Relations of Liquids and their Saturated Vapours. 89 



tubes throughout was satisfactory. The series of vapour-den- 

 sities derived from them led to the discovery of the law; the 

 points are laid down on the chart in the Archives of the 

 Royal Society for 1852. The line produced downwards will 

 be found thereon to cut the axis of temperature at —79°, being 

 exactly the same as the line of water-vapour in Plate II. It is 

 marked off ou Plate III. ; and a cross mark at the upper end 

 denotes the point of transition found afterwards by another set 

 of observations in short tubes. 



§ 24. Another specimen of dilute alcohol (62 per cent, water 

 and boiling-point 86°*5) was tried in four tubes up to 190°. 

 The points will be found in the same chart, and to range fairly 

 in a straight line that, when produced, cuts the axis at —73°. 



§ 25. A specimen of sulphide of carbon that boiled at 49° was 

 tried up to 160°; the points and line drawn through them are 

 also to be found on the chart. It is curious that the line pro- 

 duced downwards cuts the zero ordinate at 0*715, almost the 

 same as the line drawn through the points of Regnault's obser- 

 vations below the boiling-point. Both in its node and its upper 

 limit this liquid is exceptional. 



§ 26. This is no proof of my temperatures being at all to be 

 compared in point of standard accuracy with those of Regnault, 

 which indeed it would be absurd to imagine, as the thermome- 

 ters employed were common German chemical thermometers 

 with glass scales, and the correction to reduce to the air-thermo- 

 meter was taken from a Table afterwards published in the Phil. 

 Mag. vol. xxi. (June 1861). Coincidence of node may accom- 

 pany considerable divergence of temperature if that divergence 

 is gradual. The following is an example. 



§ 27. M. Avogadro's observations of the tension of the vapour 

 of mercury under the boiling-point * are to be found projected 

 on the original chart of 1852. I have again projected the points 

 on chart R, Plate III. (which has the same scales as Plate I.), 

 with original temperatures by thermometer of mercury, and the 

 same corrected to air-thermometer. A line was drawn through 

 the latter as shown; and this line was afterwards transferred to 

 the small-scale general chart, Plate III. The exact direction of 

 this line to the ether-node is curious, as the boiling-point is 

 nearly 20° below that of Regnault's mercury projected along- 

 side. The temperatures are by air-thermometer. 



§ 28. A few other liquids were tried, each in two tubes. One 

 of these, the chloride of sulphur (a small specimen of which w T as 

 prepared for me by a professional chemist), shows a very exact 

 conformity with the law from 160° to 210°; and a curious ano- 

 maly presents itself when this line is produced dowmwards. It 

 * Ann. de Chim. et de Phys. vol. xlix. p. 369. 



