834 REPORT— 1898. 



law of Cailletet and Matbias holds good with great accuracy right up to the 

 critical point (with normal pentfine observations were taken to within 0°'05 of 

 the critical teai])erature) i'or all the compounds examined except the alcohols. It 

 has, in view of these results, been suggested, I think justly, by M. Guye, that a 

 serious deviation from the law may be taken as a proof of the existence of 

 molecular dissociation or polymerisation. 



It is true that a few years ago I was under the impression that a direct 

 determination of the critical volume was possible, and the values obtained do, 

 indeed, bear as nearly constant ratio to the true critical volume, but they are 

 about 14 per cent, too low. The probable explanation of the error has been given 

 by M. Gouy,who pointed out that, at the critical point, a substance is so extremely 

 compressible that in a long column the density increases very considerably from 

 top to bottom, owing to the pressure exerted by the substance itself. 



In the course of these researches ample proof has been obtained that the 

 views of Andrews regarding the behaviour of a substance in the neighbourhood 

 of the critical point are correct, and also that the vapour pressure of a pure sub- 

 stance is quite independent of the relative volumes of liquid and vapour. These 

 points are referred to because they have been called in question by several 

 observers during the last few years. 



Two of the .substances examined attack mercury at high temperatures, and it 

 was therefore impossible to determine eitlier their vapour pressures or specific 

 volumes by the methods employed for the other liquids. The difficulty, as 

 regards vapour pressure, was overcome by sealing a wider tube to the lower end 

 of the volume tube, and using such a quantity of liquid that during the observa- 

 tions the lower end of the column was always in the wider and cold part of the 

 tube. The height of the column of mercury in the tube must, under these condi- 

 tions, be calculated.^ 



A method of determining the specific volumes of both liquid and saturated 

 vapour in a sealed tube was also devised.- When the specific volumes of the 

 liquid are already known, this method, in a simplified form, is very convenient for 

 determining the specific volumes of saturated vapour.'^ 



It is obviously necessary that, in order to obtain trustworthy results, pure 

 substances must be employed and, indeed, more time has been spent in the prepara- 

 tion of the pure substances than in the determination of their physical constants. 

 The difficulties met with in the fractional distillation of liquids, more especially 

 in the separation of pure hydrocarbons from petroleum, have led to an extended 

 study of this subject, and both new apparatus ■* and new methods of procedure ' 

 have been devised ; it has thus been possible to separate perfectly pure normal 

 and iso-pentane (B. P. 27°-95 and 36°-3) from the complex mixture of hydro- 

 carbons in American petroleum. 



2. On the Action of certain Metals and Organic Bodies on a PhotograjoMc 

 Plate. By W. J. Russell, Ph.D., F.R.S. 



The author demonstrated that printers' ink is capable of acting, in the dark, on 

 a photographic plate ; also that wood, dry copal varnish, &c., can act in the same 

 way ; that among liquids turpentine, drying oil, the essential oils, &c., act in like 

 manner. In addition to these and many other organic bodies, certain metals have 

 the same property, and either in contact, or at a distance from the photographic 

 plate, can act upon it. In this way pictures of thin surface or pictures of opaque 

 bodies, such as skeleton leaves, lace, or paper, can be obtained. It was also shown 

 that ordinary writing ink was perfectly opaque to this kind of action, and that 

 even old and much faded ink was capable of stopping the action and producing a 



' Tran.i. Clicm. Soc, vol. lix. p. !)17. 



^ Ihid., vol lix. p. .37. 



•■• Ihid., vol. lix. p. 12.5 ; Proc. Plnj.-i. Soc, vol. xiii. p. 617. 



* Chem. Ne?f;<, vol. Ixxi. p. 177 ; Trans. Chem. Soc, vol. Ixxi. p. 410. 



• Phil. Mag., 1894 p. 8. 



