404 Intelligence and Miscellaneous Articles. 



having led to different results, I thought it necessary to resume 

 my experiments, considerably improving my apparatus, especially 

 in what concerns the measurement of the pressures, which is the 

 only difficulty peculiar to this investigation. The method employed 

 having been already described in the Annates de Chimie et de 

 Physique, t. viii. (1876), I shall only dwell on the modification 

 •which the differential barometer has undergone ; it is the essentially 

 delicate part of the apparatus, all the other parts of which have 

 also been considerably improved. This barometer consists of a 

 single glass tube bifurcating, at about 70 centim. above the level of 

 the mercury in the cistern, into two wider cylindrical branches, one 

 of which forms the barometric chamber, and the other is put into 

 communication with the space filled with the gas the pressure of 

 which is to be measured. The immediate result of this disposition 

 is that there is no need to attend to the difference of temperature 

 between the two mercurial columns, which are here joined into one 

 at a little distance below the meuiscuses. The branches of the 

 bifurcation are prolonged upwards by stems of very small diameter, 

 having each a glass cock, and joining again to form a single stem. 

 The rest of the apparatus is disposed so that the manometer can be 

 charged in place by the process generally adopted nowadays, which 

 consists in first exhausting it with a Sprengel pump ; this was 

 worked, moreover, during all the time of the filling, so as to main- 

 tain the vacuum continually dry by the intervention of a tube 

 containing phosphoric acid. All suction of air through the slender 

 point was avoided by covering the surface of the mercury with a 

 layer of sulphuric acid, which remained in the cistern during all 

 the experiments. Above the lower single branch was a glass cock, 

 by closing which the differential barometer could be transformed 

 into an ordinary truncated barometer, and thus the errors due to 

 variations of the atmospheric pressure be eliminated — which is 

 extremely important. 



In order to avoid as far as possible the errors due to refraction 

 and capillarity, the two branches of the manometer, before being 

 soldered to the single stem, were rounded and polished inside with 

 the same copper mandrel, so as to be rendered perfectly cylindrical ; 

 a plane facet was then cut on the exterior, quite parallel to the 

 generating lines of the interior cylinder. This done, the pieces 

 were soldered, the necessary precautions being taken to keep the 

 plane facets rigorously in the same plane. 



These pieces are very difficult to obtain : a large number of them 

 break or split, either during the rounding, or the soldering, or even 

 after these operations are finished. The cylinders were smoothed 

 and cut by M. Lutz ; the manometers were afterwards finished by 

 M. Alvergniat ; that is to say, they were made with all the skill and 

 perfection that could be wished for. 



To eliminate capillarity-errors, an internal diameter of 2 centim. 

 was given to the cylinders ; and they were evidently perfectly 

 equal: it was easy to verify that the mercury in them was in per- 

 fect equilibrium under the thread of the cathetometer. 



