304 eepoet — 1879. 



5. A Historical Sketch of the various Vapour Density Methods. 

 By James T. Brown, F.G.S. 



Although Southern in 1803 made some very careful experiments to determine 

 how much water was required to furnish one cubic foot of steam at various pres- 

 sures, still the foundation of vapour-density methods was laid by Gay-Lussac. 

 He, in 1811, started on the correct basis of accurate work when he heated a 

 weighed quantity of substance over mercury in a graduated vessel. In 1822, 

 Oagniard de la Tour determined the combined effects of heat and pressure on 

 certain volatile liquids, but as his results were on the question of maximum vapour- 

 densitv, they hardly enter the domain of the present sketch. In the same year, 

 Despretz, who gave no drawing, and only a very imperfect description of his appa- 

 ratus, published a method in which he used a 9-litre exhausted globe, and made 

 his determinations at atmospheric temperatures, employing only a small quantity of 

 substance. In 1826, Dumas, wishing to operate on substances which attack mer- 

 cury, worked out and published his well-known method, in which the volume is 

 definite, but the amount of substance required to fill that volume with vapour has 

 to be subsequently determined. 



In 1833, Mitscherlich proposed using tubes, sealed at one end, and drawn to 

 a neck at the other, instead of bulbs, and gave details and drawings of the appa- 

 ratus for heating them ; but Dumas, two years later, objected to the proposed 

 alteration in his method, for he wrote : — ' We must then leave to this operation 

 all its simplicity to make it essentially practical, and such, in fact, that with an 

 ordinary cast-iron pot, and some pieces of iron wire, we can perform it. This is 

 what I have done from the first, and what I persist in doing, my aim never having 

 been to make a piece of apparatus for the cupboard of the physicist, but to give 

 chemists a simple and eminently practical, and yet exact process. After all they 

 are the only ones to be considered.' Deville and Troost, however, in 1860, in 

 referring to that same apparatus, called it, 'La m^thode si elegante de M. 

 Mitscherlich.' 



Bineau, in 1838, published an elaborate paper, but, unfortunately, without any 

 drawings, for when we read the following paragraph — ' The bodies on which I 

 have worked have been volatilised, sometimes by the aid of heat, by following the 

 process of Dumas or that of Gay-Lussac, sometimes without elevation of tempera- 

 ture, by working in the barometric vacuum, or by allowing the vaporisation to 

 take place in dry air or hydrogen ' — we cannot but feel that an enormous amount 

 of valuable work has been lost for want of details. In 1844, we find Oahours, as 

 well as Bineau, at work at the same subject. In 1846, the latter repeated the 

 experiments of Despretz with slight modifications, but called attention to the fact 

 that the result was seriously affected by very small errors in reading off the mer- 

 curial column. 



In 1849, Regnault used an apparatus very similar to that of Mitscherlich, but 

 arranged the tube supports so that the two could be withdrawn simultaneously ; 

 he also dispensed with sealing the tube containing air, by providing it with a stop- 

 cock. Bineau, in 1859, in order to operate at high temperatures, coated the glass 

 tubes with clay, and heated them in a sand-bath. 



Regnault, in 1861, to obtain the same result, used iron tubes, and to ensure 

 uniformity of temperature, heated them in a cast-iron tube, which was made to 

 revolve over gas-burners. The tube which served as air-thermometer, was fur- 

 nished with a stop-cock, but that containing the substance only terminated in a 

 small aperture, and was not closed, as a sufficient quantity was introduced before 

 the heating to allow it to be taken for granted that during the experiment there 

 was no residual air. Another method of Regnault's was to have two iron bottles, of 

 as nearly as possible the same size, cast in one piece. In one of these the substance 

 was placed, and in the other a small quantity of mercury. The necks were then 

 partially closed by loose stoppers, and the system was heated in a muffle. After 

 heating, it was withdrawn, and allowed to cool, and the quantities remaining in 

 the bottles were determined by suitable means. 



Grabowski, in 1866, did much to shorten the Dumas calculation, while he 



