330 BOILING OF LIQUIDS IN GLASS VESSELS. 
perienced in the process for preparing the diluted hydro- 
cyanic acid of the London Pharmacopoeia, and to a much 
greater extent in distilling oils and resins with spirit or 
water. My attention was more particularly directed to the 
subject in connexion with a series of investigations I had 
occasion to make with the view of determining the quality 
of some specimens of halsam of copaiha. Some of the 
results of these investigations are published in the journal, 
vol. vi., page 13. In attempting to effect the separation of 
the essential oil from the resin of the oleo-resinoiis balsam, 
by distilling it with water in a glass retort or flask, I found 
it impossible to continue the distillation for a sufficient 
length of time to complete the analysis, in consequence of 
the bumping which took place, and which became more 
and more violent as the process proceeded. 
On repeating the experiments with other oils and resins, 
similar phenomena were found to occur. Thus, for in- 
stance, if a piece of common resin or of shellac be intro- 
duced into a globular flask nearly filled with distilled water, 
and if the water be boiled over the fiame of a gas lamp, the 
ebullition will at first take place pretty uniformly ; but after 
some time it will be observed that the bubbles of steam 
which are formed at the bottom of the flask, in contact with 
the most heated part of the glass, will be larger than they 
were in the first instance and instead of their passing con- 
tinuously through the liquid, there will be frequent inter- 
missions, during which ebullition will entirely cease. After 
each of these intermissions the disengagement of steam will 
take place with increased violence, and as the process is 
continued, the length of the intermissions will become 
greater. If a thermometer be introduced into the flask, it 
will be found that this irregularity of ebullition is accom- 
panied by great variations of temperature. While the 
bubbles of steam are passing freely through the liquid, the 
temperature will be from 212° to 214° Fahr., but when a 
