242 Mr H. Blackadder on the Action of certain Fluids on 
fill up the interstitial spaces between the particles or fibres of 
such hydrhophic substances, without entering into the substance 
of the particles themselves; and that water, according to circum- 
stances, not only enters into the interstices, but also penetrates 
into, or combines with, the substance of these particles or fibres, 
and even at the time when the interstitial spaces are occupied by 
an oily fluid : For the presence of oil does not materially influ- 
ence the absorption of aqueous fluid, provided the surface of the 
solid body is not so coated with oil as that it shall act the part 
of a varnish. 
We are thus enabled to explain how it is that the human hair 
becomes so quickly affected by exposure on a damp evening, 
though oil may have been previously pretty freely applied to it ; 
and how it is that leather, after it has been soaked in oil, freely 
absorbs, and transmits moisture, when exposed to its influence ; 
for example, in walking over moist grass. 
Experience had taught, that an occasional sponging of har- 
ness with fresh water, had the effect of keeping the leather soft 
and pliant, without injuring its cohesion, much better than oily 
fluids. When fresh water is applied to dry leather, or when 
the latter is exposed to damp air, its fibres absorb a certain por- 
tion of water, and are thereby expanded and rendered more 
pliant and elastic ; and, as the fibres are thus brought into closer 
contact, the cohesive force is rather increased than diminished, 
provided too much water has not been received into the inter-, 
stitial spaces. When, on the other hand, oil is applied to dry 
leather, its pliancy is increased ; but, as this is produced merely 
by the lubricating property of the oil, and as the fibres remain 
crisp and contracted as before, the cohesive force, if not dimi- 
nished, cannot at least be increased. This, however, I have not 
had an opportunity of determining with sufficient exactness, by 
experiment. There is an artificial substance, of vegetable ori- 
gin, and of foreign manufacture, known by the name of Papier 
vegetal , which is exceedingly sensible to variations of atmosphe- 
ric humidity ; and, in this respect, is well fitted for the construc- 
tion of expansion hygrometers. Its texture is very compact ; 
and, though very thin, its particles or fibres evince a very strong 
cohesive force. It is more transparent than oiled paper, and when 
oiled has nearly the translucency of glass, I have often used 
