so that the force of the ascending sap could be 
measured by its effect in elevating the quicksilver. 
In a few days it was found, that the sap had been 
propelled forwards with so much force as to raise 
the quicksilver to 38 inches, which is a force con- 
siderably superior to that of the usual pressure of 
the atmosphere. Capillary attraction can only be 
exerted by the surfaces of small vessels, and can 
never raise a fluid into tubes above the vessels 
themselves. 
I referred in the beginning of the Third Lecture 
to Mr. Knight’s opinion, that the contractions and 
expansions of the silver grain in the alburnum are 
the most efficient cause of the ascent of the fluids 
contained in its pores and vessels. The views of 
this excellent physiologist are rendered extremely 
probable by the facts he has brought forwards in 
support of them. Mr. Knight found that a very 
small increase of temperature was sufficient to 
cause the fibres of the silver grain to separate from 
each other, and that a very slight diminution of 
heat produced their contraction. The sap rises 
most vigorously in spring and autumn, at the time 
the temperature is variable ; and if it be supposed, 
that in expanding and contracting, the elastic 
fibres of the silver grain exercise a pressure upon 
the cells and tubes containing the fluid absorbed 
by the capillary attraction of the roots, this fluid 
must constantly move upwards towards the points 
where a supply is needed. 
The experiments of Montgolfier, the celebrated 
inventor of the balloon, have shown that water 
may be raised almost to an indefinite height by a 
