COMPOSITION OF THE ATMOSPHERE. 15 



It was with these results before me that I began the 

 inquiry; and althoftgh not expected to speak of the composi- 

 tion of the air generally, it seemed quite necessary to make 

 some experiments, that it might be compared with the air 

 of mines. 



Having this great array of analyses, an abstract of the 

 life-labours of many men, we seem to have before us proof 

 sufficient of a decided variation in the composition of the air. 

 The variation is really so great in some places, that we must 

 admit some powerful local cause. In filthy places, and in 

 marshy spots at a high temperature, the cause cannot be 

 doubted ; and how can we doubt the same change to occur 

 in places that are badly ventilated ? When carbonic acid 

 increases, is it wonderful that oxygen should diminish ? 

 Notwithstanding all this, there is no firmly founded faith 

 amongst scientific men regarding the subject. The evidence 

 has not been brought fully before them, and probably some 

 hnks may be wanting. Seeing the question in this state, 

 I was desirous of throwing some light upon it, and indeed 

 had already come to believe strongly in the variations, 

 because of some which had been observed when making 

 occasional analyses for the trial of new apparatus, or for 

 instruction and trial of skiU. 



Under this impression, specimens of air were collected 

 from the front of the laboratory and from behind, near an 

 ash-pit, each at the same time; and the analyses, along 

 with some others, are given in the following Table. 

 Afterwards it will be seen that the carbonic acid of the 

 same spot was also estimated ; and the result is that not 

 only is there a diminution of oxygen in the less pure spot, 

 but the carbonic acid, although greater than in the pure air, 

 is not sufficient to make up the vacancy left by the deficiency 

 of oxygen, leading us to look for other gases also that tend 

 to increase the impurity. This is an unexpected result, 

 and, to my mind, one which has in it much value. But, 



