Page:Encyclopædia Britannica, Ninth Edition, v. 8.djvu/63

Rh RESISTANCE.] ELECTEICI T Y 53 continuously with the concentration, an approach to zero for infinitely weak solutions being indicated iu all cases. The chlorides may be divided into two classes. (1.) CaCl, and MgCL reach maximum conductivities 1968 x IQ- 8, 1310 x 10-*, at 18 C. for percentages 24 and 19 8 respectively, in each case short of saturation. LiOl pro bably does the same, and NaCl appears to reach a maximum between 23 &quot;9 p.c. and its saturation percentage 26 - 5. (2.) KU1, NH 4 C1, SrClj, and BaCl 2 increase in conducting power up to the point of saturation. Taking the best conducting solutions, the order of conductivities is NH 4 C1, KC1, NaCl, LiCl, CaCL,, SrCl a, BaCl 2 , MgCl,, the alkaliue chlorides heading the list. A 25 p.c. solution of NH 4 C1 is in fact half as good a conductor as the best acid solution known. It was found that, if the conductivity for small percentages be represented by k = xp - KjP, so that K may be called the ^ccific con ductivity in watery solutions, then K varies inversely as the specific gravity, that is, directly as the &quot; specific volume.&quot; The temperature coefficients for the chlorides are very nearly in dependent of the temperature. There is a slight increase for higher temperatures, which is most marked in the case of highly concen trated and viscous solutions of CaCl 2, MgCl 2. For weak solutions the coefficients are all very nearly equal ; at 18 C. the extreme value for 5 p.c. solutions lies between -% (for LiCl) and -^ (for NH 4 C1). There is a tendency, as seen by the curves, to a value T, or &quot;022 for very weak solutions. It will be noticed (see table below) that this coefficient is much larger than 0039, which is about the corresponding number for a pure metal. When the percentage is increased from five upwards, the tem perature coefficient for 1 8 C. decreases at first for all the chlorides ; it reaches a minimum for NaCl, CaCl 2, MgCl 2 , which belong to class (1) ; but there is no minimum for KC1, NH 4 C1, BaCl 2 , which belong to class (2), and have no maximum conductivity. The acids investigated were nitric, hydrochloric, sulphuric, phos phoric, oxalic, tartaric, and acetic. In every case, except that of oxalic acid, a maximum conductivity was obtained. The order in which we have named the acids is that of the conductivity of the best conducting solutions at 18 C. ; for the first three we have respectively fr 18 10 8 = 7330, 7174, 6914, the corresponding percen tages being 297, 18 &quot;3, 30 4, so that the maxima are very nearly equal, and the maximum percentages not far apart. The curve for sulphuric acid is exceedingly remarkable. Between and 100 p.c. of H 2 S0 4, it shows two maxima. The first minimum occurs at the percentage corresponding to the hydrate H. 2 S0 4 + H 2 0. The conductivity corresponding to H. 2 S0 4 is also a minimum ; for when S0 3 is added, causing supersaturation, the conductivity again in creases, there must therefore be at least one more maximum, since melted S0 3 is a non-conductor. There is no peculiarity in the curve corresponding to the hydrate 2H, 2 + II 2 S0 4, which is distinguished from H 2 + H 2 S0 4 in not being crystallizable. A striking simi larity in the case of sulphuric and acetic acid is remarked between the curves of resistance and of solidification temperature; wherever the latter is high, the former is so also ; there is a maximum in both cases for H 2 + H 2 S0 4 and for H 2 SO 4, and a minimum in both cases near 92 5 p.c.; the other minima do not agree so well. A remarkable delation is given, which appears to connect the resistance of the monobasic acids HC1, HBr, HI, and HN0 3. If any percentage be multiplied by the specific gravity of the solution, and divided by the molecular weight of the acid, the result is the num ber of molecules (n) in unit of volume of the solution. On forming a table of resistances with n for argument, it was found that for solutions with the same n, whether of HC1, HBr, HI, 01 HN0 3 , the conductivity is the same. This appears very clearly from the dotted curve in fig. 3 of the diagram, calculated from the different acids, the regularity of the curve, and in parts the coincidences, are very marked. This result may be stated thus : In solutions containing an equal number of molecules, whether of HN0 3, HC1, HBr, or HI, the components of electrolysis under equal electromotive forces pass in opiwsite directions with equal relative velocities. 1 The temperature coefficients for the four monobasic acids are nearly equal, and nearly independent of the concentration. The same increase of temperature coefficient with increase of concentration as was noticed in the case of viscous chloride solutions appears also in the viscous acid solutions of phosphoric, tartaric, and sulphuric acid. It is also found that where the conductivity is a minimum, the temperature coefficient is correspondingly great; so that, with increasing temperature the maxima and minima tend to get smoothed out. It appears also that the proximity of the maxima for H 2 S0 4 , HN0 3, HC1, becomes more marked as the temperature rises. The existence of the maxima in most cases, and of the minima in the sulphuric acid curve, led Kohlrausch to suggest the principle that no stable chemical compound in a pure state is a conductor, and that mixture of at least two such compounds is necessaiy for conduction. He mentions many instances of this principle, e.g., water, sulphurous acid, carbonic acid, acetic acid, melted boracic 1 A similar law might be stated for the chlorides, but it holds only for very weak solutions. acid, chromic acid, anhydrous S0 3, &c. In a recent paper 2 he gives some very interesting results concerning the conductivity of pure water and other bad conductors. The lowest conductivity he got for water was 71 (Hg = 10 12 ). This was after careful purifica tion and repeated distillation in glass, and finally in platinum vessels. After standing under a glass bell jar for 4 3, 20, 78, and 1060 hours, the water rose in conductivity from 78 to 133, 350, 850, and 3000 respectively. He calculates that, if pure water were a non-conductor, the presence of O l mgr. per litre of HC1 would be sufficient to account for the observed conductivity. He also found conductivities for SnCl 4 (&amp;lt;) 200, alcohol (commercial distilled) 30, acetic acid (glacial melted) 4, ether (&amp;lt;) - 8. Among recent researches of interest may be mentioned Braun s attempt 3 to measure the con ductivity of melted salts, and Grotrian s 4 on the relation between the viscosity and the electric conductivity of electrolytes. For the speculations of Kohlrausch, Hankel, Beetz, Wiedemann, and Quincke on the ultimate nature of electrolytic resistance, see the papers of the first-mentioned, or Wiedemann s Gahanismus, Bd. i. 434 sqq. Gases. We are not aware that any experiments have hitherto Gases, established that any gas or vapour at ordinary temperature and pressure is a conductor. Boltzmann 5 has arrived at the negative result that air at ordinary temperature and pressure must have a specific resistance at least 10 26 times that of copper. Sir William Thomson has, we believe, arrived at a similar result for steam ; and recent experiments by Prof. Maxwell 6 on air, steam, mercury, and sodium vapour (at high temperatures) have led him to a similar negative conclusion. It was found, however, that the heated air from a Bunsen s burner conducts remarkably well. 7 The so-called unipolar conductivity of flames presents many anomalies, which have been examined by various experimenters. For the literature see F. Braun, Pogg. Ann., 1875. It would appear, therefore, that the loss of electricity from in sulated conductors at moderate potentials, observed by Coulomb and Riess, cannot be due to conduction or convection by the air, but must arise almost wholly from the insulating supports. War- burg, who has experimented much on this subject, appears to be of the same opinion (vide Boltzmann, I.e. p. 415). Varley has lately investigated the passage of the current of a large number of Daniell s cells through a Geissler s (hydrogen ?) tube. He found that it required 323 cells to start the current, but that once it was started it could be maintained by 308 cells ; the current which flowed was proportional to the excess of the number of cells over 304. Thus, for 317 = 304 + 13 the current was proportional to 25 J, for 330 = 304 + 26 it was proportional to 51. Accordingly, if E be a con stant, and II another constant (the resistance of the gas?) we get for the electromotive force E, required to send a current I, E = E + RI. E is analogous to the electromotive force of polarization. For further details about the resistance of dielectrics we refer the reader to Maxwell s Electricity and Magnetism, vol. i. 366 sqq. The following table will give an idea of the conducting power of General different bodies ; r denotes the specific resistance in C.G.S. units (to table, reduce to ohms divide by 10 9 ) ; o is the percentage of itself that r increases in the case of metals and decreases in the case of elec trolytes per deg. C. ; t is the temperature at which r is given. t r a Silver (annealed) 20 1521 37 Copper (annealed) 20 1615 38 ,, (hard drawn 20 1659 Platinum (annealed) 20 9158 Iron (annealed) 20 9827 Lead (pressed) 20 19850 38 Mercury (liquid) . 20 21170 04 German silver 20 96190 07 H.,S0 4 (max. soln.) 18 1 39 x 10 9 1-5 NH 4 C1 (sat.) 18 2 55 x 10 9 1 5 ZnS0 4 (max. soln.) 10 26 60 x 10 9 2-3 H 2 S0 4 (pure) 18 120 20 x 10* 4/2 H./) (pure) 18 135 xlO 13 Glass. 200 227 x 10 14 400 735 x 10 11 Gutta percha 24 353 x 10 ai

7 x 10 24 2 The residual conductivity he would attribute to residual impari ties, or, as in the case of H.,SO 4 and melted salts, to dissociation, where by the solution becomes in reality a mixture of different compounds. Pogg. Ann., clviii. 1876. 3 Pogg. Ann., cliy., 1875. 4 Pogg. Ann., clvii., 1876; clx., 1877. 5 Pogg. Ann., civ., 1875. 8 Unpublished results. 7 Herwig (Pogg. Ann., 1874) has recently concluded from some ex periments that Hg vapour does conduct in a certain anomalous way. His experiments were complicated by the conductivity of the glnss tubes containing the heated vapour ; steps were taken, however, to eliminate this. Considerable doubt hangs over the whole subject.