REVIEWS. The object of this dcpartmeizt of the Journal is to issue as pro'ofn$f~as possible crifical digests of all J o w z a l articles which bear zipoa a?y'phase of Physical Chenzisf?y. ~-
~
Qzmlifative EqziiZibrizim.
1,H. Poyztiizg.
Phil. Mag. [5],42, 289 ( ( an extra pressure produced by the motion of the dissolved molecules ) ) the author finds himself ( ( surrounded by the difficulties of the dissociation hypothesis ) ) . He therefore tries to account for osmotic pressure in another way. By imagining a liquid to be really a solid which i s continually breaking down and changing its internal structure the author. reaches the conclusion that osmotic pressure, lowering of the freezing point, etc., may be explained by an association of solvent and solute. His reasoning is aided by several intermediate hypotheses which arc! so chosen as to make the result agree with known facts. T h e difficulties of the ( ( dissociation hypothesis ) 1 are most apparent to those who are most firmly attached to some other hypothesis. As a matter of fact the idea of electrolytic dissociation rests on a basis exactly like that of gaseous dissociation. I n the latter case we know that under some conditions a gas undergoes a change which increases its pressure and alters some of its physical properties, such as color. I n the former we know that a dissolved electrolyte undergoes a process which increases its osmotic pressure and changes its color, conductivity, etc. If the statement of these experimental results brings difficulties to the pseudoscientists who try to explain everything by means of molecules so much the worse for the pseudoscientists. To any one who regards,facts as of the Osmotic Pressure.
(r896). Starting with the idea that osmotic pressure is
.
first importance to science this article of Professor Poynting’s is pure rubbish. E. ~!?ZLCkillgh~llZ.
An Alteration in the Shape of Reasuring Flasks. H. Bilti.. Ber. c I Z E ~ . Ges. Gerliu. 29, 2082 ( r 8 9 6 ) . A bulb is blown in the neck of the flask, above the mark ; this increases the volume of air in the flask and makes it easier to thoroughly mix the contents. A funnel must be used in filling. Wgrnibrunn and Quilitz in Berlin manufacture flasks of this model to measure one litre, 500, zoo and 100 cc. LV. L . M. On Sublimation in the Vacuum of a Crooke’s Tube. F. K y a f f Ber. chenz. Ges. &rlz’z. 29, 2280 ( r 8 9 6 ) . T h e experiments were carried out as in the case of the boiling poiiit determinations [Ber. 29, 1316 (18g6)]. Nineteen new boiling and sublimation temperatures are given, a large proportion of the substances experimented on subliming below their melting points. T h e paper contains an illustrated description of a modified form of v. Babo’s air pump, which the authors have found serviceable in their work. L. M.
m d H. l17tilandt.
w.
On Adsorption of Vapors.
LV. Miiller-E?*zbach. Wied. Anit.
58, 736 ( r 8 9 6 ) a i d 28,684 (1896) : aZso Wieiz. AKad. Sifzungsber. 98, ( r r A )327 (1889). Beginning in 1885 Muller-Erzbach has pub-
lished a series of papers on the adsorption of the vapors of water, carbon disulphide, benzene, tetrachlormethane, etc., by alumina, ferric oxide, cobaltic oxide and charcoal : as this work is not referred to in the chapter on adsorption in Ostwald’s Lehrbuch, a fuller account than otherwise necessary will be given in this place. The adsorptive power of several of the substances experimented with is very considerable, ferric oxide suitably prepared can take up 40 per cent of its weight of water at 14OC,or 35 per cent of carbon disulphide at 8°C ; the author has determined the tensions of the vapors given off by mixtures of known composition, calculating them in the majority of cases from measurements of the rate of evaporation of the adsorbed substance [details and control experiments, Sifmngsber. 3281. A static method is described in Wied.Ann. 58, 7 4 9 (1896 1
I 86
Reviews
T h e following (interpretation ) of the phenomena is put forward in the papers under review : I. T h e adsorbed vapor does not penetrate into the interior of the adsorbing molecule : becazise if ferric oxide saturated with the vapor of carbon disulphide be mixed with water, the carbon disulphide separates in the form of large drops ; 2. consequently it forms a layer round the molecule, attracted to the latter by the ( ( molecular forces ) ) ;
3. the thickness of the layer, other circnmstances equal, is proportional to the weight of vapor adsorbed ; 4. determination of the ( ( strength of the adsorptive force ) ) must therefore afford a means of measuring the space through which the molecular attraction can be exerted ; and of determining the law according to which the force of attraction depends on the distance from the attracting centre.
After trying and rejecting several other plans, the author has succeeded in inventing the following ( ( completely new scale for measuring the force of adsorption ) ) . ( ( If one imagine a unit weight of uncombined water to be cooled from a certain initial temperature, until by the loss of heat its vapor tension has become equal to that of the adsorbed water at the initial temperature, then from the difference in temperature may be calculated the number of units of heat which exert the same influence on the water as the adsorption does, and which consequently may be considered equivalent to the latter . . , as the adsorbed water must be considered to be solid, its specific heat must be set equal to that of ice, viz. 0.502 ) ) . The application of all this may perhaps best be gathered from a n example. T h e vapor tension of a mixture of alumina with 16 per cent of its weight of water, was 8.19 mm at 15.g'C. The temperature at .which pure water has a vapor tension of 8.19 mm is 8.4OC. Hence the ((value of the force of adsorption in calories)) is (15.9-8.4)~0.502=3.8 cal. T h e thickness e, of the film of adsorbed water may be set equal to 16 arbitrary units. I n a second ,experiment the force was found to be 34.2 calories for a thickness e2
Reviews
187
of 5.4 units (i. e., 5.4 94 of water was adsorbed). If the force of .attraction fall off with the x-1IL power of the distance, then
whence x = ~ . o . I n all of the author’s numerous experiments the value of x determined as above comes fairly close to 2 , consequently ( ( the strength of the molecular force varies inversely as the square of the distance . . . and we can trace the energy of an individual molecule as far, in proportion to its size, as the gravitation energy of the sun has been observed to extend (in attracting the planet Neptune) ) ) . Readers not infatuated with molecular-theoretical speculations of this kind, may well wonder how calculations based on such arbitrary assumptions can lead to the discovery of any regularity whatever. T h e explanation lies in a well known property of the vapor tension curves for pure liquids, z i z . : that the logarithms of small differences in the tension are proportional to the corresponding differences in teniperature. T h u s the quantities cal, and ca12 in the equation above may by replaced by log r I and log z2 (where i: is the tension of the adsorbed water in terms of that of pure water at the same temperature) and the relation discovered by Muller-Erzbach may be reduced to the form
cZ=Klog
T;
or, for constant temperature, c 2= A’ log p
+const.
{where c is the percentage of vapor adsorbed, p its vapor tension, and K a constant). A recalculation of a few of the author’s tables shows that his results may be expressed by this latter formula at least as well as by the use of the ( ( calorific values 1 ) . L . M.
w.
On the Genesis of Dalton’s Atomic Theory. H. Debzls. Phil. Mag. [ 5 ] 42, 350 ( 1 8 9 6 ) ; also Zeit #/p. Chent. 20, 259 (1896). With the aid of liberal quotations from the writings of Dalton, Thomson, Avogadro and others, the author seeks to support the following version of the origin of Dalton’s atomic theory.
‘
Dalton had long been accustonied to explain the expansion, diffusion, solution, etc., of gases in a mechanical way, by nieans of the old atomic theory, just as Boyle did a hundred and twenty years before him ; in 1801 in order to explain the existence of equilibrium in a mixture of gases, he adopted the hypothesis (now known as Avogadro’s hjvpothesis) .
moleczdai, zL~eight.-co7zsin~~t specific g m si
