Oct. 9, 1879J 



NATURE 



567 



posed using plain ones in the following manner : — In the bottom 

 of the mercurial trough he placed a piece of sheet india-rubber 

 attached to an iron plate, and provided with a groove on its 

 upper surface ; the iron plate was furnished w ith a handle. 

 During the heating the inner tube stood over the groove 

 to allow of the escape of the mercury. When the level became 

 stationary communication with the mercury in the trough 

 was cut off by shifting the india-rubber disk until the inner tube 

 rested on the flat surface. The height of the column in the inner 

 tube was then noted by means of a cathetometer ; the outer tube 

 was then removed and a gummed label attached to the inner one 

 to indicate the mercury level. After cooling, the volume of the 

 vapour is determined from direct measurement. (3) In order to 

 avoid the cracking of the tubes in cases where liquids of high 

 boiling-point were used, he proposed connecting the lower end 

 of the outer tube with the inner one by a cork through which 

 two tubes leading to the flask or boiler passed. One of 

 them led below the liquid, while the other, which was pro- 

 vided with a stop-cock, reached only just below the cork. 

 If this stop-cock be closed while the liquid is being heated, a 

 portion of it is forced up the space between the two glass tubes, 

 and thus the mercurial column is heated more gradually. When 

 the liquid reaches the boiling-point the stop-cock is opened, and 

 the circulation of the steam proceeds as usual. The upper part 

 of the outer tube must be sufficiently elongated or provided with 

 a small tube leading to a condenser. 



Briihl ' proposed working the Ilofniann method at a very low 

 pressure by employing a tube I "5 metres long, with only a small 

 quantity of substance, and was therefore able to make determi- 

 nations at temperatures far below its boiling-point. He also 

 made the following suggestions : — 



1. In order to eliminate the troublesome element of the 

 tension of mercury vapour (without using two tubes as Grabowski 

 did), heat the column to the required temperature, note its 

 height, then allow it to cool, introduce the substance and heat 

 again to the same temperature till the height is constant. To 

 ensure uniformity of level in the bulb, keep it full to over- 

 flowing. 



2. Before the first reading of the mercurial colunm a small 

 piece of thin glass is passed up to liberate any air that may be 

 contained in the mercury. 



3. To make a mark on the tube a little above the vacuum 

 mercury level and then only to calibrate about 150 mm. down 

 from that point ; then, to find the total volume, add the variable 

 volume below the mark to the fixed volume above the mark. 



Muir and Suguira,' in 1877, finding that sometimes the weight 

 of the inner tube caused the groove in the india-rubber disk to 

 so far close as to prevent the escape of the mercury while heating 

 the substance, used a plain india-rubber disk which was fastened 

 to the bottom of the trough, a disk of cork intervening. Com- 

 munication between the mercury in the tube, and that in the 

 trough was maintained by means of a short piece of glass tubing 

 bent at right angles. A second tube long enough to stand 

 slightly above the level of the mercury in the trough served to 

 carry off, from the space between the two tubes, the condensed 

 liquid and excess of vapour. They adopted Hofmann's original 

 method of passing the steam in at the top of the outer tube, but 

 iLsed a small tube passing through a perforated cork in preference 

 to one fused to the end. 



Briihl ' has, this year, proved by most carefully conducted 

 experiments that the Ilofmann method cannot be used above 

 220° owing to the great and rapidly increasing vapour-tension of 

 mercury, but has omitted the grave objection to his own method. 

 Playfair and Wanklyn* called attention, in 1861, to the fact 

 that Bineau,' in 1846, pointed out that in vapour-density methods 

 at very reduced pressures slight errors in the readings of the 

 mercurial level introduce very serious errors into the result ; this 

 remark also applies to Croullebols. 



In the overflow methods, which are in reality modifications of 

 the Gay-I.ussac, seeing that they are performed with known 

 weights of substance, the first name is Ilofmann,' who, in i860, 

 gave a very meagre description of his apparatus, when he wrote 

 that he used a U-tnbe heated in a paraffin bath, and estimated 



' Dcul. Chtm. Get. Ber., 9, 1368 (1876) ; Jcum. Clum. Soc, 1E77, vol. i., 

 165 ; Moniteur tcicnti/.^ Janu-iry. 1S78, 14. 



' Jcurn. Chtm. S(K., 1877, vol. ii., 140. 



3 /)<-«/. Citin. Get. Ber., n, 197 (1879) ; youm. Chtm. Sx., 1879, *t»t., 

 •499- 



« Tram. Roy. Sx. Edin., 72 [3], 441 (1661). 



5 Ann, Chtm. Phys. [3J, 18. 330. 



« rut. Tram., ijo, 4i4(i8So}; AnH. Chtm. Pharm., Supp., i, 10 (l>6i). 



the volume of the vapour by the mercury expelled. Wertheim,* 

 in 1862-64, '" li's papers on Coniin, gave very full details of his 

 method, in which he used two tubes suspended side by side in a 

 flask. 



Watts,- in 1867, employed a globe with a ground neck, into 

 which an outlet-tube reaching nearly to the bottom of the globe 

 was accurately fitted. The globe being filled with mercury and 

 the substance introduced, the quantity of mercury expelled on 

 heating served as a basis for calculating the volume occupied by 

 the vapour. Victor M.yer,^ in 1876, introduced t».o very im- 

 portant alterations, he avoided the vapour-tension of mercury by 

 using fusible metal and placed the outlet at the bottom of the 

 bulb. His experiments at that time were all made in the vapour 

 of boiling sulphur, but Graebe,'* last year, wishing to employ a 

 higher temperature, used phosphorus pentasulphide, which boils 

 at 530°. 



Frerichs,' in 1876, used mercury in an apparatus similar in 

 principle to that of Watts's, but employed an inverted flask, and 

 brought the exit-tube, which was furnished with an inverted 

 syphon, through a suitable outlet in the bottom of the bath. 



Goldschmiedt and Ciamician,' in 1877, used mercury with the 

 simpler bulb of Victor Meyer, but added a small side-tube to the 

 outlet, so that the mercury expelled could be weighed from time 

 to time during the heating. Victor Meyer,' in the same year, 

 modified the shape of the bulb, but heated it in a tube similar 

 to that employed by Greville Williams in Gay-Lussac's determina- 

 tions, but of sufficient length for the upper part of the tube to 

 serve as condenser. 



Pfaundler's method,^ of which a preliminary notice appeared 

 in 1870, but which was not brought prominently forward till 

 this year, is based on the increased tension of the air in an 

 elongated bulb produced by heating after the introduction of the 

 substance as compared with a similar determination on air in a 

 bulb of the same size. A very short description appeared in 

 1874 of a method devised by Dulong' which is based on the 

 same principle. 



Last year Hofmann '" proposed two methods ; in one of these 

 he heated the weighed substance over mercury in the closed limb 

 of a U-tube, and marked the level of the mercury in the open 

 limb by sliding a pointed tube through a loosely fitting perforated 

 cork until it touched the surface. When the apparatus was cool, 

 the volume of the vapour was calculated from the weight of 

 mercury required to restore the level to that same point. The 

 other consisted in introducing into a tube a small but weighed 

 quantity of substance, then exhausting it and sealing it, and 

 he.iting in a jacketed tube. At the required temperature the 

 point of the glass tube is opened to allow air to enter, and then 

 at once sealed again. After cooling, the point is opened under 

 mercury or water, and the volume occupied by the vapour is 

 measured. 



In Meyer's '^ method, which is so recent and well-known 

 as not to require any explanation, the principle is that of 

 Pfaundler's, but by having the neck elongated and the outlet as 

 a side-tube, the substance is introduced afler the bulb is heated 

 to the required temperature, and by allowing the air expelled by 

 the vapour free egress into a graduated tube, it can be measured 

 under atmospheric conditions. It is, therefore, so simple that 

 the operation only requires a very short time from first to last. 



Dewar and Scott '^ have lately determined the vapour-densities 

 of potassium and sodium in a modified form of Meyer's 

 apparatus. 



In this sketch I have purposely kept off the very enticing 

 ground of fonnuliT, as they of themselves open up so wide a 

 field that they could not be dove-tailed into the history of the 

 subjec', which from any point of view is interesting. 



* Ann. Chem. Pharm., 123, 173 (1862) ; 127, 81 (1863) : 130, 269 (1864). 

 ^ Laboratory, i. 225(1867)1: Jakresbericht, 1867, 31. 



3 Deut. C/tem. Ges. Ber., 9, 1216(1876) ; Monit. scienty., J sinuary, 1878,7. 



* Deut. Chem. Ges. Ber., ii, 1646 (1878): Jo»rn. Chem. Soc., March, 

 1879, 260. 



5 Ann. Chem. Pharm., 185, 199(1877). 



<> Deut. Chem. Ges. Ber., 10, 641 (1877) ; Monit. scienti/., January, 

 1878, 13. 

 ' JJeut. Chem. Ges. Ber., 10, «o68 (1877). 

 » IbiJ., 3, 825 (1870); 12, 165 (1879): youm. Cliem. Soc., 1879, »•»••• 



499- 



N.S. 650 



' Compt. Rend., 78, 536 1874)): Journ. Chem. Soc., 

 (•374)- 



'" vcut. Chem. Ges. Ber., 11, 1684 (1878); Joum. Chem. Soc., Marcb, 

 1879, 196. 



" Deut. Chem. Ges. Ber., ji, 1867 (1878); 11, 2253 (1878); Chtmical 

 News, February 14, 1879 ; Deut, Chem. Ges. Ber., 12, 609 (1879) ; is, ma 

 (1879). 



" Proc Roy. Soc., 29, 2c6(iS79X 



