A 



ATMOSPHERE. 



Volta's cu 

 diometer. 



Armo- riment, we found it pretty exact, provided the diame- 

 ter of the glass vessel employed be not less than three 

 or four inches. In narrow vessels it is very inaccu- 

 rate. 



The second kind of eudiometer was first propo- 

 sed by Volta, and hence is usually known by the 

 name of Volta's eudiometer. It consists in mixing 

 100 parts of the air to be analysed with 100 parts of 

 hydrogen gas in a graduated tube, and passing an 

 electric spark through the mixture. A detonation 

 takes place ; the whole of the oxygen, and part of 

 the hydrogen, being converted into water. This me- 

 thod is very easy, and is susceptible of great preci- 

 sion. From numerous and decisive experiments, it 

 follows, that one part of oxygen combines with two 

 parts of hydrogen, when the experiment is made in 

 this way. It appears, too, that the whole oxygen 

 disappears, provided the quantity of hydrogen pre- 

 sent be sufficient. We have only to mix 100 parts 

 of air and 100 parts of hydrogen gas together, deto- 

 nate the mixture, and observe the diminution of 

 bulk. The third part of that diminution indicates the 

 quantity of oxygen present. Suppose, with the pre- 

 ceding proportions, that the diminution of bulk 

 amounts to 63, the third part of that number, or 21, 

 indicates the quantity of oxygen gas in 100 parts of 

 air. He must be a careless experimenter, that, with 

 this eudiometer, commits an error of 1 per cent. We 

 therefore consider it as one of the best means of de- 

 termining the proportion of oxygen gas present in 

 any gaseous mixture. It does not answer quite so 

 well when we use it to ascertain the purity of oxy- 

 gen gas, or of a gas composed chiefiy of oxygen ; be- 

 cause, in that case, the diminution of bulk in the ga- 

 seous mixture is so sudden and so great, that a va- 

 cuum is formed, and the water over which the expe- 

 riment is made lets go a portion of air, which mixes 

 with the residue, and makes it appear greater than it 

 otherwise would be, and, of course, diminishes the 

 proportion of oxygen which the gas really contained. 

 This error is diminished if the water has been recent- 

 ly boiled. We cannot state the amount of this er- 

 ror ; though we have convinced ourselves that it of- 

 ten exceeds 2 per cent. 

 De Marti's Another method of analysing air, is to expose 

 sndDavy's. 100 measures of it to a solution of sulphuret of lime, 

 or of sulphate of iron saturated with nitrous gas. 

 The first of these liquids was recommended by De 

 Marti, the second by Mr Davy. They both answer 

 very well ; they gradually absorb the whole of the 

 oxygen, and leave the azote : Hence the diminution 

 of bulk gives the oxygen in the air examined. The 

 absorption may be made in a graduated tube, or in a 

 eudiometrical instrument contrived for the purpose by 

 Mr Pepys, and described in the Philosophical Trans- 

 actions for 1808. 

 Berthol- '^'' ie ^ a8t mctno d f analysing air which we shall 



let'*, mention, is that of Berthollet. It consists in pla- 



cing 100 measures of air in contact with a cylin- 

 der of phosphorus. This method succeeds very well 

 in warm weather ; but it does not answer at all in 

 winter. We have kept a cylinder of phosphorus for 

 a fortnight in contact with air, at a temperature a 

 little above the freezing point, and the air only lost 



about 3 per cent, of its bulk ; but if you bring it At*. 

 near the lire, the phosphorus soon absorbs the whole W nr "* 

 oxygen. When the thermometer stands ;it To 1 , the 1 v ' 

 absorption is completed in a few hours. You know 

 the completion by the phosphorus ceasing to smo!. 

 The oxygen, by this process, is removed ; but as the 

 azote dissolves a portion of the phosphorus, its bulk 

 is a little greater than it ought to be. According 

 to Berthollet, to obtain the true bulk of the residua- 

 ry azote, you must diminish it by ^ T d part. This 

 iod is not so convenient as the preceding, at 

 least in this country, because it is so tedious ; but it 

 is sufficiently accurate, 



The third constituent of the atmosphere is car- Proportion 

 bonic acid gas. Its presence in the atmosphere of carbonic 

 was recognised as soon as Dr Black had ascertained aCKl in the 

 the cause of the difference between mild and caustic atmo, P here 

 alkalies : For it was known, that a caustic alkali 

 soon becomes mild by exposure to the air. Dr 

 Black ascertained, that the mildness is owing to the 

 ah :<>rption of carbonic acid. From the observations 

 of Saussure we learn, that this gas exists in the at- 

 mosphere on the summit of Mount Blanc, which is 

 nearly 16,000 feet above the level of the sea ; for lime- 

 water soon deposited its lime in the state of carbon- 

 ate, when exposed upon the summit of that mountain, 

 (Saussure's Voyages, iv. 199.) Humboldt found 

 it in a quantity of air brought down by Garneria 

 from a height of 42S0 feet, to which he had ascend- 

 ed in an air balloon, (Jour, de P/iys. xlvii. 202.) 

 It appears, therefore, to constitute a part of every 

 portion of the atmosphere to which we have access. 



As this acid gas is produced in great quantities by * 



combustion, respiration, fermentation, and many other 

 of the most common processes of nature, one would 

 be disposed to believe, at first view, that its quantity 

 must be constantly increasing. But this does not ap- 

 pear to be the case, it must therefore be decompo- 

 sed and separated from air as fast as it is formed. 

 It is of so deleterious a nature, that, if it were to 

 accumulate to any extent, it would render air inca- 

 pable of supporting life. A candle will not burn 

 in air contaminated with one-ninth of carbonic acid 

 gas. 



The quantity of this gas in air is small. Many 

 attempts have been made to ascertain it ; but the pro- 

 cess is so difficult, that absolute precision cannot be 

 looked for. It was long believed that the carbo- 

 nic acid present in the atmosphere amounted to one 

 per cent. Humboldt made many experiments on 

 the subject, and concluded from them, that its bulk 

 varied from one per cod. to half a per cent. But 

 this determination is certainly excessive. According 

 to the experiments of Mr Dahon, a quantity of air, 

 equal in bulk to 102,400 grains of water, contains a 

 quantity of carbonic acid just capable of saturating 

 12.5 grains of lime-water: 70 measures of carbonic 

 acid gas would produce the same effect : Hence he 

 concludes, that the atmosphere contains , 4 ' 6A th part 

 of its bulk of carbonic acid gas, (Phil. Mag. xxiii. 

 354. ) This quantity we consider as rather below 

 the truth. Mr Cavendish has shewn, that lime-water 

 is not capable of depriving air completely of carbonic 

 acid gas : Hence a portion would still remain in Mr 



