366 
PHYSIOLOGY. 
BOOK II. 
it is necessary to rub the wood strongly, so as to produce heat 
enough to volatilise the matter which is locked up in the very 
compact tissue of which that plant consists. The necessity 
of producing a little heat, in order to produce an exhalation 
of the volatile matter, is further exemplified by the fragrance 
emitted by many woods, otherwise scentless, when exposed to 
the violent friction of a turner's lathe : beech is said to 
acquire, under such circumstances, the smell of roses. But 
when, on the other hand, the volatile matter is inclosed in 
wood of a loose texture, neither is heat required to elicit it, 
nor has the wood, if exposed to the air, the power of retaining 
it for any considerable time, for the oxygen of the atmosphere 
will seize upon it rapidly, and quickly leave nothing behind 
but the inodorous tissue: this happens to cassia and cin- 
namon. 
" Fugitive smells are those which, belonging to perishable 
organs, are either extremely perishable in their very nature, 
or are placed in tissue of the laxest kind, or are situated on the 
surface of plants where their volatile parts are continually ab- 
stracted by the atmosphere, or finally are secreted in quantities 
so small that a short exposure to air suffices to dissipate them. 
All these odours are produced only during the life of a plant; 
they are dispersed as they are formed, and after death leave 
no trace of their existence behind them. Like permanent 
odours, these are continually given off; and in some plants, as 
the orange and the violet, without any variation in intensity 
in different states of the atmosphere ; but in the majority of 
cases the power of the smell will vary according to the eleva- 
tion of temperature, and the dampness of the air. This fact 
must be familiar to all who are acquainted with gardens. In 
the hot, dry weather of a summer's noon, flowers either be- 
come scentless, or at least lose a large proportion of their 
useful fragrance ; and in walking through a wilderness of the 
most sweet-smelling plants, we find little sign of their odour, 
unless they are bruised or trampled upon. But if a heavy 
shower should come on, all will be changed in an hour's 
time ; every leaf, every flower, will emit its peculiar odour ; 
the musk plant (Mimulus moschatus) will fill the air with its 
