Till-; KA:(ni. 2'.10 



ous supplies ; plants receive arid return tlieir ^liare ; and ani- 

 mals, that by living upon, consnaie this general store, are found 

 to give it back in great quantities when they die.' The air 

 therefore, that we breathe, and upon which we subsist, bears 



fjas shown, that lime-uater is not capaKle of depriving- air cnmpletely of car. 

 boDic acid gas : Hence a portion would still remain in Mr Dalton's exppri. 

 ment. Perhaps we shall not err far if we state the bulk of carbonic acid gas 

 in the atmosphere at -jQ^jToth part. 



The 4th constituent of the atmosphere is water in the state of vaponr. 

 That water forms a constituent p.irt of the atmosphere, has been known in 

 all a^res, and indeed is demonstrated by the rain and dew which is continually 

 falling, and by the great quantity of moisture which sulphuric acid, potash, 

 and other bodies, absorb when e.x posed to the atmosphere. The quantity 

 of moisture in the atmosphere has been observed to vary greatly at ditferent 

 times, and various instruments have been invented to measure that quantity. 

 These instruments are called hygrometers. The most ingenious of them are 

 those of Leslie, Saussure, and De Luc. It was at first supposed, that the 

 water in the atmosphere was still in the state of water, and that it was held 

 in solution in air precisely as salts are dissolved in water. But it has been at 

 last established by satisfactory experiments, that the water in the atmosphere 

 is in the state of vapour. To De Luc, Saussure, and Daltou, we are chiefly 

 indebted for these experiments. As to the quantity of water which exists 

 in the atmosphere, it depends upon a variety of circumstances. Saussure 

 found that a cubic foot of air, saturated with moisture at 660, contains about 

 8 grains troy of water, or g^yth of its weight. Supposing air always satu- 

 'ated with moisture, the quantity always increases with the temperature, be- 

 cause the elasticity of aqueous vapour increases with the temperature. 

 Hence, in cold weather, the quantity of vapour in air is always small ; 

 whereas, in warm weather, it is often considerable. In the torrid zone the 

 aqueous vapour in the atmosphere is capable of supporting from 0.6 to I inch 

 of mercury. In Britain it is hardly ever capable of supporting 0.6 inch of 

 mercury ; but in summer it is often capable of supporting 0.5 inch, while in 

 winter it often does not exceed 0.1 inch. From these facts it follows, that 

 the weight of water present in the atmosphere varies from J^ to 3-^ of the 

 whole. Mr Daltcii supposes, that the medium quantity of vapour held iu 

 solution at once in the atmosphere, may amount to y^jth of its bulk. 

 - These four bodies, oxygen, azote, carbonic acid, and vapour, are tlie only 

 known constituents of the atmosphere. It cannot be doubted, that other 

 bodies are occasionally present iu it. The dreadful eflects of marshy situ.i. 

 tions upon the health of the inhabitants, and the fatal rapidity with which 

 certain diseases are propagated, cannot well be accounted for, without sup- 

 posing that certain substances which produce a deleterious effect on the ani. 

 mal economy, are occasionally present in the atmosphere. But hitherto no 

 method has been discovered of ascertaining the presence of these bodies, and 

 subjecting them to examination. They are too subtile for our apparatus, 

 and altogether escape the cognizance of our senses. It has been ascertainc<^ 



1 Boyle, vol. ii. p. 593. 

 6 3 



