376 Prof. H. E. Armstrong on Loic-Temperature Research 



COO H 



H OOC 



H 000 



COO H 



COOH 



COO H 



The acid of this composition is known as mellitic acid ; as the acid 

 can be obtained by oxidising amorphous carbon, there is every reason 

 to suppose that the carl^on molecule in some way corresponds in struc- 

 ture to a hydrocarbon such as is here thought of. 



Colour of Amorijhous Carlon. — This also is a property which may 

 be interpreted as proof that the molecule has a complex ethenoid 

 structure. 



In writing to Schonbein in 1852 Faraday remarks : — 



" Your letter quite excites me and I trust you will establish undeni- 

 ably your point. It would be a great thin.c; to trace the state of combined 

 oxygen by the colour of its compound, not only because it would show 

 that the oxygen had a special state which could in the compound 

 produce a special result but also because it would, as you say, make the 

 optical effect come within the category of scientific appliances and serve 

 the purpose of a philosophic indication and means of research, whereas 

 it is now simply a thing to be looked at. Believin^;- that there is nothing 

 superfluous or deficient or accidental or indifferent in nature, I agree 

 with you in believine; that colour is essentially connected with the 

 physical condition and nature of the body possessing it ; and you will 

 be doing a very great service to philosophy, if you give us a hint, 

 however small it may seem at first, in the development or, as I may 

 even say, in the perception of this connection." 



What Faraday foresaw is since come to pass. Light, it is well 

 known, is always 'more or less refracted by passage through a trans- 

 parent medium. Among carbon compounds the paraffins are the 

 least refractive substances known. If certain values be taken 

 as atomic refraction constants (comp. Briihl, these Proceedings, 

 vol. xviii. p. 122), it is easy to deduce the molecular refractive 

 power of any paraffin hydrocarbon and the value thus found is in 

 close approximation to that calculated. Carbon in the ethenoid 

 state, however, produces a greater effect ; it has the peculiarity, 

 moreover, that when two or more pairs of ethenoid carbon atoms 

 occur in dose conjunction the refractive power is not merely the sum 



