( 438 ) 
In the second place the gas enclosed between pure mercury and 
steel cannot become contaminated by volatile substances which it 
might otherwise absorb from the liquid in the pump-cylinder or from 
the lubricant used for this cylinder. 
These advantages are of especial importance if we must compress 
moderately large quantities of pure or costly gases, and for this pur- 
pose an apparatus of the same dimensions as that of CAILLETET is 
indispensable in a laboratory. 
That the compressor (to be described below) answers its purpose 
in all respects, may be proved by the fact that it has been frequently 
used during the last eight years, without having undergone the 
slightest change, or giving the least trouble. The improvements 
made upon the original CAILLETET pump in former years (especially 
in ’88) were rendered necessary by repeated disappointments, which 
so often disagreeably interrupted the progress of my work that I 
almost despaired of ever obtaining a mercury pump easily handled 
and perfectly trustworthy. 
In considering the apparatus we must bear in mind that it has 
grown by gradual improvement from the CAILLETET-pump, and 
also that traces of less successful modifications have remained, It 
would be possible to design now @ priori a compressor that from 
the point of view of mechanical design would be of better shape 
and construction owing to the greater harmony of its dimensions, 
I hope that a mechanical engineer will feel himself drawn to the 
solution of this problem. J was satisfied in having an apparatus 
which worked well from a physicist’s point of view, and, in the same 
manner and after this design every other CAILLETET-pump can be 
successfully modified. 
§ 2. Fig. 1 PL I. shows CAILLETET’s original compressor so that 
we may compare the two compressors together. Fig. 1 is a section, 
figs. 2 and 3 show the manner in which the gas to be compressed 
is admitted through the sucking-cock 4 into the pump-cylinder. 
Pl. IIL is a diagramatie representation of the new compressor 
with the accessories belonging to it. The purpose of this plate 1s 
to explain the way in which the different parts work. These are 
for the greater part drawn on the same scale in a simplified but 
yet recognizable manner, while the connections are entirely diagra- 
matical. The real form of these parts, so far as they are not suf- 
ficiently represented in this plate, may be seen on Pls. IV, V and 
VI figs. 1 and 2, while figs. 1, 3 and 2 of Pl. II show the actual 
arrangement of the different parts of the foremost and the hind- 
