Upon the Extent of the Contraction, 8c. of Timber. 437 
For this purpose, large surfaces of it may be exposed in shallow 
earthen vessels, in tubs or boxes. The lime will also exert the 
salubrious effect, of absorbing carbonic acid gas from the air. 
Cork Institution, Dec. 6, 1817. 
-*,.* To these remarks of Mr. Davy I beg to add, that the 
results of his experiments tend to establish this truth: that the 
matter of contagion is imponderable, as those substances which, 
in their state of greatest dilution, merely affect the olfactory 
organs ; but I may also add, like these too, they may be capable 
of being taken up, neutralized, and precipitated, by chemical 
agents. Not only effluvia may be added to gases and liquids, 
but gases to gases, and liquids to liquids, without increasing their 
apparent volume. It is true that by the instruments of science 
the mixed gases and liquids may be presented separate ; but it 
would be unphilosophical to infer, because our means cannot yet 
separate and weigh or measure the matter of contagion, that 
therefore it has no existence. a. 
= 
LXXIII. Upon the Extent of the Expansion and Contraction of 
Timber in different Directions relative to the Position of the 
Medulla of the Tree. By Tuomas Axpaew Knicrt, Esq. 
| F.R.S. Ina Letter addressed to the Right Hon. Sir JosErPH 
‘Banks, Bart. G.C.B.P.R.S.* 
My DEAR Str,— May attempts have been made by writers 
on vegetable physiology, to account for the force with which the 
sap of trees has been proved by Hale to ascend during the 
spring, without any hypothesis having been offered which has 
been thought satisfactory: and almost all which have been offer- 
ed have been justly rejected as wholly inadequate. I have sug- 
gested in the Philosophical Transactions of 1501, second part, 
page 333, the expansion and contraction of those cellular pro- 
cesses which proceed from the bark to the medulla, which I have 
there called the true or silver grain of the wood; and which 
have generally, though most improperly, been called medullary 
processes. | have there shown, that this substance expands and 
contracts very considerably under changes of temperature and 
moisture ; and I have stated that a board of oak, which has been 
formed by cutting across the supposed medullary processes, can 
seargely be made, by any means, to retain the same form and 
poston when subjected to various degrees of heat and moisture. 
J had not at that time ascertained, with accuracy, the compara- 
tive expansion and contraction of timber when divided in different 
# From the Transactions of the Philosophical Society for 1817, part ii. 
Ees directions 
