6?7 



VOLUMETRIC ANALYSIS. 



VORTEX. 



673 



chromates, chlorates, araenites ; the peroxides of iron, manganese, lead, 

 nickel, &o., protoxide of iron, &c. The fact on which the method is 

 based is as follows : that when iodine is brought into contact with 

 an aqueous solution of sulphurous acid, containing not more than 0'04 

 or 0-05 per cent, by weight of anhydrous acid, hydriodic and sulphuric 

 acids are respectively formed : 



I + HO + SO, = HI + SO, 



Iodine. Water. Sulphurous llydriodic Sulphuric 

 acid. acid. acid. 



The standard solution of iodine is made by dissolving five grammes 

 of perfectly pure iodine in a concentrated solution of pure iodide of 

 potassium, and making up the liquid to exactly one litre with pure 

 distilled water. This solution ( = 1000 cub. cent.) will obviously con- 

 tain O'OOS gram, of iodine in every cubic centimetre. The sulphurous 

 acid liquid-is made by adding 35 or 40 cub. cent, of a saturated solu- 

 tion of sulphurous acid to 5000 cub. cent, of water, introducing a little 

 starch paste into some of the solution, and then pouring in the 

 standard solution of iodine until after brisk agitation a distinct blue 

 coloration remains. The value of this sulphurous solution having been 

 calculated according to the above equation, it must be diluted till it 

 contains 0'03 gram, of sulphurous acid in 100 cub. cent. The third 

 test-liquid necessary is one containing about one gramme of pure iodide 

 of potassium in 10 cub. cent, of water. In using these standard 

 solutions for estimating an unknown quantity of free iodine in any 

 substance, a weighed quantity of the latter is taken, the iodide of 

 potassium solution added to dissolve the iodine, (if not already in 

 solution) and the sulphurous acid solution then added until the brown 

 colour of the iodine haa disappeared. To effect this, an excess of the 

 sulphurous acid is necessary, but that excess is determined by mixing 

 a small quantity of starch paste with the liquid and then adding the 

 standard solution of iodine until the blue colour appears and remains 

 permanent. The quantity of iodine equivalent to the total amount of 

 sulphurous acid used, less the quantity of iodine afterwards added to 

 neutralise excess of sulphurous acid, will be the amount of iodine 

 present in the portion of substance that was submitted to examination. 

 The great value of this volumetric method of Bunsen depends upon the 

 circumstances that so many bodies may be made directly or indirectly 

 to liberate iodine from iodide of potassium, and that the reactions 

 which occur in the processes are constant, accurately defined, and well 

 known. Any body also which is decomposed by iodine in a known 

 manner can be thus estimated. For example, an unknown quantity 

 of sulphuretted hydrogen in any fluid is at once determined on adding 

 to a given amount of the fluid, first a little starch paste and then the 

 standard iodine solution until a permanent blue (iodide of starch) is 

 produced : as the equivalent of iodine is to that of sulphuretted 

 hydrogen, so is the amount of iodine used to the amount of sulphu- 

 retted hydrogen sought for. Hypochloritea are valued according to 

 this method, by mixing a solution of a known weight of the salt with 

 the teat solution of iodide of potassium ; adding hydrochloric acid till 

 an acid reaction is obtained, and then determining the amount of 

 liberated iodine in the manner already described. Besides a large 

 number of substances which directly liberate chlorine, and indirectly 

 therefore, iodine, those substances which are readily and perfectly 

 oxidised by chlorine also come within the range of this method : such 

 bodies may be heated with a known weight of bichromate of potash 

 and hydrochloric acid ; every two equivalents of the chromic acid yield 

 under these circumstances three equivalents of chlorine, excess of 

 which must of course be used, and that excess determined, as above 

 indicated, by the amount of iodine it will liberate. For other examples 

 of the adaptation of Bunsen's method, see the English translation of 

 Fresenius's ' Quantitative Analysis,' third edition. 



Gay-Lussac's volumetric method of estimating silver by a standard 

 solution of chloride of sodium, has already been described under 

 ASSAVIXO. The converse is of course equally easy, namely, the deter- 

 mination of the amount of chlorine in any chloride by a standard 

 solution of nitrate of silver. Mohr has shown that this method admits 

 of very general application : thus ammonia may be estimated by adding 

 excess of hydrochloric acid, evaporating to dryness. and determining 

 the amount of chlorine in the residue; the resulting chloride of 

 ammonium being a definite salt, it is only necessary to know the 

 amount of chlorine in the residue and then a simple calculation will 

 give the quantity of ammonia experimented on. Nitrogen may be 

 determined in a similar manner after conversion into ammonia by soda 

 lime. Carbonic acid may be absorbed in a mixture of chloride of 

 barium and ammonia, the carbonate of baryta collected on a filter, 

 washed, dissolved in hydrochloric acid, the solution evaporated to 

 dryness, and the chlorine estimated as before ; the weight of the latter 

 being, of course, transformed by calculation into an equivalent quantity 

 of carbonic acid. Carbonates or nitrates are converted into chlorides, 

 the chlorine estimated, and the equivalent of nitrate or carbonate 

 calculated. Salts of organic acids may be altered to carbonates by 

 ignition, and the carbonates treated as already described. Chlorates, 

 iod.ttes, and bromates are reduced to chlorides, iodides, and bromides, 

 and also acted upon in the way indicated. 



Permanganate of potash is a re-agent of considerable value in the 

 volumetric analysis of substances which readily absorb definite quan- 



of oxygen. Possessed of an intense purple colour, its solution 

 freely gives up its oxygen and becomes almost colourless. It is best 

 standarised by noting the quantity necessary to be added to a known 

 amount of a definite protosalt of iron, such as the ammonio-protosul- 

 phate, before a permanent purple tint is produced : this of course only 

 occurs when the protoxide of iron is wholly converted into peroxide. 

 After the strength of the permanganate of potash solution has been 

 thus fixed, it can be used for estimating an unknown amount of pro- 

 toxide of iron in a weighed quantity of a mixture of the two salts and 

 for other similar operations. 



In all the above described processes of volumetric analysis, as well 

 as in those only referred to, a balance and weights or saturated solu- 

 tions of definite strength are necessary at some stage of the operations. 

 Quantitative determinations can, however, be made without the use of 

 weights, with volumetric solutions of unknown strength, and the 

 strength of which need not be ascertained or regulated. For example, 

 let it be required to know the amount of carbonate of soda in a 

 specimen of crude soda-ash. Place on one pan of a balance a piece of 

 chemically pure carbonate of soda and counterpoise it by the crude 

 ash ; dissolve in separate quantities of water, tinge the solutions by 

 litmus, then add dilute acid from a graduated measure until both are 

 saturated ; suppose, now, that twenty-two volumes of acid were 

 required for the pure soda and sixteen for the impure, then as 

 22 : 16 : : 100 : x, a calculation that shows the crude ash to contain 

 72'7 per cent, of pure carbonate of soda. Many other determinations 

 may obviously be made on the same principle. The substance to be 

 determined need not even be available in the pure state ; one of its 

 compounds must, however, be attainable in that condition, and both 

 that compound and the impure body must be capable of being 

 similarly acted upon by the test-liquid employed. For instance, in the 

 example above worked out, pure carbonate of lime may be substituted 

 for carbonate of soda, but the number of volumes of test-liquid used 

 must be added to or subtracted from according as the equivalent of 

 the substituting body is higher or lower than that of the body 

 substituted. 



VOLUNTARY SETTLEMENT. [SETTLEMENT.] 

 VOLUTE, a rolling or spiral curve, a name particularly given to the 

 spirals which appear in architectural columns as ornaments of the 

 capitals. The Ionic volute (figured in COLUMN, col. 49) is that which 

 is of most interest. There has been, we believe, some discussion as to 

 what the form of this curve really was. Whether the architect of a 

 Greek temple employed anything but his eye to give an agreeable iorm, 

 we do not know ; but a mathematician would say beforehand that it 

 would be hardly possible to draw such a number of concentric spirals 

 not interfering with each other as are seen in the diagram above cited, 

 unless each of them was tolerably near .to a logarithmic spiral, in 

 which the tangent always makes the same angle with the radius. We 

 examined with particular attention a cast made at Athena by Professor 

 Donaldson, and found the following result : Taking the diagram in 

 COLUMN (coL 52), we found that each spiral, as far as c, or there- 

 abouts, was remarkably true to the logarithmic spiral : but that from 

 c the law of the curve changed, and the acute angle made by the 

 tangent with the radius vector began to increase, until it became a 

 right angle at A. 



VOM1CINE. [Nux VOMICA, ALKALOIDS OF ; Urucine.] 

 VORTEX. The theory of Descartes on the formation and mechanical 

 laws of the universe was first published in 1637, in his ' Principia 

 Philosophise.' One part of this theory, namely, the hypothesis of 

 vortices, is almost the only one which generally passes by the name of 

 Des Cartes. But it should be remembered that this is only a part, and 

 a small part, of the system which rendered the Newtonian view of 

 astronomy for a long tune unacceptable to the continental philosophers. 

 The metaphysics, the mechanics, and the astronomy, of the once cele- 

 brated Cartesian system, combined as they were by one writer, and 

 that writer a most skilful and elegant proposer of his own views, are to 

 be looked at together as that which Newton's philosophy had to meet. 

 Perhaps we should not be wrong in saying that the impossibility of a 

 vacuum, maintained by Des Cartes as self-evident, was a greater ob- 

 stacle hi the way of the theory of gravitation, with which it seemed 

 incompatible, than the theory of vortices, which Des Cartes proposed 

 as an hypothesis, and which did not necessarily contradict Newton's 

 deductions. We shall here present a brief sketch of the system, so far 

 as is necessary, from the third book of the ' Principia Philosophise : ' 

 this sketch is, as far as it goes, only a table of contents of the work 

 itself. 



The human imagination must not either limit the power of God or 

 unduly exalt its own : and it must not suppose that all things were 

 made for man's use only. In enumerating phenomena Deacartea pre- 

 fers rather to deduce them from causes than to make them serve in 

 finding causes. He then describes the relative distances of the planets, 

 and asserts the immensity of the distances of the fixed stars. After 

 the usual statements relative to the light of the sun, planets, and fixed 

 stars, he rejects the Ptolemaic hypothesis, and observes that those of 

 Copernicus and Tycho Brahd differ very little as hypotheses, and ex- 

 plain phenomena in the same manner. He says also that Tycho, 

 though he denies the motion of the earth, yet in reality gives it more 

 motion than the former (with Descartes, relative motion was a most 

 absolute idea) ; whence, differing from both, he will, with more truth 



