101 



C II E M I S T R V. 





PART II. 



CHEMICAL EXAMINATION OF NATURK. 



Chemical riAViN'oin the Fir.-t Part of this article given a stifficier.t 

 El *of"v " etai l f tne principles of Chemistry, and described the 

 substances which are usually employed as instruments of 

 investigation, it remains for us now to take a view of the 

 different substances, as they exist in nature, constituting 

 the material world, that we may ascertain how far the 

 science of Chemistry will serve towards explaining their 

 nature, and accounting for the various changes which 

 they produce on each other. Now the different substan- 

 ces of which the material world, as far as we have access 

 to it, is composed, may be arranged under the five fol- 

 lowing heads : 



1. The Atmosphere. 4. Vegetables. 



2. Waters. 5. Animal?. 



3. Minerals. 



We shall take a view of each of these in succession. 



BOOK I. 

 OF THE ATMOSPHERE. 



Of the at. 

 nmpheir. 



Composi- 

 tion of 

 the alroo- 

 ipbcre. 



THE atmosphere is that invisible fluid which surrounds 

 the earth to an indeterminate height, and which got its 

 name from the Greeks, in consequence of the vapours 

 that are continually floating in it. Two things respect- 

 ing it claim our attention ; 1. Its composition. 2. The 

 changes which take place in it. 



CHAP. I. 



Of the Composition of the Atmosphere. 



FOR all the knowledge of the atmosphere which we 

 possess, we are indebted to the moderns. The ancients 

 made no progress in the investigation. Its weight was 

 determined by Galileo, and his disciple Torricelli ; its 

 density, elasticity, and most of its mechanical properties, 

 by Boyle ; its relation to light and sound, by Hooke, 

 Newton, and Derham : but for our knowledge of its com- 

 position, we arc indebted to the labours of the chemists 

 of the last century. Hales began the subject. Dr Black 

 made an additional step. Dr Priestley discovered the 

 substances of which it is composed. But Schcclc and 

 Lavoisier were the philosophers who first placed the sub- 



ject in its true light ; and the matter was in some mea- Chemical 

 sure completed by Mr Cavendish. The atmosphere con- . 

 sists always of the three following distinct parts. 1 . Air. "" u c> ' 

 2. Water. 3. Carbonic acid. \ 



SECT. I. Of Air. 



The word \m is often used to denote the atmosphere Air. 

 in general ; but in strict language it is applied to an elas- 

 tic fluid, which constitutes by far the greatest portion of 

 the atmosphere, and which is distinguished from the 

 moisture and carbonic acid with which it is always mixed. 



Air is an invisible elastic fluid. Its specific gravity h lt> pro- 

 always reckoned 1.000; and its weight is such, that, at pcrtict. 

 the temperature of 60, and when the barometer stands 

 at 30 inches, 100 cubic inches of dry air weigh 30.5 

 grains troy. 



Its density always increases with the pressure, so that 

 its bulk is always inversely as the pressure. Air itself, 

 though it has the property of refracting light, is invisi- 

 ble. "The blue colour of the sky is owing to the vapours 

 with which the air is always mixed ; and it depends up- 

 on the quantity of these vapours. 



Air was determined by Scheele to be a mixture of oxy- 

 gen gas and azotic gas ; and this conclusion was confirm- 

 ed by the experiments of Lavoisier, and of all subsequent 

 chemists. Scheele determined the proportion of these two 

 gases to be 27 by bulk of oxygen gas, and 73 of azotic 

 gas. In these proportions M. Lavoisier acquiesced. 

 Their method was to absorb the oxygen from a given 

 bulk of atmospheric air, and to determine its quantity by 

 the diminution of bulk. Mr Cavcnduh first demonstra- 

 ted, that the proportion determined by Scheele and La- 

 voisier was inaccurate. He found air a mixture of 21 

 parts by bulk of oxygen gas, and 79 of azotic, very 

 nearly. These proportions were confirmed by Berthol- 

 let, Davy, De Marti, and many ether chemists, and are 

 now universally acceded to. 



The method of analysing atmospheric air is to abstract Emlio- 

 its oxygen, by means of some substance which has a strong tmu-r. 

 affinity for it. The apparatus is usually called a einliii- 

 mcter ; because it was supposed at first that the propor- 

 tion of oxygen in air is variable, and that the goodnes* 

 of air depends upon the greatness of this proportion. 

 And the name has been retained after this opinion hat 

 been given up. 



