139 
of Edinburgh, Session 1875-76. 
5. On a Glass Digester in which to Heat Substances under 
Pressure. By Dr E. A. Letts. 
The objections to the use of sealed tubes are known to every 
practical chemist, and are a serious drawback to their employ- 
ment. The chief of these are the time expended in the manu- 
facture of the tubes, the amount of skill in glass-blowing re- 
quired, the danger experienced in opening them, and above all, 
the fact that only a small quantity of material can be heated at 
one operation. Moreover, the same tube can seldom be used for 
more than three or four experiments, as each time it is sealed up 
its neck must be drawn out, and its length thus considerably 
decreased. These disadvantages were especially felt by me whilst 
preparing bromacetic acid, which was required in considerable 
quantities, and where as many as half a dozen tubes of bromine 
and acetic acid had to be heated before 100 grammes of the acid 
could be obtained. To obviate these objections I have had an 
apparatus constructed, which consists of a cylinder of glass, the 
walls of which are about half an inch thick. Its length is fifteen 
inches, its external diameter three inches, and its capacity about 
600 cubic centimetres. At one end it is drawn out to a tube, 
whose aperture is only about one-sixth of an inch in diameter, 
though its walls are as thick as the rest of the apparatus. Origin- 
ally this tube was provided with a stopcock, but at Professor Brown’s 
suggestion, I have substituted a glass plate, which is ground fiat, 
and accurately adapted to the top of the tube. 
In order to keep the glass plate pressing on the tube the whole 
apparatus is placed in a frame, consisting of three brass wires 
arranged symmetrically around the cylinder, and attached by means 
of nuts, below, to a brass ring, and above, to a brass plate, through 
which latter a screw passes, which, when turned, presses on a brass 
plate placed on the glass cap. 
As any experiments with such an apparatus would be attended 
