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proved this method of measuring the purity of air. A description of bis eudiometer was published by Ingenhousz in the first volume of his Experiments; but it was Mr.Cavendish who first brought this eudiometer to such a state of precision as to be enabled to ascertain correctly the constituents of air. Ilis method was to put 125 measures of nitrous gas into a glass vessel, and to let up into it very slowly 100 measures of the air to be examined, agitating the vessel containing the nitrous gas during the whole time. The diminution of bulk when the process was conducted in this way was almost uniform, The greatest was 110, the least 106.8; the mean 108.2. The variation he found to depend, not upon the air examined, but upon the state of the water in which the experiment was made. If this experiment was reversed, by letting up the nitrous gas to common air, he used 100 measures of each, and the diminution in that case was only 90 measures.

This constancy in the diminution of the bulk of all the different specimens of common air examined, induced Mr. Cavendish to con. clade that the proportion between the oxygen and azote in com. mon air does not vary. To find the absolute quantity of oxygen in air, he mixed together oxygen gas and azote in various propor. tions, and at last found that a mixture of 10 measures of the purest oxygen which he could procure with 38 measures of azote, was just as much diminished by the nitrous gas as the same bulk of common air. Hence he concluded that air is composed of 10 parts by bulk of oxygen and 38 of azote, which gives us for its composition per cent.

79.16 azote

20:84 oxygen

100.00 or very nearly 21 per cent. of oxygen gas *.

Other philosophers, who did not pay that rigid attention to pre. cision which characterises all Mr. Cavendish's experiments, obtained variable results from the nitrous gas eudiometer. Most of the circumstances which occasion the variation were pointed out by Cavendish ; but they seem to have escaped the observation of succeeding chemists. Humboldt's attempt to render the eudiome. ter of Fontana accurate did not succeed *. But Mr. Dalton has lately explained the anomalies in a very luminous manner. Ac. cording to this philosopher, oxygen gas and nitrous gas are capable of uniting in two proportions: 21 measures of oxygen gas upiting either with 36 measures of nitrous gas, or with twice 36, = 72 measures. Both of these compounds are soluble in water. If the tube be wide, a considerable portion of nitrous gas comes at once in contact with the oxygen. Hence the latter gas combines with a maxi. mom of nitrous, especially if agitation be employed. In a narrow tube the oxygen combines with the minimum of nitrous gas, provided no agitation be employed, and the residue be poured soon into another vessel. When intermediate proportions are used, the absorption will be intermediate. Mr. Dalton recommends a narrow tube; the nitrous gas is to be only in the proportion requisite to form the minimam combination; no agitation is to be employed ; and when the diminution is completed, the gas must be transferred to another tube. To 100 measures of air add about 36 of nitrous gas; note the diminution of bulk, and multiply it by 7.19ths; the product gives the bulk of oxygen in the air examined +.

* Phil. Trans. 1808, p. 107.

In order to get rid of the anomalies which had perplexed former experimenters, Mr. Davy proposed to employ the nitrous gas in a different state. He caused sulphate or muriate of iron to absorb this gas to saturation, and employed the dark brown liquid thus obtained to deprive air of its oxygen. A small graduated glass tube, filled with the air to be examined, is plunged into the nitrous solution, and moved a little backwards and forwards. The whole of the oxygen is absorbed in a few minutes. The state of greatest absorption ought to be marked, as the mixture afterwards emits a little gas, which would alter the result. By means of this Mr. Davy examined the air at Bristol, and found it always to contain about 0.21 of oxygen. Air sent to Dr. Beddoes from the coast of Guinea gave exactly the same result.

2. For the second kind of eudiometer we are indebted to Scheele. It is merely a graduated glass vessel, containing a given quantity of air exposed to newly prepared liquid alkaline or earthy sulphurets, or to a mixture of iron filings and sulphur, formed into a paste with water. These substances absorb the whole of the

Ann. xxvii. p.

+ Dalton, Phil. Mag. xxiii. 331.

oxygen of the air, which converts a portion of the sulphur into an acid. The oxygen contained in the air thus examined, is judged of by the diminution of bulk which the air has undergone. This method is not only exceedingly simple, but it requires very little address, and yet is susceptible of as great accuracy as any other whatever. The only objection to which it is liable is its slowness; for when the quantity of air operated on is considerable, several days elapse before the diminution has reached its maximum.

But this objection has been completely obviated by Mr. De Marti, who has brought Scheele's eudiometer to a state of perfec.

He found that a mixture of iron filings and sulphur does not answer well, because it emits a small quantity of hydrogen gas, evolved by the action of the sulpuric acid formed upon the iron; but the hydrogureted sulphurets, formed by boiling together sul. phur and liquid potash or lime water, answered the purpose per. fectly. These substances, indeed, when newly prepared, have the property of absorbing a small portion of azotic gas; but they lose this property when saturated with that gas, which is easily effected by agitating them for a few minutes with a small portion of at. mospheric air. His apparatus is merely a glass tube, ten inches long, and rather less than half an inch in diameter, open at one end, and hermetically sealed at the other. The close end is di. vided into 100 equal parts, having an interval of one line between each division. The use of this tube is to measure the portion of air to be employed in the experiment. The tube is filled with wa. ter; and by allowing the water to run out gradually while the tube is inverted, and the open end kept shut with the finger, the gradu, ated part is exactly filled with air. These hundred parts of air are introduced into a glass bottle filled with liquid sulphuret of lime previously saturated with azotic gas, and capable of holding from two to four times the bulk of the air introduced. The bottle is then to be corked with a ground glass stopper, and agitated for five minutes. After this the cork is to he withdrawn while the mouth of the phial is under water; and for the greater security, it may be corked and agitated again. After this, the air is to be again trans. ferred to the graduated glass tube, in order to ascertain the dimibution of its bulk *.

• Jour. de Phys. lii. 176.

Air examined by this process suffers precisely the same dimina. iion in whatever circumstances the experiments are made : no va. riation is observed whether the wind be high or low, or from what quarter soever it blows ; whether the air tried be moist or dry, hot or cold; whether the barometer be high or low. Neither the sea. son of the year, nor the situation of the place, its vicinity to the sea, to marshes, or to mountains, make any difference., Mr. De Marti found the diminution always between 0.21 and 0.23.

3. The third kind of eudiometer was proposed by Volta. The substance employed by that philosopher to separate the oxygen from the air was hydrogen gas. His method was to mix given proportions of the air to be examined and hydrogen gas in a graduated glast tube ; to fire the mixture by an electric spark; and to judge of the purity of the air by the bulk of the residuum. This method has been lately examined by Gay Lussac and Humboldt. 'They have found it susceptible of great precision. It is one of the simplest and most elegant methods of estimating the proportion of bxygen air. When 100 measures of hydrogen are mixed with 200, or any greater bulk of oxygen, up to 900 measures, the di, minution of bulk after detonation is always 146 measures. The same diminution is obtained if the hydrogen be increased up to a certain quantity. The result of their trials is, that 100 measures of oxygen gas require 200 of hydrogen for complete combustion, which coincides very well with the trials previously made in this country. Hence the method of using this eudiometer is very simple: Mix together equal bukks of the air to be examined and of hydrogen gas, ascertain the diminution of bulk after combus., tion, divide it by three, the quotient represents the number of measures of oxygen in the air. A great number of trials, in dif. ferent seasons of the year, of mixtures of 200 measures of air and as much hydrogen, gave almost uniformly a diminution of bulk amounting to 126 measures. Now the third of 126 is 42, the quantity of oxygen is 200 measures of air. Hence 100 parts of air, according to these trials, contain 21 of oxygen *.

4. In the fourth kind of eudiometer, the abstraction of the oxygen of air is accomplished by means of phosphorus. This eudiometer was first proposed by Achard +. It was considerably

* Jour. de Phys. Ix. 129.

+ Ibid. 1784, vol. i.

improved by Reboul*, and by Seguin and Lavoisier t; but Ber. thollet has lately brought it to a state of perfection.

Instead of the rapid combustion of phosphorus, this last philosopher has substituted its spontaneous combastion, which absorbs the oxygen of air completely; and when the quantity of air opea rated on is small, the process is over in a short time. The whole apparatus consists in a narrow graduated tube of glass containing the air to be examined, into which is introduced a cylinder of phosphorus fixed upon a glass rod, while the tube stands inverted over water. The phosphorus should be so long as to traverse nearly the whole of the air. Immediately white vapours rise from the phosphorus and fill the tube. These contique till the whole of the oxygen combines with phosphorus. They consist of phos. phorous acid, which falls by its weight to the bottom of the vessel, and is absorbed by the water. The residuum is merely the azotic gas of the air, holding a portion of phosphorus in solution. Berthollet has ascertained, that by this foreign body its bulk is increased 1.40th part. Consequently the bulk of the residuum, diminished by 1.40th, gives us the bulk of the azotic gas of the air examined; which bulk, substracted from the original mass of air, gives us the proportion of oxygen gas contained in it.

All the different experiments which have been made by means of this eudiometer agree precisely in their result, and indicate that the proportions of the ingredients of air are always the same; namely, about 0.21 parts of oxygen gas, and 0·79 of azotic gas. Berthollet found these proportions in Egypt and in France, and they have been found constantly in Edinburgh, in all the different seasons of the year.

Thus it appears, that whatever method is employed to abstract oxygen from air, the result is uniform, provided the experiment may be precisely made. They all indicate that common air consists very nearly of 21 parts of oxygen and 79 of azote. Scheele and Lavoisier found 27 per cent. of oxygen, but their methods

• Ann. de Chim. xiii. 38.
+ Ann. de Chim. ix. 293.
# Ibid. xxxiv. 78. and Jous. de l'Ecole Polytecho. I. iii. 274.

A very convenient apparatus for making eudiometrical experiments, has lately been invented by Mr. Pepys, and described by him in Pail. Trans, for 1807.

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