iv THE SENSE OF SMELL 175 



tlu 1 elements that form odoriferous compounds belong almost 

 exclusively to the fifth, sixth, and seventh groups. 



The Hfth group contains nitrogen, phosphorus, vanadium, 

 arsenic, niobium, antimony, didymium, tantalum, bismuth. The 

 sixth group consists of oxygen, sulphur, chromium, selenium, 

 molybdium, tellurium, wulfranium, uranium. The seventh group 

 consists of fluorine, chlorine, manganese, bromine, iodine. It is 

 undeniable that many of these elements form odoriferous com- 

 pounds, and that a certain periodicity in the appearance of 

 odorous and non-odorous substances exists within each series. 



Another interesting fact is that in some series of homologous 



o o 



chemical compounds, e.g. in those of the fatty acids and the 

 alcohols, there is a regular and continuous change in the odour. 

 It is particularly remarkable that the lowest members of these 

 homologous series have very faint smells, and that the intensity 

 of the smell continuously increases in higher members (formic, 

 acetic, propionic, butyric, valerianic, caproic acid, etc.). In the 

 highest members the series of odours is interrupted ; stearic 

 acid, e.g., has no smell. Another series of regularly changing 

 smells consists of benzol, toluol, xylol, etc. 



Undue importance was given in the past to the so-called 

 odoroscopic researches of B. Prevost (1799) on the physical 

 quality of odours. He observed that many odoriferous substances 

 assume a characteristic rotary or vortex movement on the surface 

 of water, which he interpreted as the effect of the discharge and 

 diffusion of odorous particles into the atmosphere. Liegeois 

 brought forward other odoroscopic phenomena, but expressly 

 noted that they only appeared in substances of vegetable and 

 animal origin, while those of mineral origin show no movement 

 on contact with water (e.g. ammonia, hydrogen sulphate and 

 phosphate). On the other hand, he found that some completely 

 inodorous substances, such as sulphuric acid, potash, and soda, 

 exhibit the same phenomenon. Obviously these "odoroscopic 

 phenomena " afford no explanation why odorous substances excite 

 the organ of smell : the movements are due to the surface tensions 

 of the different compounds, and are not a specific property of 

 odours. 



Tyndall also showed that the vapours of odorous substances 

 possess a remarkable power of absorbing thermal rays, but it 

 is very doubtful J if it is owing to this property that they are 

 odorous. 



Erdmann's researches on the solubility of certain essential oils 

 (cedar, rose, geranium) in liquid air are extremely interesting. 

 In comparison with other chemical compounds, these odorous 

 substances have a very high specific solubility in liquid and 

 possibly also in gaseous air. It is probable that this property is 

 common to all odorous substances and that it is one of the 



