CHEMISTRY. 



105 



Chemical 

 Examina- 

 tion of 

 Mature. 



First used 

 by Dr 

 Priestley. 



Mr Dal- 

 ton's eudio 

 meter. 





The firit eudiometer used was by Dr Priestley. It 

 consisted in mixing 100 measures of air and 100 of ni- 

 trous gas in a tube. The goodness of the air was jud- 

 ged of by the greatness of the diminution. This me- 

 thod was improved by Fontana, and is usually distin- 

 guished by the name of the eudiometer of Fontana. 

 Many attempts have been made to render it accurate ; 

 but it is in reality not much to be trusted, because the 

 diminution of bulk varies with a variety of circumstan- 

 ces, which cannot easily be appreciated. It is greatest 

 when a wide vessel is used, and least when the mixture 

 is made in a narrow tube. According to Dalton, 21 

 measures of oxygen unite either with 36 measures of ni- 

 trous gas, or with 36x2=72 measures. The first pro- 

 portion takes place in a narrow tube ; the second in a 

 wide vessel ; in intermediate vessels the proportions are 

 intermediate. Hence he uses a narrow tube. He lets 

 up 100 measures of air, and then adds 50, or any great- 

 er number of measures of nitrous gas. The mixture 

 takes place slowly, and the bulk gradually diminishes. 

 In a few minutes, the diminution of bulk will have reach- 

 ed its maximum. Note it down, and multiply it by 

 T V=0.3684'. The product is the quantity of oxygen 

 which the air examined contained. Suppose 100 mea- 

 sures of air to be examined, and the diminution of bulk 

 to be 57 i 



57X0.3681=20.9988, or very nearly 21. 



This would indicate the oxygen in the air examined to 

 be 21 measures. This method affords a tolerable ap- 

 proximation ; but the error is generally greater than 1 

 per cent. 



Mr Davy's To avoid the ambiguity occasioned by the unequal 

 eudiometer, diminution of bulk which takes place when air and ni- 

 trous gas are mixed, Mr Davy takes a strong solution 

 of green sulphate of iron, makes it absorb a quantity of 

 nitrous gas, and then agitates this liquid in the air to be 

 examined. The whole oxygen is absorbed, and the di- 

 minution of bulk determines exactly how much oxygen 

 the air contained. This method may be used, and is 

 susceptible of great accuracy. The only objection to 

 it, is the slowness with which the absorption of the oxy- 

 gen takes place. 



The liquid, made by boiling a mixture of lime and 

 sulphur in a phial of water, which is yellow, may also 

 be used with considerable precision. It was employed 

 by Scheele, and in an improved form by De Morti. A 

 stick of phosphorus in summer, may likewise be used 

 with advantage. The residual gas ought to be agitated 

 in water, to deprive it of a little phosphorus, wnich it 

 usually holds in solution, and which may increase its 

 bulk a little. 



But the best method of analysing air is the follov.-ing, 

 which was first put in practice by Vclta, and is therefore 

 called Volta'e eudiometer. Mix equal bulks of air and 

 hydrogen gas together, and pass an elrctric spark through 

 the mixture. Combustion takes place. Note the dimi- 

 nution of bulk ; one-third of that diminution is equal to 

 the bulk of oxygen which the air examined contained. 

 The result of all the trials hitherto made, gives us 



Volta's 



eudiome- 

 ter. 



Composi- 

 tion of air. a j r a mixture of 21 measures of oxygen, and 79 e? azote ; 

 or by weight, of 



22.91 oxygen, 

 77.09 azote. 



100.00 



VOL. VI. PART I. 



SECT. II. Of Water. 



That water is always present in the atmosphere, is 

 obvious from the clouds which constantly form in it, 

 and from the rain and dew which are so frequently fall- 

 ing from it. Various instruments have been contrived 

 to point out the quantity of water which the atmosphere 

 contains. These instruments are called hygrometers. 

 None of them are unexceptionable ; but, upon the whole, 

 the one contrived by Professor Leslie seems to be the 

 best. From these instruments we learn, that the quan- 

 tity of water in the atmosphere is exceedingly various. 

 Sometimes very little can be detected, and sometimes the 

 air is quite saturated with it. 



From the experiments of Saussure, we learn, that a 

 cubic foot of air, at the temperature of 66, is capable of 

 holding only about 8 grains troy, or jV'h its weight of 

 water. The quantity increases with the temperature, 

 and, according to Leslie, doubles for every additional 

 27 of temperature. Hence it is obvious, that the quan- 

 tity of water in air depends, in some degree, on the tem- 

 perature. At low temperatures, very little water can 

 be present ; but at high temperatures, air can contain a 

 considerable quantity. 



With respect to the state in which the water is con- 

 tained in the air, there are two opinions which have di- 

 vided philosophers. The first opinion is, that the water 

 it dissolved by the air, and held in solution by it precise- 

 ly as sugar or salt is by water. This opinion was started 

 by Dr Hooke, explained at considerable length by Dr 

 Halley, and fully discussed and supported by Leroy and 

 Hamilton. The second opinion is, that the water is first 

 converted into vapour, and that in that state it mixes 

 with air precisely as one elastic fluid with another. This 

 opinion, though with different modifications, was sup- 

 ported both by Saussure and Deluc, and has lately been 

 maintained by Dalton, with arguments that appear deci- 

 sive. When water evaporates or enters into air, it al- 

 ways produces cold, indicating a conversion into vapour. 

 The same quantity of water evaporates, and continues in 

 the state of vapour under an exhausted receiver, and in a 

 receiver full of air. Vapour increases the bulk of air 

 exactly in proportion to its quantity. 



From the experiments of Dalton we learn, that in the 

 torrid zone the quantity of vapour in the atmosphere va- 

 ries from being able to support a column of mercury 0.6 

 inch in height, to being able to mpport a column 1 inch 

 high. In Britain it seldom amounts to 0.6, but in sum- 

 mer is frequently at 0.5 inch. In winter it is often as 

 low as 0.1 inch. 



From this it appears, that the quantity of vapour 



Chemicai 

 Examina- 

 tion of 

 Nature. 



Water. 



Hygrome- 

 ter. 



in the atmosphere varies from 7 ;7 tn to 



P art 



the atmosphere by weight. Mr Dalton supposes the 

 medium quantity at once in the atmosphere to amount 



to about y'jth of its bulk. 



SECT. III. Of Carbonic Acid. 



The existence of carbonic acid in the atmosphere be- Carbonic 

 came manifest, as soon as it had been determined by Dr acid. 

 Black, that alkalies owe their mild state to the presence 

 of that gas. For it was known that a caustic alkali be- 

 comes gradually mild, when left exposed to the atmo- 

 sphere. It haj been found in air, upon the tops of moun- 

 tains as well as in plains. Even air, brought by means 

 t;f a balloon from the height of 4280 feet above the sur- 

 face of the earth, was found to contain it. Hence we 

 o 



