of Edinburgh, Session 1878 - 74 . 258 
— first, that the upward motion through a liquid of a microscopic 
bubble must necessarily be very slow, even although under a high 
magnifying power it may seem otherwise ; and next, that in space 
containing only vapour of a liquid in contact with the liquid itself, 
evaporation and recondensation may proceed with excessive rapidity. 
The action of Wollaston’s cryophorus, to which reference has just 
been made, and Dalton’s experiments on vapours, made by passing 
liquids up into a Torricellian vacuum, alike exhibit the facility 
with which vapours form and recondense in spaces void of gases 
which are permanent at the existing temperature. Add to these 
considerations the information derived from the experiments of 
Cagniard de la Tour, Faraday, and Andrews, as to the enormous 
celerity with which substances pass from the liquid to the gaseous, or 
from the gaseous to the liquid condition, when near their critical tem- 
peratures, which for different substances range probably between the 
very remote limits 773° and - 166° Fahrenheit, and the explanation 
which I have ventured to propose of the motion of a vapour bubble 
in a liquid conveying a heat current becomes sufficiently feasible. 
I have to-day IHen at some pains to verify the result obtained 
yesterday, namely, that a piece of metal at the same temperature as 
the Iceland spar has no power to move the globules of vapour in 
the fluid cavities. Placing a shilling on the microscope stage 
beside the crystal, I left it for about ten minutes. Then holding it 
in forceps to avoid heating it by the hand, I moved it up into the 
field touching the spar, and so as almost to cover a fluid cavity from 
view. No motion of the bubble ensued. But, on putting my finger 
on the top of the shilling, by-and-by the bubble began to move, and 
slowly but steadily crossed the cavity towards the shilling. The 
same experiment was repeated with a bit of sheet lead about an 
inch square and 008 inch thick, with precisely the same result. I 
do not find lead notably less active than silver ; but the experi- 
ments made were necessarily too hasty and imperfect to settle the 
point as to whether any difference exists. The relative thermal 
conductivities of silver and lead being in air as 100 to 8'5, accord- 
ing to Wiedemann and Franz’s experiments, we might expect, 
when heat was conducted from the hand into the crystal through a 
piece of metal, that silver would produce more energetic effects than 
lead. May the effects be due, in part at least, to radiant heat, the 
