102 Dr Brewster on the General Fro'periies of Tahaslieer, 
this pore is filled with air ; and that when a ray of light MN 
enters the separating surface ab at e, and quits it at h, it 
suffers so little refraction, and is therefore so little scattered, 
that the tabasheer appears transparent, and allows us to see 
objects distinctly through it. This effect, which could not 
take place in any other porous substance, arises from the 
small difference between the refractive power of air and of 
tabasheer. Let us now suppose that a small quantity of wa- 
ter is introduced into the pore abed, so as not to fill it, but 
merely to line its circumference with a film contained between 
abed and i»/9y Then the light which was formerly scatter- 
ed by the slight refraction at e and h in passing from tabasheer 
into air, will now be a little less scattered at these points, 
since it passes from tabasheer into water, where the difference 
.. of refractive power is less ; but in passing from the film of wa- 
ter into the air at f and in entering the water again at g, the 
scattering of the rays will be very considerable, from the great 
difference in the refractive powers of air and water. In passing 
through every pore, therefore, the light is refracted, and conse- 
quently scattered no less than four times ; and hence the piece 
of tabasheer must appear to be opaque. If we now saturate it 
with water, the pore abed wdl be completely filled ; the two 
great refractions which took place at f and g, will no longer 
exist ; and the light will suffer only a slight refraction at e and 
Ji, by which it will be less scattered than when the tabasheer 
was dry. Hence it follows, that when the tabasheer is saturated 
with water, it ought to transmit the incident light freely, and 
to be more transparent than when it is quite dry, — a result 
which is perfectly conformable to observation 
Art. 11.— -Account qf the Ayrshire Rose. In a Communi- 
cation from Mr Neill, Secretary of the Caledonian Hor- 
ticultural Society, 
I^oii a number of years past, a very rampant climbing rose- 
bush has been cultivated in Scotland, under the name of the 
^ The reader is referred to the original paper in the Philosophical Transactif»m 
fov iTxore detailed inforrhation respecting the properties of Tabasheer. 
