July 19, 1888] 



NATURE 



285 



chlorine or other soluble substance, &c, upon the electromotive 

 force of a voltaic couple (see Nature, vol. xxxviii. p. 117). 



Take a voltaic couple, composed of an unamalgamated strip of 

 zinc or magnesium (the latter is usually the most sensitive), and 

 a small sheet of platinum, immersed in distilled water ; balance 

 its electric potential through an ordinary galvanometer of about 

 100 ohms resistance by that of a precisely similar couple com- 

 posed of portions of the same specimens of the same metals, 

 immersed the same moment as the other pair in a separate 

 quantity of the same water ; and gradually add to one of the two 

 cells sufficiently small and known quantities of an adequately 

 weak solution of known strength in a portion of the same water, 

 of the substance to be used, until the balance is upset, and take 

 note of the proportions of the substance and of the water then 

 contained in that cell. In the present experiments a magnesium 

 platinum couple was employed. 



The minimum proportions required with several substances 

 were as follows : potassic chloride, between I part in 3875 and 

 4650 parts of water ; potassic chlorate, between I in 4650 

 and 5166; hydrochloric acid, between 1 in 516,666 and 

 664,285 ; and with chlorine between 1 in 15,656,500,000 and 

 19,565,210,000. 



The proportion required of each different substance is 

 dependent upon very simple conditions, viz. unchanged com- 

 position of the voltaic couple, a uniform temperature, and 

 employing the same galvanometer. The apparently constant 

 numbers thus obtained may probably be used as tests of the 

 purity or of the uniformity of composition of the dissolved 

 substances. 



The "minimum-point" varies with — (1) the chemical com- 

 position of the liquid ; (2) the kind of positive metal ; (3) to 

 a less degree with the kind of negative metal ; (4) the tempera- 

 ture at the surface of the positive metal, and at that of the 

 negative one ; and (5) with the kind of galvanometer employed. 



The or. ler of the degree of sensitiveness is manifestly related 

 to that of the degree of free chemical energy of the liquid ; also 

 to the atomic and molecular weights of the dissolved substances, 

 and to the ordinary chemical groups of halogens. The greater 

 the degree of free chemical energy of the dissolved substance, 

 and the greater its action upon the positive metal, the smaller 

 the proportion of it required to change the potential. 



As the "minimum point" of a chemically active substance 

 dissolved in water is usually much altered by adding almost any 

 soluble substance to the mixture, measurements of that point in 

 a number of liquids at a given temperature with the same voltaic 

 pair and galvanometer will probably throw some light upon the 

 degree of chemical freedom of substances dissolved in water. 



" On the Change of Potential of a Voltaic Couple by Variation 

 of Strength of its Liquid." By Dr. G. Gore, F. R. S. 



This paper contains a series of tables of measurements of the 

 electromotive forces of a voltaic couple composed of unamal- 

 gamated zinc and platinum in distilled water, and in aqueous 

 solutions of different strengths of the following substances : 

 potassic chlorate, potassic chloride, hydrochloric acid, and 

 bromine. The measurements were made by balancing the 

 potential of the couple by that of a suitable thermo-electric pile 

 (Proc. Birm. Phil. Soc. vol. iv. p. 130) through an ordinary 

 astatic galvanometer of about 100 ohms resistance. 



The following are the minimum proportions of those substances 

 required to change the potential of the couple in water : potassic 

 chlorate, between 1 in 221 and 258 parts of water ; potassic 

 chloride, between I in 695,067 and 1,390,134; hydrochloric 

 acid, between 1 in 9,300,000 and 9,388,185; and of bromine, 

 between 1 in 77,500,000 and 84,545,000 parts. 



With each of these substances a gradual and uniform increase 

 of strength of the solution from the weakest up to a saturated 

 one was attended by a more or less irregular change of electro- 

 motive force. 



By plotting the quantities of dissolved substance as ordinates 

 to the electromotive forces as abscissae, each substance yielded 

 a different curve of variation of electromotive force by uniformly 

 changing the strength of its solution, and the curve was charac- 

 teristic of the substance. As the least addition of a foreign 

 soluble substance greatly changed the "minimum-point," and 

 altered the curve of variation of potential, both the curve and 

 the minimum proportion of a substance required to upset the 

 balance of the couple in water may probably be used as tests of 

 the chemical composition of the substance, and as means of 

 examining its state of combination when dissolved. By varying 

 the strength of the solution at each of the metals separately, a 



curve of change of potential was obtained for each positive metal, 

 but not for every negative one. 



" Influence of the Chemical Energy of Electrolytes upon the 

 Minimum-point and Change of Potential of a Voltaic Couple in 

 Water." By Dr. G. Gore, F.K.S. 



By means of a zinc-platinum voltaic couple in distilled water, 

 with its electromotive force balanced by that of a suitable 

 thermo-electric pile l (Birm. Phil. Soc. Proc. vol. iv. p. 130), 

 the effect of several groups of chemical substances upon the 

 potential of the couple was examined. Measurements were 

 made of the electromotive forces of a series of strengths of 

 solution of each substance, and the results are given in a series of 

 tables. 



The minimum proportions of substance required to change the 

 potential of the couple in water were as follows : — 



Potassic iodate, between 1 in 443 and 494 



,, bromate ,, 1 ,, 344 ,, 384 



,, chlorate ,, 1 ,, 221 ,, 258 



,, iodide ,, 1 ,, 15,500 ,, 17,222 



,, bromide ,, I ,, 66,428 ,, 67,391 



,, chloride ,, I ,, 695,067 ,, 704,540 



Iodine ... ,, I ,, 3,100,000 ,, 3,521,970 



Bromine ... ,, I ,, 77,500,000 ,, 84,545,000 



Chlorine ... ,, 1 ,, 1,264,000,000 ,, 1,300,000,000 



On comparing these numbers we find that the proportion of 

 substance required to upset the voltaic balance was largest with 

 the oxygen salts, intermediate with the haloid ones, and least 

 with the free elementary halogens. It was smaller the greater the 

 degree of chemical energy of the substance ; thus it was about 400 

 times less with chlorine than with iodine. And it was smaller 

 the greater the degree of freedom to exert that energy ; thus it 

 was about 5,416,000 times less with free chlorine than with 

 potassic chlorate, or 1,570,000 times less than with the combined 

 chlorine of the chlorate, and about 185 times smaller than with 

 potassic chloride, or 88 times less than with the combined 

 chlorine of that salt. 



The order or curve of variation of potential by uniform increase 

 of strength of the solution was different with each substance, 

 and was apparently characteristic of the body in each case. A 

 great number of such representative curves might be obtained 

 with a zinc platinum or other voltaic couple in different 

 electrolytes. 



June 21. — "Further Researches on the Physiology of the 

 Invertebrata." By A. B. Griffiths, Ph.D., F.R.S. (Edin.), 

 F. C.S. (Lond. and Paris), Principal and Lecturer on Chemistry 

 and Biology, School of Science, Lincoln ; Member of the 

 Physico-Chemical Society of St. Petersburg. Communicated 

 by Sir Richard Owen, K.C.B., F.R.S. 



I. The Renal Organs of the Asleridea. 



The digestive apparatus of Uraster rubens (one of the Aste- 

 ridea) is briefly described as follows : — The capacious mouth, 

 found upon the oral side, leads into a short oesophagus, which 

 opens into a wider sacculated stomach with thin distensible walls. 

 There are five large stomach sacs ; each of these is situated in 

 radial position and passes into the base of the corresponding ray. 

 Each sac or pouch is kept in its place by two retractor muscles 

 fixed to the median ridge of the ray, which lie between the two 

 ampulla? or water-sacs. Passing towards the aboral side, the 

 stomach forms the well-known pentagonal "pyloric sac." The 

 pyloric sac gives off five radial ducts, each of which divides into 

 two tubules bearing a number of lateral pancreatic follicles, 

 whose secretions are poured into the pyloric sac and intestine. 

 The author has proved the nature of this secretion to be similar 

 to the pancreatic fluid of the Vertebrata (Edinburgh Roy. Soc. 

 Proc, No. 125, p. 120). Recently, the secretion found in the 

 five pouches of the stomach (of Uraster) has been submitted to 

 a careful chemical and microscopical examination. With a 

 quantity of the secretion uric acid crystals were extracted by the 

 same methods as described in his previous papers (Proc. Roy. 

 Soc, vol. xlii. p. 392, vol. xxxviii. p. 187). 



The tests proved the entire absence of urea in the secretion 

 under examination. No guanin or calcium phosphate could be 

 detected in the secretion, although the author has found the 

 latter compound as an ingredient in the renal secretions of the 

 Cephalopoda and the Lamellibranchiata (Edinburgh Roy. Soc. 

 Proc, vol. xiv. p. 230). 



» This instrument i manufactured by Messrs. Nalder Brcs., Horseferry 

 Road, Westminster* 



