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closed test-tube, part of the clear upper liquor can be poured off and tested by a drop of the reagent. The extract is prepared for titration by acidifying and gently warming to drive off alcohol. If any precipitate separates it is filtered off and washed with dilute acid. The liquid should finally contain about 1 per cent. of free sulphuric acid. M. J. S.

Tanret's Reaction for Albumin, Peptone, and Alkaloïds in Urine. By L. BRASSE (Compt. rend. Soc. Biol. [8], 4, 369-370).— Albumin, peptone, and alkaloïds are precipitated from urine in the cold by Tanret's reagent, potassium mercury iodide. On heating, the peptone and alkaloidal precipitate dissolves, leaving the albumin insoluble. The alkaloïdal precipitate can be easily separated from the peptone precipitate by reason of its solubility in ether.

No insoluble combinations are formed with any of the ordinary constituents of urine, such as creatine, creatinine, xanthine, or hypoxanthine, but bile salts give rise to a precipitate insoluble, like albumin, in both cold and hot solutions. This precipitate, however, can be differentiated from the ordinary albumin precipitate inasmuch as it is soluble in ether.

J. P. L.

Hæmatoscopic Study of Blood. By A. HENOCQUE (Compt. rend. Soc. Biol. [8], 4, 283—284). The chief advantage in the use of the hæmatoscope (Abstr., 1887, 312), for the spectroscopic examination of blood is the possibility of examining the blood undiluted.

J. P. L.

205

General and Physical Chemistry.

Chemical Action of Light on an Explosive Mixture of Chlorine and Hydrogen. By E. PRINGSHEIM (Ann. Phys. Chem. [2], 32, 384-428).—Bunsen and Roscoe (Ann. Phys. Chem., 96, 373; 100, 43, 481; 101, 235; 108, 193; 117, 529) propounded the question"Whether in photochemical combination an amount of work is done for which an equivalent of luminous energy disappears, or whether the action effected by the chemical rays is merely started by the light without the expenditure of a sensible amount of luminous energy." They endeavoured to solve this problem in the case of hydrogen and chlorine, by comparing the quantity of light absorbed by the mixture with that absorbed by dry chlorine, or as they call it, the optical extinction in chlorine. They assumed that this optical extinction would be of the same amount in the mixture, and therefore concluded that the considerable additional extinction observed in the mixture was due to the transformation of luminous energy into heat energy during the chemical action. The author objects to this reasoning, that the light absorbed in chlorine alone is itself transformed into heat, thereby raising the temperature of the gas, whilst in the case of the mixture, the luminous energy may very probably be transformed directly into chemical energy. In support of this view, he adduces Bunsen and Roscoe's observation that the action of light was considerable when the range of temperature was from 18° to 26°.

The luminous energy, indeed, may either increase the molecular motion directly and the atomic motion indirectly, or vice versâ, or more probably both processes may go on simultaneously. From the kinetic theory of gases, the introduction of energy by conduction of heat must increase the molecular velocities, which are of a very irregular character. Now it is found that the absorption of light is a function of the wave-length, and it therefore seems probable that the chemical action of light consists in the direct transfer of energy to the atoms, whose motions consist of vibrations of definite periods, and is therefore the direct cause of dissociation. When in equilibrium, there is a definite relation between the atomic and the molecular energy of a gas, and if this is disturbed by the absorption of light, there will be a change in the number of molecular impacts, and before a fresh state of equilibrium is attained, the direct increase in the atomic energy may be more or less expended; in the present case in effecting chemical change; in other cases, in producing fluorescence, phosphorescence, or electrical phenomena. The assumption, therefore, that what Bunsen and Roscoe call the optically absorbed light simply increases the temperature, and that the additional absorption due to the presence of hydrogen increases the chemical absorption, is not justifiable. If the light absorbed by the chlorine molecules in the mixture produces chemical action, this, as shown later, will probably

VOL. LIV.

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consist in the formation of intermediate unstable products whose coefficient of absorption of light differs from that of the mixture of chlorine and hydrogen, so that "chemical extinction" must be regarded as the consequence rather than the cause of the chemical change. The existence of chemical extinction is therefore no criterion as to whether transformation of luminous into chemical energy takes place, so that Bunsen and Roscoe's method is incapable of solving the question proposed by them, but a result obtained in another portion of their researches, namely, that the chemical action is proportional to the intensity of the light, although not so applied by them, seems to give the required solution; for if the action of the light were simply to disturb a state of unstable equilibrium, the quantity of hydrogen chloride formed would be independent of the initial impulse. It must, therefore, be assumed that a definite absorption of luminous energy will take place for every molecule of hydrogen chloride formed, but it does not follow that more light must be absorbed than with the chlorine alone.

The determination of the cause of the increase of the extinctioncoefficient, which may be due merely to a change in the condition of the mixture, leads to an investigation of the physical and chemical phenomena which accompany the formation of hydrogen chloride, and therefore to the consideration of " Photochemical Induction." Bunsen and Roscoe found that when light falls on a freshly prepared mixture, or on one which has been for a time in darkness after previous exposure to light, there is at first no appreciable formation of hydrogen chloride; after a certain interval, a slight action begins, gradually increases to a maximum, and then remains constantly proportional to the time. The act of overcoming the resistance here indicated and bringing the gas into a state of readiness for combination, they call chemical induction, and when due to the action of light, photochemical induction. They found the duration of the seemingly inactive interval to increase with the thickness of gas traversed by the light, and to decrease with increasing intensity of light more rapidly than the intensity increases. Both of these results are confirmed by the author, who further finds that the rate of formation of hydrogen chloride depends only on the intensity, and not on the wave-length of the light, from which he concludes that the inductive action is probably a chemical one. By instantaneous exposure to a stronger source of light, he finds that the gas undergoes a momentary expansion proportional to the intensity of the light, which is evidently not due to the heat of formation of hydrogen chloride, from the rapidity with which it appears and disappears, and also because it is found to be independent of previous induction and of the amount of hydrogen chloride formed. Neither can it, for the former reason, be due to the heat which Budde (Ann. Phys. Chem., 144, 213) has shown to be produced by the absorption of light by chlorine.

He therefore concludes that some intermediate product is formed, that the expansion is due to momentary dissociation, and that the quantity formed is proportional to the intensity of the light. It dojes not seem reasonable to suppose that this may consist of molecules of H.Cl and Cl or HCl, and H; the author therefore seeks for the cause in

the presence of water-vapour (the gas being in contact with water during the experiments), in accordance with the influence known to be excited by water-vapour on other explosive mixtures, and in confirmation of this he finds that when a solution of hydrogen chloride is substituted for the water, and the gas dried as completely as possible, much less hydrogen chloride is formed by the action of light of given intensity, and with the feeble light used in the earlier experiments, no action at all takes place.

The author assumes the action to take place in two stages, for example:

(1.) H2+ Cl2 + H2O = Cl2O + 2H2;
(2.) Cl2O + 2H2 = 2HCl + H2O;

or some similar action, the exact nature of which may be determined by chemical analysis. He supposes the action to be somewhat as follows:-The intermediate substance is first formed, the process continuing until a certain proportion of the gas is transformed. Mass actions then come into play between chlorine, hydrogen, water and the intermediate product, and hydrogen chloride begins to be formed. The second stage of induction now begins. The amount of the intermediate product increases more rapidly than it is used up in forming hydrogen chloride, which is therefore formed at an increasing rate, until the intermediate product is destroyed as fast as it is produced, introducing the third stage, in which the formation of hydrogen chloride is proportional to the time. When removed from the action of light, the quantity of intermediate product will soon fall below the minimum required for the production of hydrogen chloride, and the remainder will break up again into hydrogen, chlorine, and water. This gradual decomposition of the intermediate product into its original constituents will go on even during induction, which explains why, with diminishing intensity of light, the duration of induction. increases more rapidly than the intensity diminishes, for since the nature of the change undergone by the intermediate product depends on its actual amount, therefore the less the intensity the greater will be the ratio of the amount destroyed to that newly formed in each G. W. T.

moment.

Anomalous Dispersion produced by Glowing Vapours. By A. WINKELMANN (Ann. Phys. Chem. [2], 32, 439-442).-Kundt (Ann. Phys. Chem. [2], 10, 231) discovered the anomalous dispersion produced by sodium vapour. The method he used was to allow a ray of light to pass through the conical flame of a Bunsen burner coloured with sodium vapour, so as to be refracted by the flame, which acted like a prism with its refracting angle horizontal and upwards, and then through a vertital prism, by which means he obtained crossed spectra which showed the anomalous dispersion. In the hope of making the action stronger, Kundt attempted, but without success, to reduce the flame to a prismatic form by the use of glass or mica plates. The author has succeeded in doing this by using as the burner a tube of triangular section and provided with an air-blast.

On the top of the tube is placed a double thickness of wire-gauze, and on that again a triangular support smaller than the section of the burner, made of iron wire, which carries an iron cup of the same dimensions as the support, in which the sodium is placed.

With this apparatus Kundt's phenomenon was exhibited very clearly, and it was also obtained, although less distinctly, with potassium. Lithium and thallium chlorides were also tried but with negative results; this the author attributes to the insufficient density of the vapour. G. W. T.

Components of the Rare Earths yielding Absorption-spectra. By G. KRÜSS and L. F. NILSON (Ber., 20, 3067-3072).-A reply to G. H. Bailey (this vol., p. 1), in which the authors maintain the correctness of their views on the components of the rare earths (Abstr., 1887, 890).

Components of the Rare Earths yielding Absorption-spectra. By G. H. BAILEY (Ber., 20, 3325–3327).—A rejoinder to the preceding Abstract.

Theory of Researches on Contact-electricity. By F. EXNER (Ann. Phys. Chem. [2], 32, 515-520).-The author's recent researches in support of his denial of contact-electrification (Ann. Phys. Chem. [2], 32, 53) were founded on his experimental result, that if an insulated conductor is connected with an electrometer, and the latter then disconnected from the earth, and if the capacity of the conductor is then altered, the electrometer will remain unaffected, whilst according to the contact-theory there should be a flow of electricity. The present note is a reply to objections which Hallwachs (ibid., 64) has advanced against the author's conclusions.

In reply to these objections he states that

(1.) The potential of the grating of oxidised iron which surrounded the apparatus differed by less than 0.05 of a Daniell from that of the walls of the room, which would justify his neglecting it; moreover, from (4), its potential, whatever its value, would be without influence on the results.

(2.) The capacity of the conductor was measured inside the grating, and not outside as assumed by Hallwachs.

(3.) He is justified in neglecting the change of capacity in the quadrants during the movement of the needle, since for the maximum deflection to be expected the error so arising would not exceed 1 per cent., and as no deflection was observed, the objection is entirely irrele

vaut.

(4.) The potentials would naturally be understood to refer to that of the grating as zero, without an express statement to that effect; moreover, the author shows by a simple mathematical proof that the result is independent of the absolute values of the potential, the essence of his method consisting in eliminating the influence of the grating by comparative observations on different metals. The dif ference of deflections which should have been obtained according to

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