appears practically to eliminate molecular aggregation of the atoms as a factor in determining the crystalline structure; that is to say, the distance separating two neighbouring atom centres is the same whether those atoms belong to the same or to different molecules. Another interesting fact is that, whilst the elements and binary compounds for the most part crystallise in the cubic or hexagonal systems, substances of greater molecular complexity rarely crystallise in these highly symmetrical systems; thus, of a great number of organic compounds examined, 2.5 and 40 per cent only belong to the cubic and hexagonal crystalline systems respectively (Table I.). This observation is important as one of many indications that the cells into which the crystal structure of a complex compound are partitionable are not, in general, all of the same volume. Further investigation shows that the volumes of the polyhedral cells representing the atomic

domains of the several elements present in a complex crystalline compound are governed by the law of valency volumes to which reference has already been made. The correctness of this conclusion concerning the proportionality between the numbers expressing the fundamental valencies of the elements and the volumes of the corresponding spheres of atomic influence has been abundantly verified, not only by the laborious process of working out a large number of cases, but in several other ways which may be more rapidly indicated. The following are illustrations of the latter kind of verification.

Table III. states the composition and axial ratios, a: bc, of a series of four crystalline minerals which differ in composition by the increment, MgSiO4; the sums of the valencies of the atoms composing the different molecular aggregates are stated under the heading W. The increment, Mg2SiO4, also occurs as the crystalline

2.869 2.867

10802 1: 44033 Clinohumite.. 108031: 5.6588

Values for the increment, forsterite. Observed. 10757: I: 12601 2449: 2:277: Calculated 10823 I 12775 2429: 2.245:

..

mineral forsterite, of which the axial ratios have been determined. It is evident that the ratio, alb, has approximately the same value of 108 for all four members of the series, and that practically all differences in relative dimensions are expressed by the ratio, c/b. On dividing the valency volume, W, by the corresponding value for cb in each case, the quotients 117, 121, 123, 124, and 12.7 are obtained respectively for the substances prolectite, chondrodite, humite, clinohumite, and forsterite. The relative dimension, c/b, is thus roughly proportional to the sum of the valencies in this set of minerals. The comparison may, however, be made more accurately by including the changes in both relative dimensions, a/b and c/b, in the calculation in the following manner. The " equivalence parameters" are the rectangular dimensions, x, y, and z, of a rectangular block having the volume W, and are in the ratio of the axial ratios, ab: c. The parameters x and y preserve almost constant values throughout the series, and addition of the increment, Mg2SiO4, leads to a practically constant increase of about 2.86 in the dimension z, on passing from one mineral to the next in the series. The mineral forsterite also gives nearly the same x and y values as before, and its z value, 2.87, is equal to the differences between consecutive pairs of z values in the main series; these differences vary between 2.85 and 2.88. The axial ratios and equivalence parameters of forsterite can indeed be calculated with considerable accuracy from the data available for the series of four minerals.

The relations here displayed may be rendered more obvious by a series of models (Fig. 5). Rectangular blocks having as the horizontal dimensions the x and y values, and as vertical dimension the z value, for forsterite, when superposed upon a similar set of blocks having the corresponding dimensions for prolectite, form a stack exhibiting the equivalence parameters of chondrodite; superposing on this a second set of forsterite blocks leads to a stack showing the equvalence parameters of humite, and on again repeating the operation, a stack with the dimensions of clinohumite results. From the numerical data and the models exhibited it must be regarded as definitely proved that, in this series, the volumes appropriated by the constituent atoms are, in any one member, directly proportional to the valency numbers of the corresponding elements,

(Mean value of S=2'970).

Experimental determinations of the molecular volumes of a long series of normal paraffins, made on the liquid substances at temperatures at which the materials are in physically similar conditions, are stated in column 4. Since the valency of carbon is four times that of hydrogen it would be anticipated from the crystallographic conclusions previously drawn, that each carbon atom should appropriate four times as large a space for occupation as one hydrogen atom; the quotient of the molecular volume by the valency the same value, S, in the case of all the hydrocarbons. sum or valency volume, W, should consequently lead to

The mean value of S, namely, the atomic volume of hydrogen, is thus calculated as 2.970, and that it is constant within very narrow limits is seen on comparing volume 4 and 5, the latter of which states the product of the valency volume, W, by the value 2.970. The simple relation between the atomic volumes of carbon and hydrogen in the liquid normal paraffins indicated in the abundantly confirmed by numerous series of determinations above table was recently pointed out by Le Bas, and is that the law of valency volumes, first enunciated on the in addition to that now quoted. It is thus definitely proved ground of the crystallographic evidence, holds rigidly in the case of these liquid substances.

Sufficient has been said to demonstrate that a method has now been devised by means of which the vast stores of accurate goniometric measurements collected by crystallographers during the past century can be interpreted and that the requisite interpretation has in many cases already been given. Prof. Liveing, in a discourse delivered in this room nineteen years ago, suggested that crystalline forms are the outcome of the accepted principles of mechanics; the aid of these, and of these alone, has been invoked to show that crystalline structures result from the equillibrium of the attractive and repulsive forces radiating from the atomic centres.

Royal Institution.-A General Monthly Meeting of Members of the Royal Institution was held on the 3rd inst., The Duke of Northumberland, K.G., President, in the Chair. Mr. J. Devonshire, Mr. G. H. Griffin, Mr. W. E. Lawson Johnston, Mrs. Guy Pym, and Mr. A. E. Reed were elected Members. The Chairman reported the decease of Professor J. H. van't Hoff, an Honorary Member of the Royal Institution, and a resolution of condolence with the family was passed.

TABLE II.-Gas from NH4Cl and Concentrated H2SO4 Rubber Grease on Taps.

(Weight of a normal litre no correction for adsorption.)

1.63958

1.63921

1.63927

1.63987

1.63986

1.63936

Mean.. 1.63952

Even in presence of a lubricant which is not chemically attacked there is still the possibility of an appreciable error resulting from solution. The tap grease dissolves the gas, and part of this is given off when the density bulb is pumped out. In practice this effect will tend to make the density determinations too high. During the filling and adjustment of the temperature and pressure the grease partially saturates itself and causes the full bulb to weigh more than it should. On exhaustion all or part of the gas in solution is given up and the subsequent weighing approximates closely to the initial one, so that the non-variation in weight of the empty bulb during a series of determinations is no proof of the absence of this error. That solution actually occurs is proved both by Scheuer's results (loc. cit., 1910, p. 601) and our own (loc. cit., p. 1643), but the amount of gas dissolved or given up varies so much with the experimental conditions that it is not possible to apply a constant correction.

(c) Adsorption.-The adsorption or condensation of compressible gases on glass surfaces has often been pointed out as a source of error of uncertain magnitude in exact determinatious. Most experimenters have sought to determine its influence by varying the capacity of the bulb in which the gas is weighed. This method has yielded no very definite results for gases beyond showing that the magnitude of the effect must be small (G. Baume, Journ. Chim. Phys., 1908, vi, 1. Guye and Davila, Mém. de la Soc. de Phys. et d'Hist. Nat. Genève, 1908, xxxv., 635). It will generally be admitted that when a given mass of gas is introduced into a glass vessel the pressure exerted by the gas on the walls will, on account of adsorption, be slightly diminished. The greater the ratio of surface to volume the greater will be the diminution of pressure, and conversely the pressure corresponding to the total number of molecules in the mass of gas will only be reached when this ratio is infinitely small. As McBain (Zeit. Phys. Chem., 1909, lxviii., 497) has shown in the case of hydrogen adsorbed by cocoa-nut charcoal, the total quantity of condensed gas exists in two states-(a) a surface film, (b) a solution in the material of the condensing surface. When equilibrium between the gas and its condensing surface has been obtained a constant fraction of the total mass of gas is present in these two states, but the magnitude of this fraction, surface remaining constant, must clearly depend on the concentration of the gas producing adsorption and also on the temperature. In other words, the actual mass adsorbed will vary with the pressure and temperature of the gas in the vessel.

In the case of hydrogen and charcoal, McBain has shown that equilibrium between the free gas and that adsorbed as a surface layer is quickly reached, whilst adjustment of equilibrium of the gas in solution proceeds slowly and is only complete after a considerable interval of time. Bearing in mind the possible existence of these two effects, we have investigated the adsorption of HCl on glass surfaces in the following way:

The method, of which only a brief account has appeared before, was originated by a suggestion of Prof. W. Ostwald, and is based on the assumption that when gas is displaced a constant pressure from a glass vessel by means of mercury the adsorbed gas remains behind between the mercury and the glass surface, and can afterwards be

collected by lowering the mercury so as to produce a Torricellian vacuum. The adsorbed gas given up is then removed, and the process can be repeated until further evolution ceases. The apparatus consisted of two cylinders A and B, of about 300 sq. cm. internal surface, connected with each other through a two-way stopcock c. The cylinders could be filled with mercury from the reservoirs D and E, which were attached to a cord running over the pulleys G and H. This system of adjustment enabled one to transfer the gas from one cylinder to the other at constant pressure. The cylinder в was not used in the measurements, and merely served as a reservoir for the collection of the displaced gas.

The measuring cylinder A was surrounded with ice and water contained in a tin bath, and the capillary tube K, which was carefully calibrated, was used for the measurement of the volume of the adsorbed gas.

Before the hydrogen chloride entered the apparatus the greatest care was taken to ensure the complete absence of moisture, for both Scheuer and ourselves found that condensation is greatly increased if the vessels are not thoroughly dry. The gas was left in contact with the glass for twenty-four hours, and then displaced into the cylinder B at constant pressure. The tap c was then turned off, and a short length of the mercury thread was frozen in the calibrated capillary by means of solid carbon dioxide contained in the paper vessel L. This was necessary because the gas dissolved in the grease of the tap, and afterwards slowly evolved in a vacuum. mercury was next lowered so as to produce a vacuum, into which the adsorbed gas escaped, and on allowing mercury

The

CHEMICAL NEWS, April 7, 1911

to refill the cylinder this gas was collected and measured, and afterwards removed through the tap c. This series of operations was repeated a number of times, allowing definite intervals for the adsorbed gas to accumulate in the vacuum, untii the volume obtained became negligibly small. The increments of gas were summed, and in this way the total gas volume given off from the walls of the cylinder in a given time was obtained. Cylinder A was allowed to saturate itself with gas at four different initial pressures. The results are given in Table III.

163

appreciable fraction continues to be given off at a gradually decreasing rate, which does not reach zero even after five hours. Thus at a saturation pressure of 732 mm. 0·0086 cc. is given off after thirty-three minutes, and only 0.0018 cc. more is yielded after five hours. The quantity of gas condensed is very nearly directly proportional to the pressure, and lends no support to the view of Scheuer that the adsorbed layer is permanent, and does not vary with the degree of evacuation of the bulb. Very similar curves to ours have been obtained by McBain in the research already cited, so that it is legitimate to conclude that our results establish for HCl on glass surfaces the existence of the two kinds of adsorption. From these curves the total quantity of gas adsorbed per sq. cm. surface at a saturation pressure of 760 mm. after equilibrium has been obtained can be calculated if the adsorption be assumed to be directly proportional to the saturation pressure. This is for the glass of the cylinders o'036 cub. mm. at 0° and 760 mm., for the the glass of the moulded bulb o'146 cub. mm. at o° and 760 mm. The adsorption, even on the surface of the moulded bulb, is hence very small, and need only be taken into account when results of the highest degree of accuracy are desired. The actual adsorption on the glass density bulb itself was not measured, but experiments in a similar bulb showed that it approximated to the higher figure, and the correction applied in

The volumes of gas evolved were then plotted against the times of exposure to vacuum, and yielded a series of four curves, which are here reproduced (Fig. 2).

In addition, the total adsorption was measured on a different glass surface by replacing cylinder a by a thick walled bulb of moulded glass. The volume of gas condensed was in this case considerably larger, being o'047 c.c. at normal temperature pressure for a surface of 320 cm. for a saturation pressure of 782 mm. It might be supposed that part of the gas set free in this way had been mechanically imprisoned in cavities in the glass by the rising mercury. To test this point similar experiments were carried out on air and also on hydrogen. The volume of air adsorbed was very much smaller, but curves of exactly the same type resulted.

The adsorption with hydrogen was barely detectable. Had the gas been mechanically imprisoned the volume of air or hydrogen should have been of the same order of magnitude as that of the hydrogen chloride.

The form of the curves shows that the larger portion of the condensed gas is very quickly evolved in vacuo. An

our previous research was calculated from this. Objection to the correction might be taken on the ground that it is smaller than the probable experimental error of each individual determination. The probable accuracy of the mean of a series of values is, however, very much greater, and since an error of known magnitude is introduced by adsorption, the correction seemed to us legitimate. Our glass density bulb had an internal surface of 291 sq. cm. The correction per litre is hence o'00014 grm., which only changes the density by one part in 10,300. It is, however, to be noted that a very small trace of moisture on the glass surfaces of the bulb would increase greatly the magnitude of this correction.

With regard to the sign of the adsorption correction, we find ourselves totally at variance with Scheuer (loc. cit., p. 613). Our results prove conclusively the adsorbed layer of gas is not of constant mass, and varies with the pressure in the density bulb, temperature remaining constant. In consequence the apparent density, whether determined by the method of Regnault or by the charcoal adsorption method, will be, in all cases, slightly too large, and must be