Imatges de pàgina

Chemical News, I I Jan. 4, 1882. I

Chemical Researches on the Assay of Silver.

The answer is No, for exactly so much excess of salt has been added. At the neutral point, the temperature being 15° to 17c C, the liquid contains J milligramme of silver, and therewith an equivalent quantity of salt—viz., the half of 0-54. = 0*27 milligramme.

At this neutral point the quantity of salt used was quite sufficient, which quantity of salt is chemically equivalent to all the silver, even when some silver remains dissolved in the liquid. At 150 to 170 C. the last half cubic centimetre of decimal salt solution added is not used to form chloride of silver. But that is not now the question. What is to be answered here is, whether the total quantity of salt added does exactly express the quantity of silver, and the answer is in the affirmative—viz., it is so.

Let us now return and review the first method, in which just so much salt has been added that a larger quantity of salt would not have yielded a precipitate; and it will be quite evident from what has just been stated, that at 150 to 17° a quantity of 0-27 milligramme of salt has been added in excess, that therefore 0*5 mil igramm?of silver too much has been calculated. The assay made according to the first method at from 15° to 170 gives the quantity of silver o-5 milligramme too high, and the second method does the same, leaving out of the question the chance of greater error by applying too much salt if the experiment be somewhat roughly conducted. Common salt in solution fulfils a very

Peculiar function when it is added to chloride of silver issolved in nitrate of soda after the point of neutrality has been reached. The function alfuded to is not a chemical function either in the case of common suit or nitrate of silver, for the silver which is in solution requires no additional chlorine, and the chlorine in solution requires no additional silver.

It is of the more importance to consider the functions of each of these re-agents in this case, since chloride of silver is somewhat soluble in common salt. Curiously enough, this property is suspended in this case, and chloride of sodium exerts the power of expelling chloride of silver from its solution in nitrate of soda. A solvent, therefore, expels a dissolved substance from its solution without, however, exerting any chemical force upon it. 1 ask leave to tarry awhile and discuss this phenomenon so as to point out its great interest. For instance, let us assume that we have brought an assay experiment to the neutral point. The liquid then contains chloride of sodium and nitrate of silver, or, if it is preferred, chloride of silver dissolved in nitrate of soda. What, now, is the action exerted by the common salt when it expels chloride of silver? The most simple expression of the fact is this, that the peculiar solubility of chloride of silver in nitrate of soda becomes suspended by the addition of common salt, and also by nitrate of silver, and that there is formed in small quantity a similar combination between nitrate of soda and common salt (or nitrate of silver) as was previously formed between nitrate of soda and chloride of silver—a combination, however, in which chloride of silver is not soluble. In order to elucidate this, I will express the phenomenon in the form ol a chemical formula.

At the neutral point there exists in solution CI. Ag. and N05 NaO. The addition of common salt, while it expels Cl.Ag., produces Cl.Na and NO, NaO. Or the addition of nitrate of silver produces, while it expels CI Ag., N05 Ag.O and N06 NaO.

For those who prefer the explanation according to Bcrthollet's views:—

1. The solution at the neutral point contains Cl.Na + No5 AgO + NO,NaO.

2. Addition of chloride of sodium produces, while Cl.Ag. is expelled, Cl.Na+ NOs NaO.

3. Addition of nitrate of silver produces, while it expels Cl.Ag., NOs Ag.O+ N05 NaO.

According to the last formula;, which I think the correct and true one, there is expressed in 1 the state of equilibrium between chemical force and power of solution: while in 2 and 3 there is expressed the creation of a new state of equilibrium between nitrate of soda and common salt or nitrate of silver, whereby the Cl.Ag. is expelled in equivalent proportion. If I may attach some value to any facts occurring in this treatise, I think that the last mentioned peculiarity is by far the most interesting, for if I do not mistake I have proved that there is here a relation between what we call physical and chemical action and one which may even be expressed in chemical equivalents. The fact is, that the solubility of chloride of silver in nitrate of soda can become suspended by a quantity of chloride of sodium or of nitrate of silver equivalent to the quantity of chloride of silver dissolved. I do not in the least doubt but this is generally so in all cases where it refers to the solution of such substances, the nature of which when in solution can be referred to w7hat was first called attention to by Berthollet.

From the above mentioned, it is clear that in the first and second methods of assaying more common salt is used than is required for the formation of chloride of silver, the last portions of common salt being applied to expel from its solution the chloride of silver already formed. According to the third method, the quantity of chloride of sodium applied is just equivalent with the silver submitted to experiment. Viewed, therefore, from a chemical point of view, the third method is the only true and correct one.

Those circumstances which could disturb and influence in one of the three methods the final result, viz. the quantity of silver present, have been generally treated of in the foregoing section. They are the influence due to the quantity of silver submitted to experiment, the quantity of the nitric acid, and the quantity of that frequently occurring substance, copper. Finally, the influence of the temperature of the test bottle has to be considered. It is now our duty to look into these influences more deeply as far as regards methods 1 and 3, while it is not necessary to do this for the second method, since it is closely connected with the first.

Quantity of Silver.—I have previously observed that the quantity of chloride of Bilver which becomes dissolved in the liquid is three times larger in case 3 grammes of silver, from 3 to 5 cubic centimetres of nitric acid and 300 cubic centimetres of standard solution are used, than when only 1 gramme of silver is applied. That this, however, does not affect the result is proved by the following experiment for the first and third methods, performed at 15° C.

a 1 gramme of silver required 1003-15 c.c. salt
* 3 .. .. 3°°9-5 »

The experiment was finished by the addition of salt unto the limit.

The quantity of decimal solution required to come to the utmost limit of the silver was,—

For a, 10 drops = 1 cubic centime re
„ 6, 60 „ =3 „ „

I then added to a 10 drops of decimal solution of salt

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= 0-5 cubic centimetre; to 6, 30 drops of the same solution = 1-5 cubic centimetre. The liquid having been thus brought to the point of neutrality, I found that in order to make a and 6 give equally large precipitates, a ■would require half a drop of decimal solution of salt, and b one drop of decimal solution of silver, after the liquids of each of them had been divided into two equal portions, to which in the one case five drops of nitrate of silver in the other five drops of salt, both in decimal solution, had been added.

It is, therefore, clear, that neither of the results of these methods is in any way dependent upon the quantity of chloride of silver in solution. The quantity of silver is this :—

Precipitate unto the limit. Neutral point.

a, ioo3-T5 + 0-025 = i°°3'i75 1001-675

6, 3009-5 — 0-05 = 3009-45 3°07-95

$ of 3009-45 = 1003-15; i of 3007-95 = 1002-65.

a and b, or one gramme of silver and three grammes of silver, yield the same result, while the quantity of chloride of silver dissolved in the liquid from 6 was three times larger than in that from a, while also in 6 three times more common salt had not given rise to any precipitate than was the case in a.

Nitric Aria and Copper.—Three experiments were mentioned before, which had been made for the express purpose of investigating the influence of this acid. I dissolved a, 1 gramme of pure silver in 5 cubic centimetres of nitric acid, 6, 1 gramme in 10 cubic centimetres, and c, 1 gramme of pure silver with 1 gramme of pure copper in 7-5 cubic centimetres nitric acid; when all three were precipitated with salt solution unto the limit, and respectively required :—

a, 1003 c, c, b, 1003 c. c. and e, 1002-75 c. c. salt.

From which results it is sufficiently evident that nitric acid, although its quantity be doubled, has no influence on the results, notwithstanding we have seen that chloride of silver is somewhat soluble in stronger as well as in weaker nitric acid.

As regards the copper, it is not easy to see with great accuracy in such a case as this where one gramme of each metal was used; for that reason, and for one which I will presently mention, I feel inclined to attribute the J milligramme difference to difficulty of observation; while the quantities of decimal silver solution also to reach the limit of precipitability required were as follows:—a, 20 drops: b, 22 drops; c, 20 drops. I added of decimal solution of salt, a, 10 drops; b, 11 drops ; c, 10 drops. The three solutions had been brought so perfectly to the point of neutrality that at first one single drop only of solution of silver appeared to be required; but this having been added, a drop of decimal solution of salt appeared necessary again. There may be consequently a difference of half a drop.

It is clear, therefore, that nitric acid and copper do not change the neutral point, which, however was not to be expected. The results arc recorded below, according to first and third method:—

First Third

Method. Method.

cub. cents, cub cents.

Pure silver in the ordinary quantity

of nitric acid. .... 1003 1002-5

Pure silver in double the quantity

of nitric acid .... 1003 1002-45

Silver and copper .... 1002-75 100225

TUe Temperature.—It is now absolutely necessary to consider the influence of the temperature of the testbottle on the final result in the application of either the first or the third method. In the whole aeries of the following experiments i gramme of pure silver was dissolved in 5 cubic centimetres of nitric acid and precipitated with 100 cubic centimetres of standard solution at 16^5" C, the bottles having been brought to the undermentioned temperatures, at which they were kept as much as possible throughout the whole experiment. I also employed the decimal solution of salt. The quantity of solution of salt required to reach the limit of precipitability, which may, therefore, indicate the quantity of silver, was:—


It is, consequently, unnecessary to indicate further the influence of the temperature upon the first method; for in all these experiments the standard solution had a temperature of i6'5° C, and always one gramme of pure silver was used. The differences are only due to the difference of temperature of the test-bottle. Some of the above-mentioned liquids were brought to the point of neutrality by means of decimal solution of silver, the quantities required being the following:—

a, at 50 C. . . 0-35 cubic centimetres

*f 5° • • °'37S

c, 5° . . 0325 „

/, 16° . . 0-525 „

9. i°'5° . . 0-55 „

»', 56° • -i 65 >•

k, 56- . . 1-65

Here also the influence of the temperature is veryapparent.

In order to be enabled to compare all these results of one gramme of pure silver treated with standard solution of the same temperature, according to methods the first and third, we have to subtract the last figures from the first, in which case we obtain :—

First Method. Third Method.

Temperature of Precipitation with Bringing to neutral
test-bottle. salt unto the limit. point.

cubic cents. cubic cents.

a, 50 C. 1002-65 1002-30

b, 5° 1002-50 1002-125
o, 50 1002-60 1002-275
/, 160 1002-90 1002-375
g, 16-50 1002-90 1002-35
t, 560 1003-85 1002-20

k, 56° 1003-85 1002-20

The differences here enumerated are sufficiently clear From 5* to 160 the difference, according to the first method, is one-third of a milligramme, solely in consequence of the temperature of the test-bottle j while from 16° to 56° it amounts to almost one milligramme. The third method is, it will be seen, independent of the temperature. The differences there are differences due to the experiment, as it is found impossible that this and the temperatine and other influences should not exert some slight disturbing effect over which the experimenter

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has not a perfect control. When the first method is used, and care is not taken that the temperature of all the test-bottles be the same, that method can readily give rise to very great inexactness. A contra test or witness,1 when used may readily, in this way (i. e., when the effect of the temperature is neglected), become a false test.

How is it that the influence of the temperature of the test-bottles has not been long since discovered from the results obtained from the experiments?

Mohr* has quite neglected and totally overlooked the solubility of chloride of silver, even when heat is applied. He has, therefore, given very bad advice when he said, (hut the liquid, in making a silver assay, has to be heated. At 560 C. the quantity of chloride of silver dissolved is 4-39 milligrammes, already equal to 3*3 of silver.

From the facts here alluded to, the following deductions may be drawn:—The neutral point is exactly situated in the middle of the ultimate limits of precipitation which can be produced either by salt or silver; and this holds good without any disturbing effect of lower, middling, or high temperature. If we take, therefore, invariably one gramme of pure silver as a starting-point, and have brought the precipitation with salt unto the limit, but not further, taking care to add the salt guttatim, and we find that we require at different temperatures of decimal solution of silver to reach again the limit of silver, the following quantity in drops,— viz., 14, 16, 18, zo, 2z, 14; then, on addition of 7, 8, 9, 10, xi, 12 drops of decimal solution of salt, we shall reach the neutral point again with perfect ease and certainty. (To bo continued.)

On the Estimation of the Carbon of Cast Iron, by M. le Br. E. Mulder.

M. Mulder treats, in the first place, of the causes o' the errors which may arise in working out the analysis by M. Regnault's method. Thus, he inquires whether the mixture of chromate of lead and chlorate of potash, at the high temperature of combustion, would not produce free chlorine. This is the known opinion of MM. Erdmann and Marchand.' They have even stated that nascent oxygen disengages chlorine from chlorate of potash. M. Marignac has also observed that oxygen proceeding from the decomposition of chlorate of potass is not free from chlorine.

The author is convinced that the mixture of chromate of lead and chromate of potash, as advised by M. Kcgnault, does not develope the faintest trace of chlorine, but that this is disengaged as soon as the chlorate of potass predominates over the chromate of lead. Chlorate of lead is then formed, which, while decomposing, furnishes free chlorine. M. Mulder also observes, that in following M. Kegnault's directions there is no fear of losing the carbonic acid by the formation of carbonate of potash, the large quantity of chromate of lead preventing this. These analyses prove that chromate of lead alone is insufficient to induce the combustion of carbon, especially carbon in the state of graphite. Cast iron at first burns by the oxygen of the

1 Professor Mulder understands by this word, that whilo an opsay is performed, any with an alloy, another as?ay is at the same time made with perfectly pure silver under perfectly the same conditions.

* "Fitrir Methodo," part ii., p. J4, German Edition.

a JournalJ'ilr pnxktUche ClietiiLc, bd. xxxi., s. 275.

chlorate, and the combustion is finished by the chromate of lead; but towards the end of the operation this does not suffice for the combustion of graphite.

The results obtained by M. Kudernatsch's method of effecting combustion with oxide of copper alone are uncertain, at lesst if a ourrent of oxygen is not at the same time passed through the combustion-tube, as M. Rose advises.

From his researches, M. Mulder comes to the conclusion that M. Kegnault's method is insufficient to determine the total quantity of carbon, and that chromate of lead cannot occasion the combustion of graphite. He prefers direct combustion in an oxygen current. He fills two-thirds of the combustion-tube with pure sand, which has been heated to redness in oxygen; next he introduces a plug of asbestos and the mixture of pulverised cast iron with pumice stone j then another plug of asbestos, a small layer of oxidised copper, and closes it with another plug of asbestos. This he heats with oak charcoal. In this way M. Mulder has discovered from 5'3 ro 5''3 of carbon in oast iron which, analysed by M. Regnault'B method, yielded but 3-77 per cent.— Scheikundige Verhandelingen en Onderzoekingen, vol. iii., p, 1.

Contributions to the History of Picric Acid,
!>;/ M. Carey Lea, Philadelphia.

Solaltilltr in Kiilithuric Acid.—It is stated in the text-books that picric acid is insoluble in sulphuric acid. It is, however, soluble to a small degree in strong sulphuric acid; in a more dilute acid it is apparently wholly insoluble, until the dilution reaches a certain point, when it increases again. If picric acid be left in contact with oil of vitriol, and the latter be decanted, and mixed with two or three times its volume of water, the picric acid is deposited on cooling in what appear to be very minute square or nearly square scales.

Picric acid crystallises in the rhombic system, and if we suppose these scales to be formed by predominating « F » planes bounded at the edges by octahedral planes, they should be rhombs approaching very nearly to squares, having their axes as -o.374:1 0000.

11 cold saturated aqueous solution of picric acid be mixed with sulphuric acid diluted with an equal volume of water, the following results are obtained:—

1 vol. solution of picrate, 4) No precipitate, solution revols. dilute sulphuric acid > maining as colourless as (1 vol. acid, 2 vols, water).) water.

1 vol. sol. picric acid, 2 dilute") No precipitate, solution very

sulphuric acid, same dilu- I nearly colourless, faintest tion. r tinge of yellow only

J visible.

2 vols, solution picric acid, ) Nearly the whole of the

1 vol. sulphuric acid, same ) picric acid was precipidilution. ) tated.

The amount remaining in solution continued in further trials to diminish as the sulphuric acid became more dilute, until a maximum was reached with

3 vols, solution picric acid, 1 *

1 vol. dilute sulphuric > acid, same dilution. J

It thus appears that mixtures of sulphuric and water reach their minimum of solvent power for picric acid when the mixture consists of about 1 vol. acid to 11 vols, water. The proportion of water may be still further increased without materially increasing the solvent

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power for picric acid. If a cold saturated solution of picric acid be mixed with even but -fath of its volume of sulphuric acid, almost the whole is thrown down.

The fact that the characteristic colour of picric acid, which it maintains so persistently through all its combinations, and which is so powerful that, as I have found by actual experiment, a milligramme will distinctly tinge a kilogramme of water, or in other words, that water is coloured by one-millionth of its weight of picric acid — the fact that this colour is totally destroyed by sulphuric acid of a certain strength, without in any way decomposing the acid, is very remarkable. Four volumes of sulphuric aci 1 diluted with five volumes of water exhibit this property, and picric acid dissolves in such a mixture to a colourless solution. This peculiar property has no doubt led to the supposition of the insolubility of picric acid in sulphuric acid above referred to.

Water containing Tooootn 0l? picric acid exhibits a bright yellow colour. With itt6'a6oth the colour is still distinct, even in a stratum of not over an inch in thickness. But in large quantities a millionth gives a distinct colour as above mentioned.

Testa for Picric Acid.—The best tests for picric acid are :—Ammoniacal solution of sulphate of copper, which gives a greenish crystalline precipitate; alkaline sulphide with excess of alkali, which with heat gives a deep red liquid; alkaline cyanide with ammonia, which when heated gives also a red liquid.

The following table will exhibit the relative sensibility of these reagents:—

Strength of aqueous solu- Ammonio mil- Potash liver of Cyanide of

tion of picric phate of copper, sulphur (heat). potassium,


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Immediate precipitate.

No precipitate at first, but by standing a few minutes a distinct one.

Distinct precipi tate by standing.

No precipitate.

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The yellow colour was slightly deepened ; the cj'anide t«at is the more dolicate of the two.

Purification of Picric Acid Since my former

observations on the purification of picric acid, I have had occasion to prepare considerable quantities of the acid for my examinations, and find that all purifications, by converting into potash salt, are inapplicable except for very small quantities. The picrate of potash crystallizes out by so small a fall of temperature that the filters, even when kept heated by a double funnel, become immediately clogged, and the operation becomes to the last degree tedious and troublesome. As the picrate of lime is very soluble, it seemed probable that it might afford a convenient means of solution; it has, indeed, been already recommended for that purpose.1 But I find it wholly inadmissible. A basic salt is formed which falls to the bottom with the excess, of hydrate of lime, and great waste*ensues. The insolubility of alkaline picrates in cold alkaline solutions, which 1 have described_in a previous number of this Journal, furnished me with an excellent process. The crude acid is saturated with carbonate of soda, an excess of which is It be avoided, as it tends to dissolve resinous matter. The hot solution is then easily filtered, and into the

• See Gmelin, Bng. Ed., vol. xi. p. 114.

filtrate a few clear crystals of carbonate of soda are placed. On cooling, the picrate of soda crystallizes out almost as completely as the potash salt would have done, and all the wearisome delay in filtration is avoided. From the mother water mo-e picric acid may be recovered by the addition of a little carbonate of potash. In decomposing alkaline picrates to separate the acid, sulphuric (and not as usually recommended, chlorhydric) acid should be used, because a moderate excess of sulphuric acid throws down a great portion of the acid which would otherwise remain in the mother water. A. moderate but decided excess of acid is absolutely necessary, because otherwise a portion of alkaline picrate escapes decomposition. Even then, it is advisable to recrystallize the acid from alcohol. If picric acid be dissolved by the aid of heat in a solution of sulphate, nitrate, or almost any salt of potash, more or less picrate of potash will crystallize out on cooling. 1 have thought this process not devoid of interest, because picric must become more extensively known in the laboratory and in the arts than hitherto.

Effect of Reducing: Ag-enta.—The effects of reducing agents when alkali is not present, or not present in excess (in presence of excess of alkali, picramic acid is formed), are very variable, depending upon slight differences which it is very difficult to seize. I subjoin some of the best marked results obtained.

A mixture of picric acid, alcohol, iron filings, and acetic acid, were digested for an hour at a heat a little below Iii°. The filtrate was intensely blue, by standing for half-an-hour or less, became brown and muddy, deporting a blackish powder in small quantity, and without trace of crystallization. This filtrate was not changed in colour by acids, or apparently affected by them. Alkalies decolorized it. Its shade of blue varied considerably in different experiments, sometimes full blue, sometimes violet, sometimes greenish.

Other experiments were made by acting on picric acid by zinc and dilute sulphuric acid. After an action of some hours, the solution was mixed with alcohol and filtered. The filtrate heated with bicarbonate of potash in successive portions, gave a fine violet liquid, which with further addition of alkali became deep blue with a tinge of violet. According as acid or alkali were present in excess, there was more of the violet or blue shade. The colours were always very fugitive, and changed to dirty brown by standing, with deposit of an amorphous blackish powder (very small in quantity compared with the picric acid used), which was soluble in acids, and insoluble in alkalies.

These experiment!', although many times repeated, did not lead to the isolation of any substance of interest. There is a certain amount of resemblance between these reactions and those of some of the decomposition products of phenylamine: the latter contains the radical C,2 II.-„ which exists in a substituted form in picric acid'. American Journal of Science, No. 95.

On an Arsenical Thermal Water, by M. OuTON.

The author gives an analysis of water from a thermal source situated in the village of Bou-Chater, anciently Utica (rendered famous by the death of Cato). This water contains, besides the ordinary mineralizing elements, particularly sea salt (four to five thousandths), a comparatively enormous proportion of arsenic.

The numbers given by M. Guyon are o-io6 gramme per litre of arsenic acid, or 0-1684 of arseniates of potash

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and soda, in a total weight of 0-9689 of salts; that is to Bay, more than a sixth of the dry residue. The temperature of the Bou-Chater water is 400; it is clear, limpid, without any unpleasant taste; the inhabitants drink it after letting it cool.

The source forms a basin, about two metres in diameter; the water is confined by a dam of rough stones serving as an abode for an old tortoise (emydi leprosa) which has lived there from time immemorial, and is looked upon by the inhabitants as the genius of the place. The overflowings of the basin form a stream which, in default of a marked bed, spreads on all sides, forming a marsh of considerable extent, covered with rushes, Indian grass, and other plants peculiar to this kind of earth. The inhabitants make use of the stream for washing their linen and sheep's wool; and it is in the course of this same stream that their flocks, of all kinds, drink daily.

It would be interesting if physiologists would study the influence which these waters exercise on men and beasts, according to the recently raised questions on the economic action of arsenious acid.

M. Guyon believes this source of Bou-Chater to be that'mentioncd by Julius Coesar in his " Commentaries," as having such a terrible effect on the army of the Curio. He thinks, considering the present innocuousness of these waters, that their mineral richness must have diminished. "There is nothing," says the author, " to contradict the supposition that the waters of Bou-Chater were formerly more charged with saline principles than they are at present; and perhaps this impoverishment may take place with all known thermal waters, the origin of which can be traced to a remote period.— Repertoire de Chimie.

On the Estimation of Platinum Diffused through Metallic Beds or in the Alpine Rocks of Dauphiny and Savoy, by M. Gi/eymard.

I Have published, in the Annates des Mines and tue Cotnptes-Rendus of the Institute, five memoirs on the discovery of platinum in metallic beds or in the Alpine rocks of Dauphiny and Savoy. Platinum is there found only in small quantities; and also in veins often containing silver or gold.

I assayed 100 grammes of material by the dry way, nsing the most convenient fluxes and a very pure litharge containing neither gold nor platinum.

As only a very small quantity of platinum was found in the button of lead, I added to it a little pure silver; the residuary beads on the cupil then contained silver, platinum, and frequently a little gold.

I consulted M. Berthier on the means of estimating, in the beads of metal, a quantity of platinum so small as not to be weighable in the most accurate balance. The problem was pronounced very difficult. After many researches 1 came to the conclusion that the solution might be arrived at by standard solutions of platinum. I will describe this unexpectedly successful process.

Dissolve 10 milligrammes of platinum in aqua regia, then add sufficient distilled water to make 250 cubic centimetres in bulk.

One cubic centimetre of this solution contains Jyth of a milligramme of platinum, or 2 cubic centimetres 0*08 milligramme.

Into eight little capsules put 2 cubic centimetres of the solution, containing o-o8 milligramme; 0-04 milligramme; O'oi milligramme; 0-005 milligramme; 00025

milligramme; 0-00125 milligramme; and 0-000625 milligramme of platinum.

Into these eight capsules, placed in a row and each containing 2 cubic centimetres of standard solution, add a little powdered tin salt, mix with a glass rod, and the colour of the platinum will soon assume shades corresponding with the above figures from 0-08 to 0-000615 milligramme.

Treat the metallic beads by nitric acid and then add hydrochloric acid. Soluble chloride of platinum and insoluble chloride of silver are obtained. Add two drops of hydrochloric acid, then two cubic centimetres of distilled water. Let them stand for a few minutes aud then decant into other little capsules.

To the capsules containing the solutions of the metallic beads add also powdered tin salt, and the colour of the platinum becomes very apparent in five or six minutes. This colour I compare with that of the eight capsules, and 1 select which ever contains the identical colour produced by the metallic beads. If this happens to be that of the fifth capsule, containing 0-005 milligramme of platinum, it may be concluded that the substance treated contains 0-005 milligramme of platinum to 100 g'-ammes of material. If the colour is identical with that of the first capsule, the proportion in the material assayed will be 0-08 milligramme of platinum to too grammes.

Were the colour intermediate to those of the two capsules, Nos. 2 and 3, the platinum for 100 grammes would be

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Were estimation by weighing possible, the results would not be so precise as those given by this process, which I regret not having devised in the early part of my investigations.

When the substance assayed contains a little gold with platinum, I have succeeded also in estimating it by a standard solution prepared with 20 milligrammes of gold dissolved in aqua regia, diluting the solution sufficiently to make the bulk 250 cubic centimetres. One cubic centimetre then contains ^th of a milligramme of gold (008 milligramme), or » cubic centimetres, 0-16 milligramme.

Then take eight small capsules, into which put 2 centimetres cube of a solution containing 0-16 milligramme; 0-04 milligramme; 0-02 milligramme; o-ol milligramme; 0-605 milligramme; 0-0025 milligramme; 0-00125 milligramme of gold.

Add to each a small quantity of powdered fin salt, and mix with a glass rod. In a few seconds the colour, more or less intense, of the purple precipitate of Cassias will be obtained.

Treated in this way, the substance assayed gives the yellow colour of platinum, and, when gold is also present, in less than a quarter of an hour the purple precipitate falls to the bottom of the capsule. Decant gently, and then add water enough to make the quantity in the capsule 2 centimetres cube. Compare the colours, as with platinum, and take .which ever gives the same tint. I thus obtain, in f'ractibns of a milligramme, all the gold contained in 100 grammes of substance.

I must add that the estimation of gold by'standard solutions is less precise than the estimation o\ platinum, because the colours of the precipitate are less' easy to appreciate. Ihe colours of platinum are perfectly clear, and the estimation mnthematieally precise.

I have made several hundred analyses of platinum and

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