SMELL AND CHEMICO-PHYSICAL PROPERTIES. 1255 



This last example will best illustrate how difficult it was to correlate 

 smell with the chemico-physical properties of odorous substances. No one 

 would group the sensation of ethyl-acetate with that of amyl-acetate ; 

 they are very unlike, yet they are linked together by the smells of the 

 intermediate members of the group. It is only by studying a whole 

 group of related elements, or compounds, that it is possible to make out 

 any connection at all. We may say then that smell follows the periodic 

 law, and that the power of existing smell, when present, is found to vary 

 in the same way as the chemico-physical properties of substances. 



Such a statement is not vitiated by the fact that certain substances, having 

 dissimilar chemico-physical properties, produce the same smell. This only 

 shows that the nose has not a sufficient power of analysis to distinguish in 

 this case the two stimuli from each other. In the case of vision, two very 

 dissimilar stimuli may affect the eye, and produce identically the same 

 sensation. Thus a mixture of red and blue-green spectral rays will produce a 

 sensation called grey, identical with that produced by a mixture of orange and 

 blue. In the case of taste, where there are but a limited number of sensations, 

 we find many instances of a like nature. Our previous conclusions, although 

 they place the power of producing smells on all-fours with the chemico- 

 physical properties of chemical substances, do not commit us to the statement 

 that odorous substances act chemically upon the olfactory end-organs. This 

 is quite another matter, and must be discussed on its own merits. While it is 

 quite possible that the differences in quality of smell may be due to the 

 different chemical decomposition produced upon the protoplasm of the end- 

 organ, it does not necessarily follow that this is the case. Odorous bodies 

 have dynamical properties which might fully account for their action as 

 stimuli. Odorous bodies have no doubt definite vibratory periods, and they absorb 

 in great amount the heat rays. Spectroscopic investigation indicates, although 

 the evidence is as yet somewhat fragmentary, that the elements or their com- 

 pounds in one of MendelejefFs groups are similar in the character of their 

 molecular vibrations ; and Carnelley 1 has shown that a progressive change of 

 vibrational pitch occurs on passing from a lower to a higher member of a 

 group. Much the same is true of the alcohols, and of the fatty acids ; the 

 alcohols, for instance, have characteristic absorption-bands which shift towards 

 the red end of the spectrum, on ascending the group. William Ramsay 2 

 showed that a substance must have at least fifteen times the molecular weight 

 of hydrogen in order that it may be smelt at all. The light gases of the 

 atmosphere are odourless; so is marsh gas (8), ethane (15), olefiant gas (14), 

 hydrocyanic acid (15), methyl-alcohol (16), the latter two of which are smelt 

 by some persons and not by others. Bodies of similar constitution, but with 

 heavier molecules, are frequently odorous ; and as a rule the strength of the 

 odour increases with the molecular weight. Thus, while methane and ethane 

 are odourless, propane (22) can be smelt. While hydrocyanic acid is odourless, 

 the higher members or nitriles have characteristic smells. While methyl- 

 alcohol is odourless, the higher members have stronger and stronger odours. 

 Recently J. Passy 3 has shown by olfactometric measurements the relative 

 potencies of some of the alcohols, and his results are given in the following 

 table :- 



Methyl-alcohol ... 1 



Ethyl- . . . 4 



Propyl- ... 100 



Butyl- 1000 



Amyl- 10,000 



1 London, Edinburgh, and Dublin Phil. Mag., 1884, ser. 5, vol. xviii. 



2 Nature, London, 1882, vol. xxvi. p. 187. s Compt. rend. Soc. de biol., Paris, 1893. 



