunter showed that logwood, ebony and cocoanut charcoal exceed 

 that from boxwood in absorptive power, the very dense variety 

 prepared from cocoanut being superior to all other forms. 



Charcoal has been used in respirators as a protection against 

 infection ; it has been enclosed in covers placed over the openings of 

 sewers to prevent the escape of noxious emanations into the streets ; 

 its value as a means of purifying drinking water has long been 

 recognised ; but no scientific application was made of its extra- 

 ordinary power of condensing and holding gases until recently : 



