at the Royal Institution, 1900-1907. 879 



complex character than those represented by the conventional formulae 

 OHo. Oo and O3. 



True blacks have not yet been produced by artificial means but 

 a number of artificial dye stuffs are known the colour of which 

 borders on black. The almost complete absorption of light by such 

 materials appears to be conditioned by the conjunction of a number 

 of hght-absorbing systems and the superposition of their individual 

 absorptive effects, as they are all substances of complex benzenoid 

 structure, containing several systems each of which taken singly 

 would be more or less intensely coloured — usually yellow or red. 



It is therefore probable that the blackness of the amorphous 

 forms of carbon is due to its complex atomic structure and that it 

 is composed of ethenoid-benzenoid systems similar to those which 

 are met with in the more complex coal-tar hydrocarbons. A similar 

 argument would lead us to attribute a complex paraffinoid structure 

 to the diamond : it cannot well be supposed that six atoms of carbon 

 can form a saturated system in the manner represented by Brlihl 

 {loc. cit.), unless, being formed under great pressure, the affinities in 

 the diamond are forced to act in unusual directions, as in Tamman's 

 ice of greater density than water. The fact that two atoms of 

 nitrogen can form a saturated molecule shows the difficulty of 

 arriving at positive conclusions in such cases, however. 



Sir James De war's observations on the marvellous absorptive 

 power of charcoal at low temperatures appear, in some measure, to 

 meet with an explanation from the point of view above advocated ; 

 they at least afford strong presumptive proof that charcoal is possessed 

 of properties such as are characteristic of ethenoid compounds. 



Conductivity ix High Charcoal Vacua in relation to the 

 Theory of Chemical Change and Induction Phenomena. 



Among the observations recorded by Sir James Dewar during 

 the Septenate under discussion, there are not a few of which 

 apparently the full significance has yet to be appreciated. Eeference 

 may first be made to a recent series of experiments in which a novel 

 use has been made of the Crookes radiometer in determining small 

 gas pressures. The observations show that when helium is the 

 residuary material filling the instrument, an attached charcoal con- 

 denser cooled in liquid hydrogen is unable to absorb the gas 

 sufficiently to diminish the pressure to such an extent that the 

 radiometer will not rotate when the concentrated beam of an electric 



