162 SENSE OF SMELL. 



fusion and distillation. The fact, moreover, has been directly proved by 

 an experiment of M. Berthollet. On nearly filling a tube with mercury, 

 and placing a piece of camphor at the top of the tube, he found that, 

 after a time, the mercury descended, the camphor had diminished in size, 

 and the space above the metal was occupied by an odorous gas. 1 



But what is the cause of the disengagement of these odorous mole- 

 cules ? By most writers on this subject it has been considered to be 

 owing to the solvent action of caloric on the odorous body. The opinion 

 that all bodies are odorous is as old as Theophrastus; and it is one which 

 it is difficult not to embrace, if we add provided they are subjected 

 to the appropriate agents for disengaging the odorous particles ; and 

 the probability is, that the reason we esteem particular bodies inodor- 

 ous is, that our olfactory nerves are not organized with sufficient deli- 

 cacy to enable us to distinguish their odorous properties. Heat assists 

 the escape of odorous particles from a variety of bodies ; and hence 

 it has been maintained, that every body which is volatile must be odor- 

 ous. M. Adelon 2 asserts, that this is not the case ; but it is difficult to 

 accord with him. The fact of our not appreciating the odour is no 

 proof of its non-existence. In truth, bodies that are inodorous to one 

 animal or individual may not be so to another. In cases, too, in which 

 smell is morbidly acute, a substance may appear overwhelmingly odor- 

 ous, which may appear devoid of smell to a healthy individual. M. 

 H. Cloquet 3 refers to the case of a celebrated Parisian physician, who 

 was subject to violent attacks of hemicrania or megrim, and who was 

 dreadfully tormented, during one of the paroxysms, by the smell of 

 copper, exhaled from a pin that had been dropped on the bed ! 



Caloric seems to be only one of the causes of the disengagement of 

 odours. Some are retained by so feeble a degree of affinity, that they 

 appear to be exhaled equally at all temperatures. Light influences 

 their escape in particular cases ; some plants giving off their fragrance 

 during the day ; others perfuming the air only at night. Dampness, 

 in many instances, assists their escape, hence the fragrance of a gar- 

 den after a summer's shower; and the smell afforded by all argillaceous 

 substances when breathed upon, a fact, the knowledge of which is 

 of importance to the chemist. 



Lastly; substances, that appear to us devoid of odour, may exhale a 

 strong one, when rubbed together. All these circumstances tend greatly 

 to prove, that every substance is possessed of odorous qualities, although 

 we may not be aware of the precise mode for causing their emanation, 

 or our olfactory nerves may not be sufficiently delicate to appreciate 

 them. 



Around odorous bodies, the molecules, as they escape, form an atmo- 

 sphere, which, of course, will be denser, the nearer it is to the body. 

 These particles are diffused around, not, probably, in the same man- 

 ner as light or sound, but as one fluid mixes with another; and, when 

 the air is still, it is conceived, their strength will be inversely as the 



1 Cloquet, Art. Odeurs, Diet, des Sciences Medicales, torn, xxxvii., p. 89, Paris, 1819. 



Op. cit., i. 322. 



a Osphresiologie ou Traite des Odeurs, Paris, 1821. 



