varies at different times and in different places; and that upon this variation depended the purity or noxious qualities of air. Hence it became an object of the greatest importance to get possession of a method to determine readily the quantity of oxygen in a given portion of air. Accordingly various methods were proposed, all of them depending upon the property which many bodies possess, of absorbing the oxygen of the air without acting upon its azote. These bodies were mixed with a certain known quantity of atmospheric air in graduated glass vessels inverted over water, and the proportion of oxygen was determined by the diminution of bulk. These instruments received the name of eudiometers, because they were considered as measures of the purity of air. The eudiometers proposed by different chemists may be re duced to five. 1. The first eudiometer was made in consequence of Dr.Priestley's discovery, that when nitrous gas is mixed with air over water, the bulk of the mixture diminishes rapidly, in consequence of the combination of the gas with the oxygen of the air and the ab. sorption of the nitric acid thus formed by the water. When nitrous gas is mixed with azotic gas, no diminution at all takes place. When it is mixed with oxygen gas iu proper proportions, the absorption is complete. Hence it is evident, that in all cases of a mixture of these two gases the diminution will be proportional to the quantity of the oxygen. Of course it will indicate the proportion of oxygen in air; and by mixing it with different portions of air, will indicate the different quantities of oxygen which they contain, provided the component parts of air be susceptible of va riation. Dr. Priestley's method was to mix together equal bulks of air and nitrous gas in a low jar, and to transfer the mixture into a narrow graduated glass tube about three feet long, in order to mea. sure the diminution of bulk. He expressed this diminution by the number of hundred parts remaining. Thus, suppose he had mixed together equal parts of nitrous gas and air, the sum total of this mixture was 200 (or 2·00): suppose the residuum when mea. sured in the graduated tube to amount to 104 (or 1.04), and of course that 96 parts of the whole had disappeared, he denoted the purity of the air thus tried by 104. A more convenient instrument was invented by Dr. Falconer of Bath; and Fontana greatly im 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. His 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. clude that the proportion between the oxygen and azote in common 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 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, ob tained 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 Phil. Trans. 1808, p. 107. 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 uniting 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 maximum 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 +. 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. 351. 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 perfection. 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 perfectly. 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 atmospheric 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 divided 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 diminution of its bulk *. Jour. de Phys. lii, 176. Air examined by this process suffers precisely the same diminu. tion in whatever circumstances the experiments are made no variation 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 season 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 oxygen 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 bulks 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. |