VOL. LXIV.] PHILOSOPHICAL TRANSACTIONS. 503 



already saturated with the putrid effluvium, they cannot readily take any more; 

 but Dr. Alexander was not aware, that air thus loaded with putrid effluvium, is 

 exceedingly noxious when taken into the lungs. I have lately however had an 

 opportunity of fully ascertaining how very noxious such air is. 



Happening to use at Calne, a much larger trough of water, for the purpose 

 of my experiments, than I had done at Leeds, and not having fresh water so 

 near at hand as I had there, I neglected to change it, till it turned black, and 

 became offensive, but by no means to such a degree as to deter me from making 

 use of it. In this state of the water, I observed bubbles of air to rise from it, 

 and especially in one place, to which some shelves, that I had in it, directed 

 them ; and having set an inverted glass vessel to catch them, in a few days I 

 collected a considerable quantity of this air, which issued spontaneously from the 

 putrid water; and, putting nitrous air to it, I found that no change of colour or 

 diminution ensued ; so that it must have been, in the highest degree, noxious. 

 I repeated the same experiment several times afterwards, and always with the 

 same result. 



After this, I had the curiosity to try how wholesome air would be affected by 

 agitation in this water; when, to my real surprize, I found, that after one 

 minute only, a candle would not burn in it; and, after 3 or 4 minutes, it was in 

 the same state with the air which had issued spontaneously from the same water. 

 I also found, that common air, confined in a glass vessel, in contact only with 

 this water, and without any agitation, would not admit a candle to burn in it 

 after 2 days. 



These facts certainly demonstrate, that air, which either arises from stagnant 

 and putrid water, or which has been for some time in contact with it, must be 

 very unfit for respiration; and yet Dr. Alexander's opinion is rendered so plausible 

 by his experiments, that it is very possible that many persons may be rendered 

 secure, and thoughtless of danger, in a situation in which they must necessarily 

 breathe it. On this account, I have thought it right to make this communication 

 as early as I conveniently could; and as Dr. Alexander appears to be an 

 ingenuous and benevolent man, I doubt not but he will thank me for it. 



That air issuing from water, or rather from the soft earth, or mud, at the 

 bottom of pits containing water, is not always unwholesome, I have also had an 

 opportunity of ascertaining. Taking a walk, about 2 years ago, in the neigh- 

 bourhood of Wakefield, in Yorkshire, I observed bubbles of air to arise, in 

 remarkably great plenty, from a small pool of water, which, on inquiry, I was 

 informed had been the place where some persons had been boring the ground, in 

 order to find coal. These bubbles of air having excited my curiosity, I pre- 

 sently returned, with a basin, and other vessels proper for my purpose, and 

 having stirred the mud with a long stick, I soon got about a pint of this air; and. 



