June io, 1897] 



NA TURE 



27 



ethyl chloride boiling at 12% ethyl fluoride at - 32", 

 propyl chloride boiling at + 45% ethyl fluoride at - 2^. 



Similar observations have been previously made by 

 Paterno and Oliveri, and by V'allach and Heusler. These 

 facts can also be connected with the experiments of 

 Ciladstone on atomic refraction. Finally, although clearly 

 a member of the chlorine group, fluorine in some of its 

 properties also presents some analogies to oxygen. The 

 whole of these observations appear to clearly establish 

 that fluorine would only with difficulty be reduced to a 

 liquid, and it has already been shown by one of us that 

 at - 95', under ordinary pressure, it does not change 

 its state. 



In the new experiments that we now publish, the 

 fluorine was prepared by the electrolysis of potassium 

 fluoride in solution in anhydrous hydrofluoric acid. The 

 fluorine gas was freed from the vapours of hydrofluoric 

 acid by passing it through a small platinum spiral 

 cooled by a mixture of solid carbon dioxide and alcohol. 

 Two platinum tubes filled with well-dried sodium fluoride 

 completed this purification. The liquefaction apparatus 

 consisted of a small cylinder of thin glass, to the upper 

 part of which was joined a platinum tube. The latter 

 contained another small tube of the same metal. The 

 gas to be liquefied arrived by the annular space, passed 

 into the glass bulb, and passed out again by the inside 

 tube. This apparatus was united to the tube which led 

 in the fluorine. 



In these experiments we have used liquid oxygen as 

 the refrigerating substance. This oxygen was prepared 

 by the methods described by one of us, and these re- 

 searches have necessitated the employment of several 

 litres of this liquid. The apparatus being cooled to the 

 temperature of quietly-boiling oxygen (-183'), the cur- 

 rent of fluorine gas passed into the glass bulb without 

 liquefying ; but at this low temperature the fluorine had 

 lost its chemical activity, and no longer attacked glass. 



If now the pressure on the boiling oxygen be reduced, 

 it is seen, as soon as rapid ebullition is produced, that a 

 liquid trickles down the walls of the glass bulb, whilst 

 no gas issues from the apparatus. At this moment, the 

 exit tube is closed with the finger to prevent the entrance 

 of any air. Before long the glass bulb becomes filled 

 with clear yellow liquid possessing great mobility. The 

 colour of this liquid recalls the tint of fluorine seen 

 through a layer a metre thick. According to this experi- 

 ment, fluorine becomes a liquid at about - 185°. As soon 

 as the little condensation apparatus is removed from the 

 liquid oxygen, the temperature rises and the yellow 

 liquid begins to boil, furnishing an abundant evolution of 

 a gas which presents all the energetic reactions of fluorine. 



We have taken advantage of these experiments to 

 study some of the reactions of fluorine upon bodies 

 maintained at very low temperatures. Silicon, boron, 

 carbon, sulphur, phosphorus, and reduced iron, cooled in 

 liquid oxygen, and then projected into an atmosphere 

 of fluorine, do not become incandescent. At this low 

 temperature, fluorine does not displace iodine from 

 iodides. Its chemical energy, however, is still sufficiently 

 great to decompose turpentine or benzene with produc- 

 tion of flame even at - 180". It would seem that the 

 powerful aflinity of the fluorine for hydrogen is the last 

 to disappear. 



F"inally, there is one other experiment that we ought 

 to mention. When a current of fluorine gas is passed 

 into liquid oxygen, there is rapidly produced a white 

 flocculent deposit, which soon settles at the bottom of 

 the vessel. If the mixture is shaken and poured on a 

 filter, this precipitate is separated. It possesses the 

 curious property of deflagrating violently as soon as the 

 temperature rises. We are pursuing the study of this 

 compound, as well as that of the liquefaction and 

 solidification of fluorine, in which further experiments 

 are required. 



NO. 1 44 I, VOL. 56] 



A NEW DETERMINATION OF THE GRAVI- 

 TATION CONSTANT AND THE MEAN 

 DENSITY OF THE EARTH. 



AN account of a new determination of these quantities, 

 carried out in a very careful manner by Dr. C. 

 Braun, S.J., at Mariaschein in Bohemia, has just been 

 published in the Memoirs of the Vienna Academy (Bd. 

 Ixiv., Math. Nat. Classe). 



Dr. Braun has been engaged on the work since 1887. 

 He used the torsion-rod method, and though his apparatus 

 was considerably larger than that of Prof. Boys, it was 

 still much smaller than the older apparatus of Cavendish, 

 Reich, or Baily. The rod was about 24 cm. long, and was 

 suspended from a tripod by a brass torsion wire, nearly 

 I metre long and 0055 mm. in diameter. The whole 

 torsion arrangement was under a glass receiver, about a 

 metre high and 30 cm. in diameter, resting on a flat 

 glass plate. The receiver could be exhausted, and in the 

 later experiments the pressure was about 4 mm. of mer- 

 cury, and the disturbances due to air currents were very 

 greatly reduced. The attracted masses at the end of the 

 rod were gilded brass spheres, each weighing about 54 

 grammes. Round the upper part of the receiver, and 

 outside it, was a graduated metal ring, which could be 

 revolved about the axis of the torsion wire, and from this 

 were suspended, about 42 cm. apart, the two attracting 

 masses. Two pairs were used : one a pair of brass 

 spheres about 5 kgms. each, the other, a pair of hollow 

 iron spheres, filled with mercury, and weighing about 

 9 kgms. each. 



To determine the position of the torsion-rod, a mirror 

 was fixed on the centre of the rod, and immediately in 

 front of it was a mirror at 45° to the horizontal, throwing 

 : the reflexion down through the base plate on to the 

 horizontal objective of the observing telescope ; another 

 mirror, immediately under the lens, was inclined at 45% 

 and sent the beam horizontally on to a graduated glass 

 scale in the focal plane of the eyepiece. The object of 

 which the image was viewed was an index mark on a 

 plate placed horizontally just below the scale, and the 

 light from it was made to traverse the axis of the tele- 

 scope outwards by reflexion at a parallel plate of glass at 

 45^ to the horizontal. As the index mark was nearly at 

 the same distance from the objective as the scale, the 

 rays fell nearly parallel on to the torsion-rod mirror, and 

 the angular value of the scale divisions was determined 

 from their length and the distance of the scale from the 

 objective. It was also determined by a theodolite, view- 

 ing the scale through the object glass, and found to be 

 about 3^ min. 



The instrument was fixed on a stone slab, in the corner 

 of a room with very solid walls, and protected from 

 temperature variations and electrical effects by a casing 

 of cloth and tinplate. 



As there was a continuous creep of the torsion-rod in 

 one direction, amounting in the course of years to several 

 lengths of the scale, it was necessary to have some 

 method of moving the torsion-head. This was effected 

 from outside the receiver in a very ingenious manner. 

 A plate was fixed on a part of the torsion-head which 

 did not revolve, and to this was attached a clock from 

 which the escapement was removed, and or^ the axis of 

 the escapement-wheel was fixed a small magnet. On 

 the axis, where the driving spring had been, a pinion was 

 fixed, gearing with a large wheel attached to the torsion- 

 head. The magnet could be turned round by moving a 

 magnet outside the receiver, and so the torsion-head could 

 be slowly revolved. The gearing-down was such that, if 

 the minute finger of the clock moved one minute, the image 

 of the index in the telescope moved one scale division. 



Vibrations of the torsion-rod were started by a light 

 magnetised fork, which could be made to softly touch 

 the rod on either side by the motion of a magnet outside 

 the receiver. 



