VAPOUR PRESSURES OF TWO MISCIBLE SOLIDS. 429XLVIL-The Vapour Presswes o f Two PerfectlyMiscible Solids and their Solid Solutions.By ERNEST VANSTONE, B.Sc. (Wales) (1851 Exhibition ResearchScholar University College, Cardiff , formerly '' Isaac Roberts "Research Scholar).IN a former paper (Trans., 1909, 95, 590) it was shown thatcamphor and hydroxycamphor or borneol form a continuous seriesof solid solutions. The vapour pressures of these substances andof their solid solutions have since been measured; the methodsemployed and results obtained form the subject of the presentpaper.The Vapour Pressure of Camphor.Measurements have previously been made by Ramsay and Young(PI&?. Trans., 1884, Part I, 34) and by Allen (Trans., 1900, 77,413). Ramsay and Young determined the vapour pressures fortemperatures from Oo to 180O. The ordinary barometric metho430 VANSTONE: THE VAPOUR PRESSURES OF TWO PERFECTLYwas employed, and also a second method, in which the temperatureaof volatilisation corresponding with different pressures were readon a thermometer the bulb of which was coated with a layer ofcamphor.This method gave good results for liquids, but does notseem satisfactory when used for solids. Concerning the barometricmethod, the authors state: “We think it right t o give details asto the method of operation, as we found it a matter of extremedifficulty to expel all moisture and air.” In spite of the precautionstaken, I think it very probable that the results obtained werevitiated by the presence of a trace of air.The results obtainedby the two methods are not very concordant, thus a t 64Othe barometric method gave 6.4 mm., whereas the second methodgave 7.2 mm. a t 48.9O.H e tookthe precaution to boil the mercury, but passed the camphor up ina small tube, and applied a correction for the air admitted.The air-current method was also employed, the principle of whichis to find the weight of camphor required to saturate a known volumeof air. The saturation limit was, however, obtained by a methodof extrapolation. Allen’s work extended over temperatures fromOo to 80°. The vapour pressures measured were very, small, thegreatest being 9 mm. at 80°. There is considerable discrepancybetween Allen’s results and those of Ramsay and Young, thus at48.9O the latter obtained 7.2 mm., whereas Allen at 50° obtained1.3 mm.I therefore decided to make determinations by both thebarometric and air-current methods for temperatures from 7 8 O to1 60°.The Barometric Method.Allen also used two methods, one being the barometric.The apparatus (Fig. 1) consisted of two tubes about 80 cm. longand 12 mm. internal diameter. One of these tubes was providedwith it trap 20 cm. from the closed end. This tube served as astandard barometer, which was filled as follows :Mercury was poured in until it extended a few cm. past thetrap. The tube was then connected to the water pump, and themercury boiled. After cooling, the tube was nearly filled withmercury, and heated to the boiling point of aniline in the vapour-jacket.By connecting to the pump and repeatedly tapping thetube, most of the air bubbles were removed. After cooling, thetube was completely filled and inverted in the trough. The secondtube served as the experimental tube. A thick-walled capillarytube was sealed on one end, a piece of wider glass tubing a fewmm. diameter next, and then the stopcock.The lower end of the jacket wasclosed by a doubly-bored rubber cork covered with a layer ofBoth tubes were jacketedMISCIBLE SOLIDS AND THEIR SOLID SOLUTIONS. 431mercury; the upper end by an ordinary split cork. An experimentwas conducted as follows. The experimental tube was connectedto the pump by sealed glass joints, and the mercury pumped up.The tube was then heated to the boiling point of the liquid in thebulb, the pump kept at work, and the tube repmtedly tapped toremove bubbles from the side.It was then allowed to cool, andthe camphor introduced as a small pellet under the mercury in thetrough. - The tube was again heated, andthe pump worked. As the upper partof the tube became hot, the camphorvaporised, and some of it passed into thecapillary portion, where it condensed andclosed the tube.When the vapour was condensing wellup in the side-tube, t'he heights of themercury in the barometer and experi-mental tube were read by means of acathetometer, provided with a vernierwhich enabled readings to be made to0.01 mm. The telescope of the catheto-meter was brought into the horizontal byfocussing on the surfaces of the mercuryin the limbs of a wide U-tube clampednear the top of the apparatus. Readingswere taken until the pressure was con-stant.The apparatus was then allowedto cool, and readings again taken at roomtemperature. If the difference in levelwas now greater than 0.1 mm., thecapillary tube was gently heated; thecamphor which had condensed there waaby this means driven back into theexperimental tube or into the upperwider portion, and thus the passage tothe pump was again open. The aboveoperations were then repeated until thedesired result was obtained.This made i t certain that all the airFIG. 1.and moisture had beenremoved, the vapour pressure of camphor a t room temperaturebeing about 0.1 mm.It was found more convenient to makeobservations a t the highest temperature first, as on passing from alower t o a higher temperature air bubbles always appeared on thesides of the tube.In the early experiments a Topler pump was used, but later 432 VANSTONE: THE VAPOUR PRESSURES OF TWO PERFECTLYFleuss pump was placed a t my disposal, and the work became farless tedious. Accurate results at five temperatures were obtained,the temperatures being the boiling points of the following liquidsunder atmospheric pressure : ethyl alcohol, 78O ; propyl alcohol, 9 6 O ;toluene, 1 loo ; chlorobenzene, 130° ; and bromobenzene, 1 5 6 O .The temperatures were read on Anschutz normal thermometers(which had been previously standardised) placed inside the jacket.These could be read to 0.lo.I n some cases the temperatures wereobtained by reading the barometer, from Ramsay and Young'stable of vapour pressures.The following results were obtained :Vapour Pressure of Camphor.Temperature.78.0"96%110'9131 '1131.4152'0157.0Number of readings. Vapour pressure.30 6-40 mm.6 16'15 ,,20 33'00 ,,28 75.37 ,,16 76.00 ,,15 76.61 ,,14 181'50 ,,T h e Vapour Pressure of Borneol.It was determined in the same manner with the following results:The vapour pressure of borneol has not been previously measured.Temperature. Number of readings. Vapour pressure.77.9" 6 2'16 mm.96.8 11 6-65 ,,110'0 9 14.94 ,,131'0 16 40.92 ,,156.0 14 115.16 ,,The Air-Current Method.The apparatus was the same as that used by Perman and Daviesin finding the vapour pressures of naphthalene and dilute solidsolutions of naphthalene and &naphthol (Trans., 1907, 91, 1114).Details are given in that paper.A stopcock replaced the groundglass stopper, and a larger bulb was necessary for condensing thecamphor. The stopcock was sealed on after introducing thecamphor. A thermostat containing water and a toluene regulatorwere used for temperatures below looo; for higher temperaturesolive oil and a mercury regulator were employed. In order that thepressure of the air in the spiral should be the same before andafter the experiment, it was placed in the thermostat and a currentof air drawn through for some time; the stopcock was then closed,the spiral removed, the camphor washed out of the bulb witMlSCIBLE SOLIDS AND THEIR SOLID SOLUTIONS.433alcohol and ether, the spiral cleaned by immersing in lightpetroleum, dried, and weighed. It was again placed in thethermostat, and a known volume of air drawn over, then cleanedand weighed as before.The temperature of the aspirator and the height of the barometerwere observed a t the end of the experiment.Method of CaZcu2ation.--If w = weight in grams of camphorwithdrawn :P = pressure of atmosphere in mm.p = ,, air in aspirator in mm.Y'= absolute temperature of aspirator.P=volume of air aspirated, in litres.22.41M-u, = specific volume of camphor vapour = - ~M = molecular weight of camphor.then, assuming the truth of Dalton's Law of Partial Pressures, thefollowing relationship holds good :Pressure of camphor vapour - Volume of camphor vapourTotal pressure.Total volume. ' -orcorrecting volume V for temperature and pressure we get:wvcTP 760273pVt- 760 wvC2" pc =The following results were obtained :UI.0.41690.34390.25220.34550,33590-33550.25610'2555P (mm.).752-2z65.0r56.5763.6759-8764'9763.6759.9v.7.3055'9154.4395.9155-9155.9154.4394.439P.738.7752.6744'2751.6747.0752.2750-8746.3T. Time (mins.). to.288'9" 540 77'9287.5 370 78'0287'4 325 78.0287'1 480 78.0288.2 350 78-0287.9 330 77.9288'2 285 78.0289'0 555 78-0Pc (mm.1.6.8236.9046.7466-9236.8056.7506-8726.8000-39560.39940.39520-40160.39270.39600'40G50-39080.3959758.7 2-957z59-0 2.957161.2 2.957763'2 2'957765'6 2.957766-1 2.957764-1 2.959766.5 2.959765.4 2.959Mean vapour pressure at 78" ,..........744'9745'3747'3749'1750.2750'4748.4751.7750.8289'2289'1289.3289 -6291'0291'3292'3290-4'290.218018028025022024027025527095.295-195'195.295-195'195-195'195 '16.82815.82015.96115.81216-08315-83515 *98915.98115.71115.896Meaa vapour pressure a t 95" ............15-88434 VANSTONE: THE VAPOUR PRESSURES OF TWO PERFECTLYw0.38630-38700.38750.38751.35001 '33321-30441 -29401-01001.00641 *00900'68590.68920'69980'69281.03521.02481 -03041 *02041 '58301'58721'60142-04102-0154P (mm.).760.3764.3763.3762.2759.5762.0767.1751.6746.3744 '27485758-6759.1759 -5757 -7774'4773-9774'1774.2765.7764.0766 '4758.5764.0l? P .T. Time (mins.) to.1'4764 746.4 289.3 80 109.01'4764 750'8 288.8 80 109.01'4764 749'7 289.0 80 109-21'4764 748.4 289'2 80 109.0Mean vapour pressure a t 109" ............2.959 750-0 283'4 135 120.32.959 752.3 283-9 150 120.22.959 758.1 282.7 150 120'02.959 741.8 284.0 180 120.0Mean vapour pressure a t 120" .. . ... . . . . . .1.4764 736.1 284-6 115 130.41.4764 734'1 284'4 70 130'41'4764 738-7 283-9 70 130.4Mean vapour pressure a t 130 '4". . . . . . . . . . .1.000 744.6 2895 60 131'21.000 745-1 289.4 35 131.31.000 745.5 289'5 35 131.31.000 743'4 289.7 40 131.5Mean vapour pressure a t 131 -3".. . . . . .. . .. .1-000 765.7 282.2 55 139'91'000 764'8 282.9 60 139'91'000 764'8 283-3 50 140.11'000 765.6 282.0 50 139'6Mean vapour pressure a t 140" ... . .. .. ....1-000 756.4 282.9 90 150.01-000 754-6 283.3 60 150.01'000 757-7 282-2 45 150-1Mean vapour pressure at 150" ............1'000 744-3 298'8 60 156.01'000 750'6 288-8 60 156.0Mean vapour pressure at 166" ,...........pc (mm.).30'36630'34730.42030.43530.39050 -1 549'6748'4548-304 8 '3773-0472.7872.T272'8474.78375.09075-2737552875-17104-77104'12104'76103'40__.104.5149.83150.0315056150'13186'43184'34186.4It is seen from the above that for temperatures of 120° andupwards, considerable quantities of camphor were drawn off.Alarger spiral was used in these experiments. At 150° the camphorshowed signs of charring. The rate at which the air was drawnover was varied widely to ensure saturation.The results obtained by the two methods are compared in thefollowing table :Temperature.78.6"96%111'0131'0157.0Barometric (mm.). Temperature. Air ourrent (mm.).7 *09 78.1" 6-8316.15 95-1 15.8833-00 110.9 33.0075'00 131.1 75.20181.5 156.0 185-MISCIBLE SOLIDS AND THEIR SOLlD SOLUTIONS. 435The agreement is very close; the high values given by the air-current method at 150° and 156O are probably due to charring ofthe camphor. The vapour presures at the temperatures of Ramsayand Young's experiments and those of Allen have been obtainedby graphic interpolation.These are given in the table below:Vapour pressure Ramsay and Young AllenTemperature, (mm.).(mm.). (mm. 1.78.4" 6'8 9.5 * 7 '6280'0 7 *l - 9'1592 '4 13.1 15-4 * -100'0 19.5 22.6 -101.0 20.5 27 *2 -109'4 30.8 35 '0 -116.7 42-6 46-0 -127'4 65 -5 66.3 -132.0 76.7 78 '1 -134'2 84.2 88.6 -136.3 91'0 92 '8 -140'3 105.0 105.0 -141-7 110.0 109'4 -147'0 131.0 155'1 -154'3 165.8 197-6 -* These results were obtained by the barometric method.It is seen that the present results are a t nearly all temperaturesThe vapour pressures of borneol were also determined by the air-lower than those of the other investigators.current method, with the foIIowing results :V.p (mm.). 7'. Time(mins.). to. V.P. (mm.).5'021 742'8 285.7" 390 78.0 2.2135.021 745'8 284'5 390 78.0 2.8445'021 749.3 283.6 345 78.1 2'358,la P.0-095 753 -70.1085 755.90-1072 758.80.2912 761.60'2858 758.10'2884 752.30'2908 763.9Mean vapour pressure a t 78" ............ 2.3055.021 751.5 284'5 255 95.0 6.7085-021 747-7 284'9 270 95-1 6.5975'021 741'6 285'4 350 95-3 6'6715'021 753'2 285.4 300 95-3 6.7250.4105 749'70.4095 754'10.5918 751-10 3528 754'10.2039 755.50.2025 765.00'3344 763.30'3330 752'40'3286 764'4Mean vapour pressure a t 95.2" ... ........ , 6.6753'0024 736.8 285.7 180 1105 15.7652-9545 743'2 285.7 180 110.5 15-9204.9180 740.2 285.7 180 109.8 15-4502'6185 742.9 286'0 180 110'4 155081-4825 744.3 286'1 180 110.9 16.8291'4825 752.0 285'8 85 110.4 15.685Mean vapour pressure a t 11 0 -4".. . . . , . . . . . . 15.721.4825 752.1 286'0 60 120.0 25-6661.4825 741'6 285.5 60 120.0 2.5.4631.4825 754'1 284-8 60 120.0 25.045VOL. XCVII.Mean vapour presstire at 120" ......... .. 25.37 1G 436 VANSTONE: TRE VAPOUR PRESSURES OF TWO PERFECTLYW. P. 7. p (mm.). T. Time (mins.). to. V. P. (mm.).0.5470 765.9 1-4825 756-1 284.0 60 130.4 40.6620'5397 767.3 1'4825 757.5 284'0 60 130.0 40-148Mean vapour pressure at 130.2" ..... .... ... 40'400.6016 770.6 1-00 76@9 284'2 45 139.9 64-1810.5988 770.4 1.00 760.4 284.3 45 140'1 63.936Mean vapour pressure a t 140" ............ 64.060.4738 769.9 0*500 759.8 2845 45 150'2 96.5210'4748 769-6 0.600 759.3 284-8 45 150'2 96.814Mean vapour pressure a t 150*2"... ... .. . ... 96-660'6910 7684 0.500 T57.9 285.1 30 159.2 133-310.6560 767.7 0'500 457.2 285-1 30 158'4 127'20These results are compared below with those obtained by thebarometric method :Temperature. Air CUI rent (mm. ). Temperature. Barometric (mm. ).78.0" 3 -30 77'9" 2-1695'2 6.67 96 -8 6 5 5110.5 15-70 110.0 15.00130-2 40'4 131'0 40-92150.2 96'6 156.0 115.16158.4 127-2 - -As in the case of camphor the results agree closely,.Ramsay and Young have shown that the ratio of the boilingpoints expressed as absolute temperatures of closely related liquidsis constant. As the two solids investigated are also very closelyrelated, the ratios of the absolute temperatures corresponding withequal vapour pressures have been calculated :Pressure (nim. ).102030405060708090100110Tc.360'3373.5381-8388.3393 '3398.0401.3405-9409.041193415'3T B .376.0388.4396 4403-0407 *8411.6415-0418-0421 -2423-7426'3TBI T,.1.0431'0401.0381.0381,0571'0341 '0341.0301 *0301.0291 -026The constancy of the ratios is evident.The Vapour Pressures of Solid Solutions.One of the chief difficulties of previous investigators has beento obtain solid solutions having vapour pressures large enough tobe accurately measured. The only work of importance is that ofSperansEy ( Z ~ i t s c l i .phyysikal. Chem., 1903, 46, 70; 1905, 51, 45)MISCIBLE SOLIDS AND THEIR SOLID SOLUTIONS. 437who measured the vapour pressures of solid solutions of pdichloro-benzene and p-dibromobenzene, and p-chlorobromobenzene andp-dibromobenzene. He concluded that “the regular laws whichhold for liquid solutions also hold for solid solutions.”The Vupour Pressures of Solid Solutions b y the Air-CurrentMethod.The equation given on page 433 for calculating the vapourpressure of camphor, when applied to solid solutions, becomes :whereP8 = vapour pressure of solid solution.pc and p b =partial pressures of camphor and borneol.wc ,, wb=weights in grams of camphor and borneol in the totalweight (V) drawn off;alsowc+wb=w (2).Equations (1) and (2) contain three unknowns, namely, w,, wb,and P,, hence it seems impossible to determine the vapour pressureof a solid solution, in which both constituents have appreciablevapour pressures, by the air-current method alone.I f , however,P8 can be obtained by the barometric method, wc and Wb can becalculated, and pc and pb, the partial pressures of the constituentsobtained.Solving for wc in this way, we get:It is obvious that this involves the difference between the specificvolumes v, and v b , so that the method can only be applied to casesin which these differ fairly widely.For camphor and borneol vc = 0.1 473 and V b = 0.1 454, hence themethod is of no use in the present case; we may, however, writevc=vb in equation (l), which then becomes:WC = vps - v’b(P - Pa)/(v, - vb)(P - pg).wvc P P*=---- v+ WW,’thus the total vapour pressure of a solid solution can be obtainedapproximately by the air-current method.The error involved isabout 1-2 per cent.The difficultieswere now very much greater, as change in concentration had to beavoided as far as possible.The temperature of l l O o was chosen for two reasons: (1) Theweight of substance drawn off by aspirating a litre of air throughA series of experiments was carried out at l l O o .G G 4138 VANSTOKE: THE VAPOUR PRESSURES OF TWO PERFECTLYthe spiral was small, and hence the change in concentration of thesolution would be small; (2) the vapour pressure at that tem-perature was large enough to be accurately measured by thebarometric method, that for camphor being 33 mm.and for borneol15 mm.The solutions were made by fusing the accurately weighed quan-tities of camphor and borneol in sealed tubes. These were thenbroken, the mass removed, cut up in small pieces, dried oversulphuric acid, and introduced into the dry spiral. The spiral wasplaced in the thermostat, and a few C.C. of air drawn through; thestopcock near the condensing bulb was then turned off, the otherend of the spiral closed by means of a small india-rubber stopper,t.he spiral removed, cleaned, and weighed. It was then againplaced in the thermostat, and a known volume of air drawn through,again closed, cleaned, and weighed. It was necessary to clean outthe spiral after each experiment, and refill with a fresh quantityof solution.The following results were obtained :Mols.ofborneolPer100 Qfmixture. W.0.26320'26420'26450'24040.2406 20 {0.233230 { 0'23450'214440 { 0'21540.205450 { 0.20450.26420.2642 60 .(0.2662loo { 09682P.769.8750.6762'4771.1770.7754.2755.8747'1760.9753.7754-8763.5763.5764'3764.8765.3765.1764'3763.8758.7758.7P.765.5737.5749.0758.5757.7740.9742-4734'7747'2740'7740'0750.3750.0750.4750.7751'9750.5749.7749'1744'1744'1V.1'001 -001.001 -001 '00I -001 -001'001 moo1'001-001'48251 *4825iwm1'48251'48251'48251 '48251-48251 '00T.288.8"288'5288'9287.9288.3288.8288-9287'6289'2289'2289.3288'6289'0294.4289-7290.0290-2290'2290.3290'21-00 290'2Time(mins.).7575759075808090110110751001501001051001009010090100to.110.1"110.1110.1110.1110.1110.0110.0110'1110.1110'1110'0110.1110.1110.0110.0110.1110.1110.1110'1110.0110.0PI (mm.1305230-6230.1227'8127.9027-1027.2625'2124'8624-0023-9620.8720'8819 9319'9418 '2418-1717-9017.8115-0016-1MISCIBLE SOLIDS AND THEIR SOLID SOLUTIONS. 439Attempts were made to confirm these results by the barometricmethod. The difficulties were now even greater, and the resultscan only be regarded as approximate.To remove air and moisture, to prevent any change in con-centration by having a large quantity of substance present, andyet not so much as might obscure the mercury meniscus, was indeedextremely difficult.The experimental tube was now provided witha three-way tap in place of the ordinary stopcock previously used.The junction of the tube nearest the capillary wits well groundon the inside. This enabled the tube to be closed by a ground-glass stopper, sealed to a long glass rGd, which passed down thetube beneath the mercury in the trough, being bent at its lowerend so that it could be moved from the outside, One branch ofthe three-way tap communicated with the air pump, the other witha small reservoir of mercury.The plan was to cause the substance to sublime quite near thetop of the tube, keeping the pump at work, then to run in mercuryon the top of the grpund joint closed by the stopper, and soeffectively close the tube.Experiments with solutions containing 20, 40, 60, and 80 mole-cules of borneol per 100 molecules of mixture were made.For theexperiments with the 20 per cent. solution, a three-way tap and acapillary tube alone were used, mercury being run into the capillarytube. There wits some loss by sublimation into the portion outsidethe vapour jacket, whilst it is certain that air and moisture wereremoved.Experiments were made with the same solution at five tem-peratures, a.s in the case of the pure substances.The apparatus had to be taken down and the tube cleaned outbefore proceeding to make observations with a solution of differentconcentration, and often for the same solution as the quantity ofsubstance necessary a t the high temperatures obscured the mercurymeniscus a t the lower temperatures.Vapour Pressures of Solid Solutions of Camphor and Borneol.Barometric Method.Molecules of borneol per 100 of mixture=20.Temperature.78.6"97.097'4110.6131'6131'8156.2Number of readings.54615466Vapour pressure.6'10 mm.15-90 ,,16.04 ),28.13 ,)66'90 ,,67'50 ,,159.40 ,440 VANSTONE: THE VAPOUR PRESSURES OF TWO PERFECTLYMoEecular Concentration = 40 per cent.borneol.Temperature. Number of readings. Vapour pressure.78.4" 5 5-54 mm.97'2 5 13.27 ,,110'0 5 25-60 ),131 -0 5 63'70 ,)156.4 5 150.5 ,,Molecular C o ~ ~ c e ' l ~ t m t ' i w ~ = 60 per cent.Zlor.rLeo2.Temperature. Number of readings. Vapour pressure.78-50 7 4'83 mm.97.1 6 11.40 ) )11 0'2 5 23.05 ,)131.2 5 60-58 ,)156'0 7 140.00 J JMolecular Concentration = 80 per cent. borneol.Temperature.78 '6"96-897 '1110'6110.8131.8156.2Number of readings.64544410Vapour pressure.3'56 mm.8'80 ,,9-10 ),19-70 ) )20.00 ),56.40 ),130'20 ,,These results, as well as those for camphor and borneol, have beenplotted on a temperature-pressure diagram (Fig. 2); it is seen thatthe curves for the solutions lie between those for the pure sub-stances. The vapour pressures obtained by both methods arecompared in the following table :Barometric Air currentConcentration. Temperature.(mm. ). Temperature. (mm. ).20 mols. borneol ......... 110.6" 28-1 110'1" 27 -840 $ 2 ) ) ......... 110'0 25.6 110'1 25.060 ), ) ) ......... 110'2 23.0 110.8 20.880 ,, ,, ......... 110.6 19.7 110'1 18-2The deviations are in the direction expected, since the errors areentirely due to change in concentration. For those solutionsrelatively richer in camphor, the barometric method would give,owing to loss of the more volatile component, results which wouldbe too low, and conversely for solutions relatively richer in borneol.The agreement is as close as can be expected, considering theextreme difficulty of determining the vapour pressure of a solidsolution by the barometric method.Isothermals are shown in Fig. 3. The results given at llOo arethose obtained by the air-current method ; for other temperatures,the barometric results are given.It is seen that the isothermalMISCIBLE SOLIDS AND THEIR SOLlD SOLUTIONS. 441FIa. 2.I78" 98" 118" 138" 158"Temperature.FIG. 3.6000% bornaol 20 40 60 80 100% borneol100% camphor 0% camphorMols. of borneoZ per 100 of mixture442 VANBTONE: THE VAYOUR PRESSURES OF TWO PERFECTLYare straight lines. This leads to the important conclusion “ t h a tthe vapour pressure of a solid solution is a linear function of themolecular concentration, and can be calculated from the equation :p100- ’ 8 -where n=nurnber of mols. of borneol per 100 mols. of mixture.table :The calculated and observed results are given in the followingV a p o w Pressure of Solid Solutions at l l O o .Concentration.102030405060708090Cal c u I a t ed .30.3 nun.28.6 ,,26‘9 ,,25-2 ,,23.5 ),21-8 ,)20’1 ),16.7 ,,18-4 ,,Observed(air-current method).30.4 mm.57.8 ,I27’1 ,,25.0 ,,24’0 ,,20.9 ),19.9 ,,18.2 ,)17’9 ,,Speransky (ibid.), for solid solutions of p-chlorobromobenzene andy-dibromobenzene, obtained fairly good agreement between calcu-lated and observed vapour pressures.The pressures were measuredin mm. of paraffin oil in a differential tensimeter. Young (Trans.,1902, 81, 768) has shown that the equation given above holds formixtures of liquids which are chemically closely related, hence thepresent work is strong evidence in support of the van’t Hoff theoryof solid solutions, that they follow the same laws as liquid solutions.It is seen also from the curves in Pig.2 that the vapour pressuresof solid solutions of camphor and borneol are always greater thanthe vapour pressure of borneol. Precisely the same may be saidof the freezing points, hence, when the substance of lower freezingpoint and vapour pressure is considered, the addition of a substancewith which it forms solid solutions produces a change in theseproperties opposite to that expected, and directly contrary to thatwhich usually occurs with solutions which obey Raoult’s law. Itseems therefore futile to apply such laws to determine the molecularweight of solids.Summary and Conclusion.1. The vapour pressures of camphor and borneol have beendetermined for temperatures from 7 8 O to 156O.The resultsobtained for camphor are generally lower than those of formerinvestigators. The vapour pressure of borneol has not been pre-viously determined.2. The ratio of the absolute temperatures corresponding witMISCIBLE SOLIDS AND THEIR SOLID SOLUTIONS. 483equal vapour pressures is constant, thus Ramsay and Young’s rulefor closely related liquids also holds for closely related solids.3. The air-current method of determining vapour pressures hasbeen extensively used, and it has been shown that the resultsobtained agree closely with those obtained by the barometricmethod.4. The vapour pressures of a complete series of solid solutionshave been determined. It has been shown that the vapourpressures of solid solutions, like other physical properties, followthe ordinary mixture law :where %=number of mols. of R per 100 of mixture.5. Approximate results for solid solutions have been obtainedby the barometric method, more accurate results by the air-currentmethod.6. A method of determining the partial pressures of solutionsby combining the data obtained from barometric and air-currentmethods has been indicated.7, Since the vapour pressures of solid solutions of camphor andborneol follow the mixture law, it is highly probable that themolecular weights of the solid components are normal.8. The agreement between the results obtained by the twomethods leads t o the conclusion that the densities of the vapours ofcamphor and borneol at the temperatures employed are normal.I n conclusion, I wish to express my thanks to the Principal ofUniversity College, Cardiff, and the st& of the chemical depart-ment, for the interest taken in the work and the facilities affordedme. I am especially grateful to Dr. E. P. Perman for suggestingthe work, and for his advice and assistance in carrying it out.The expenses of the work have been defrayed by grants from theGlamorgan County Council and the College Council, to whom alsoI wish to express my thanks.UNIVERHITY COLLEGE,LONDON, W.