412 Mr. L. Fletcher on the Dilatation of Crystals 
shown by the Plate, is carried away from each of the vessels 
by the pipes h into the escape-pipe or chimney c. 
The cocks figured in the Plate show how the exhaust- stoam, 
or the steam after it has been superheated by passing- through 
the superheater K, or a mixture of the two, can be conveyed 
into, or shut off from, the steam-jackets of the vessels E and F. 
In order not to have to stop the engine and air-pump when 
it is desired to disconnect any one vessel or apparatus with the 
air-pump, a stopcock is attached to each branch of the general 
suction-pipe e, and above these stopcocks are attached the 
special mercurial gauges if desired. These stopcocks also 
enable one to moderate the suction-action on any of the 
vessels. 
Fig. 2 (Plate VII.) represents the combination of the air- 
pump and compression-pump; they can be worked together, 
or either of tliem can be thrown out of action by removing the 
pin which connects it with the rod of the steam-engine. 
We think that in the future the chemical action of bodies 
on one another under increased and diminished pressures will 
be more studied than has yet been done ; and we hope and 
expect the appliances we have described, as they will be always 
ready for use, will greatly conduce to the study of chemical 
action under either or both of these conditions ; and therefore 
it will be seen that we expect that these mechanical arrange- 
ments will have a much wider application than we have outlined. 
Our friend Mr. J. G. Douglas, B.A., who has kindly made 
the drawings for us, has also most kindly given us an approxi- 
mate estimate of the cost of the apparatus ; he thinks the whole 
of the apparatus, excluding the table, would not exceed 
£50. It will be admitted, we think, that this is by no means 
costly when all the different pieces of apparatus, such as 
stills, air-pumps, &c, that will be dispensed with are taken 
into account ; even leaving out of consideration altogether the 
saving of time and labour which will be effected by the adop- 
tion of these new laboratory appliances. 
LX. The Dilatation of Crystals on Change of Temperature, 
By L. Fletcher, M.A., of the Mineral Department, British 
Museum; late Fellow of University College, Oxford. 
[Concluded from p. 350.] 
Third Part. 
WE now proceed to the consideration of the case of an 
Anorthic crystal ; and the method we shall adopt is 
one identical in character with that indicated above for an 
Oblique crystal. 
