1254 THE SENSE OF SMELL. 



stimuli. The very uniform appearance of the olfactory end-organs 

 indicates, indeed, that they are all concerned in the reception of all the 

 stimuli which produce olfactory sensations ; and we may at any rate 

 conclude that the neo-Miillerian law requires, in the case of smell, 

 additional evidence before its acceptance can be granted. 



Relationship between smell and chemico-physical properties 

 of odorous bodies.— From general analogy we should expect that 

 there will be found to exist some sort of resemblance between the 

 sensations produced by stimuli of similar physico-chemical properties. 

 This connection is at first sight by no means apparent, and was only 

 clearly understood after Mendelejeff had classified the elements into 

 natural groups under what is known as the Periodic Law. 1 Taking one 

 of these groups, we find that they consist of elements having similar 

 general chemico-physical properties ; thus in the sixth group we have 

 oxygen, sulphur, chromium, selenium, molybdenum, and tellurium. In 

 ascending one of these groups, beginning in the sixth group with oxygen, 

 we find that the molecular weight increases, and that there is a pro- 

 gressive change in the chemico-physical properties of each member of 

 the group. We also find that the odd and even members respectively 

 of the group resemble each other more particularly. Thus in group six 

 we find that, in their chemico-physical properties, sulphur, selenium, 

 and tellurium closely resemble each other. Now what we can say as 

 to the chemico-physical properties of these elements, we can also affirm 

 as to their physiological properties, as regards the sense of smell, and 

 also of taste. Thus in group six, sulphur, selenium, and tellurium, 

 when compounded with hydrogen, with methyl, or with ethyl, etc., 

 have strong and disagreeable odours. They are not the same, although 

 there are points of resemblance, especially as regards their offensiveness. 

 The odours of the selenium compounds are, however, recognisably 

 intermediate between those of sulphur and those of tellurium, just as 

 its molecular weight occupies a position between these two. There 

 are other groups having elements which give rise to smell; thus 

 in group seven we have chlorine, bromine, and iodine. These odours 

 are not unlike each other, the odour of bromine linking the lower 

 chlorine to the higher iodine. Among these compounds may be 

 instanced chloroform, with its fragrant odour ; bromoform, with a 

 fragrant odour, mixed with another odour which we recognise as that 

 of the higher iodoform. 



Leaving Mendelejeff's groups, and studying groups of organic bodies 

 like the monatomic alcohols or the fatty acids, having similar chemico- 

 physical properties, but showing a progressive change in their characters 

 as we pass from the lower to the higher members of the group, we 

 shall find that there exists in like manner a change in respect of the 

 sensations they are capable of producing. 



Ethyl-acetate . . . Has an acetic and ethereal odour. 



Propyl- ,, . . . Has an acetic odour together with 



a slight flavour. 



Butyl- „ . . . Has a slight acetic odour with a 



pine-apple flavour. 



Amyl- „ Has no acetic odour, but has a well- 



marked flavour of pine-apples. 



1 J. B. Haycraft, "The Sense of Smell," Proc. Roy. Soc. Edin., 1887, vol. xiv. p. 207; 

 and Brain, London, 1889, vol. xi. p. 166. 



