I think it probable enough, that putrid matter, as Dr. Alexander has endeavoured to prove, will preserve other substances from putrefaction; because, being already saturated with the putrid effluvium, it 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 this water; when, to my real surprise, I found, that after only one minute's agitation in it, a candle would not burn in it; and, after three or four 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 two 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 two years ago, in the neighbourhood of Wakefield, in Yorkshire, I observed bubbles of air to arise, in remarkably great plenty, from a small pool of water, which, upon 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 presently returned, with a bason, 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, examining it, found it to be good, common air; at least a candle burned in it very well. I had not then discovered the method of ascertaining the goodness of common air, by a mixture of nitrous air. Previous to the trial, I had suspected that this air would have been found to be inflammable.
I shall conclude this letter with observing, that I have found a remarkable difference in different kinds of water, with respect to their effect on common air agitated in them, and which I am not yet able to account for. If I agitate common air in the water of a deep well, near my house in Calne, which is hard, but clear and sweet, a candle will not burn in it after three minutes. The same is the case with the rain-water, which I get from the roof of my house. But in distilled water, or the water of a spring-well near the house, I must agitate the air about twenty minutes, before it will be so much injured. It may be worth while, to make farther experiments with respect to this property of water.
In consequence of using the rain-water, and the well-water above mentioned, I was very near concluding, contrary to what I have asserted in this treatise, that common air suffers a decomposition by great rarefaction. For when I had collected a considerable quantity of air, which had been rarefied about four hundred times, by an excellent pump made for me by Mr. Smeaton, I always found, that if I filled my receivers with the water above mentioned, though I did it so gradually as to occasion as little agitation as possible, a candle would not burn in the air that remained in them. But when I used distilled water, or fresh spring-water, I undeceived myself.
I think myself honoured by the attention, which, from the first, you have given to my experiments, and am, with the greatest respect,
Dear Sir,
Your most obliged
Humble Servant,
London, 7 Dec. 1773.
J. PRIESTLEY.