37° 



NA TURE 



[August 17, 1882 



I am sure that it only needs a trial of the form indi- 

 cated to prove its superiority in every respect for astro- 

 nomical purposes. All objectors on the score of accuracy, 

 •&c, should refer to the Annals of the Harvard College 

 Observatory, vol. i., part ii., pp. xxxiv., where they will 

 find what seems a sufficient answer. 



Edward S. Holden 

 Washburn Observatory, University of Wisconsin, 

 Madison, June 30 



THE LIMIT OF THE LIQUID STATE OF 

 MATTER 



THE conditions under which an investigation is carried 

 out often predetermine the conclusions to be drawn 

 'from the observations made. That this has been the case 

 with the observations made upon the upper confines of 

 the liquid state, there is now ample evidence to show. 

 When Cagniard de Latour, on heating liquids in sealed 

 tube^, noticed the disappearance of the liquid surface, he 

 came to the conclusion that the liquid state had ceased to 

 be possible, and that the substance had passed into the 

 gaseous slate. But Latour had no means of varying the 

 volume of his liquid to observe whether or not increase 

 •of pressure might again induce liquefaction. This defect 

 was removed by Dr. Andrews, who constructed the well- 

 known apparatus for varying the volume by means of a 

 screw. And it is to the work performed with this appa- 

 ratus that the above remark is applied. By two modes of 

 observation Dr. Andrews arrived at the conclusion that 

 the liquid and gaseous states of matter were continuous. 

 The experiments being conducted in transparent glass 

 tubes, the appearance of the contained fluid constituted 

 ■one mode, and the registration of the pressure constituted 

 the other. Neither of these methods eon hi by the necessi- 

 ties of the case give any aid in determining the state of 

 matter. Dr. Andrews's method of demonstrating the 

 continuity, by passing from a lower to a higher tempera- 

 ture under a pressure which prevented the formation of 

 •vapour, ensured the homogeneity of the fluid under exa- 

 mination, and precluded the existence of a visible liquid 

 surface ; and as liquid and gas are equally transparent, 

 no tidings of the state of the fluid under examination 

 ■could come to him by observations of its appearance. 

 jHow did Dr. Andrews tell when his tubes contained 

 liquid? By lowering the pressure till a meniscus was 

 seen. Then the formation of a meniscus is the only test 

 •of the liquid slate. Dr. Andrews then obliterated the 

 ■only ocular test of the fluid's condition by increasing the 

 pressure, and raised the temperature till on again reducing 

 the pressure no meniscus was formed, showing the fluid to 

 be gaseous, and he then declared that no sudden change 

 of state had o-curred — that is to say, that it was impos- 

 sible to say that the fluid was either liquid or gaseous, 

 but that it had probably passed through an intermediate 

 state. Of course a change of state had taken place, and 

 if we only reflect that the change from cohesion to re- 

 pulsion is caused by the thermal velocity of the molecules, 

 and not by the number of them in a space, the change 

 should depend upon temperature and not upon pressure. 



The characteristic property of the liquid state is then 

 the possession of cohesion sufficient to form a surface, or 

 simply surface tension ; and could this property be re- 

 tained in a visible form at all pressures, the existence of 

 the continuity enunciated by Andrews could be put to a 

 crucial test. By compressing hydrogen over various 

 liquids in which it is insoluble, I was enabled to carry the 

 above proposition into effect, and after several hundreds 

 of experiments, detailed in a paper read before the Royal 

 Society, the conclusion was arrived at that the two states 

 are not more continuous than are the solid and liquid 

 states, but are separated by an isothermal passing through 

 the critical point. In fact by Latour's or Andrews's 

 method, where the liquid was in contact with its own 



vapour, the critical point is the only place where the 

 direct passage from liquid to gas is visible, but the em- 

 ployment of hydrogen for retaining a free surface enables 

 us to observe the passage at any pressure, and it takes 

 place as suddenly at 200 atmospheres pressure as at the 

 critical pressure. Thus the critical point is the termina- 

 tion of an isothermal line, which is the limit of the liquid 

 state. 



As to the other mode employed by Andrews — namely, 

 pressure — continuity of pressure does not prove continuity 

 of state. If it did the continuity of the solid and liquid 

 states could easily be proven. In fact, the irregularities 

 observed by Andrews in the vicinity of the critical point 

 rather lend support to the views that a change of state 

 takes place there. 



We may state the change thus : — The cohesion of the 

 liquid state is weakened as the thermal motion increases, 

 till the repulsion is in excess of the attraction, and the 

 gaseous state ensues. The evidence I have collected 

 from capillary phenomenon in the paper above referred 

 to proves this to be the case, and shows that pressure has 

 no effect in altering the occurrence of the phenomenon. 

 Thus we are led to the conclusion, that so far from the 

 liquid and gaseous states of matter being continuous and 

 indistinguishable, the liquid limit or " absolute boiling 

 point" is the only fixed point among the properties of 

 matter. The freezing point can be altered by pressure, 

 and besides, many bodies like ethyl alcohol may have no 

 freezing point, probably becoming more and more viscous 

 till absolute zero is reached. But all substances may be 

 made to pass into the gaseous state, and even delicate 

 compounds may be rendered gaseous without decomposi- 

 tion when under sufficient pressure. We see then that 

 this important change of state, for which I propose the 

 name Cohesion Limit, and which till lately was supposed 

 to have no existence, is in reality the only fixed point in 

 the relations of the states of matter, being determined by 

 temperature alone. J. B. Hannay 



INTERNATIONAL METEOROLOGY 



THE second meeting of the International Meteoro" 

 logical Committee took place at Copenhagen, August 

 1-5 inclusive. All the Members were present, except Prof. 

 Cantoni, who had resigned his seat on the Committee on 

 account of health. Prof. Tacchini was unanimously 

 elected in his place. The following brief account of the 

 more important of their proceedings is in the numerical 

 order in which the respective subjects were discussed . — 

 It was resolved — 



(a) To organise an exhibition, in connection with the 

 International Fisheries Exhibition, London, of the me- 

 thods and apparatus used in different countries for giving 

 weather intelligence and storm warnings to the coasts, 

 and of the instruments, &c, used in the study of ocean 

 meteorology. 



(b) To issue a circular to all existing organisations, 

 requesting them to supply data as to their condition and 

 operations up to the end of the current year. 



(c) To request the several institutions to be more pre- 

 cise in the information published by them as to the hour 

 of occurrence of rain and other phenomena. 



(d) To request all institutions to append to their Daily 

 Bulletins, Monthly Sheets giving the mean results for the 

 month, in the same way as the London Office has done 

 since 1880. 



(e) To request all institutions to furnish particulars of 

 any stations which may exist in distant localities, espe- 

 cially in the Torrid Zone, South America, and the Islands 

 of the Pacific, at least during the period of the Interna- 

 tional Polar Observations, and to publish the names of 

 such stations in the Polar Bulletin issued by Prof. Wild. 



(/) To express approval of the plan proposed by Capt. 

 Hoffmeyer and Dr. Neumayer to publish daily synoptic 



