48 DRS. W. RAMSAY AND S. YOUNG ON VOLATILIZATION OF SOLIDS. 
exposed to radiated heat in the same manner as the solids were. The vapour-tensions 
of water found in this way were identical with those of Begnault. 
18. This method of determining vapour-tensions has great advantages compared 
with the usual method. For it is extremely difficult to obtain a number of constant 
temperatures when a long tube is heated, and the pressures depend on those tem¬ 
peratures ; whereas by the method described, the temperature depends on the pressure, 
which can be varied at will by exhausting with the pump or by introducing air. 
19. The experiments described have shown that solids have definite temperatures of 
volatilization, as liquids have definite boiling-points, depending on the pressure to 
which they are subjected, and that these temperatures are sensibly coincident with 
those of their vapour-tensions. That they cannot be absolutely identical is evident ; 
for there must be a certain excess of pressure to produce a flow of vapour from the 
evaporating substance to the surrounding space, and consequently the evaporating 
substance must have a higher temperature corresponding to the higher pressure in its 
immediate neighbourhood. But by the ordinary method of measuring vapour-tensions, 
where the body emitting vapour is placed in the vacuous space above the mercury in 
a barometer tube, no flow is possible, and hence the level of the mercury is a true 
measure of the tension. 
Our results, we venture to think, show that with solids as with liquids this differ¬ 
ence, even when rapid evaporation is taking place, is an extremely minute one. They 
also show that the ice-steam line of Jas. Thomson (see diagram, fig. 1 , M, L) is the 
upper temperature limit of ice at pressures below the critical one. 
