I presume that by this time you are arrived in London, and I am willing to take the first opportunity of informing you, that I have niver been so busy, or so successful in making experiments, as since I had the pleasure of seeing you at leeds.

I have fully satisfied myself that air rendered in the highest degree noxious by breathing is restored by sprigs of mint growing in it. You will probably remember the flourishing state in which you saw one of my plants. I put a mouse to the air in which it was growing on the saturday after you went, which was seven days after it was put in, and it continued in it five minutes without shewing any sign of uneasiness, and was taken out quite strong and vigorous, when a mouse died after being not two seconds in a part of the same original quantity of air, which had stood in the same exposure without a plant in it. The same mouse also that lived so well in the restored air, was barely recoverable after being not more than one second in the other. I have also had another instance of a mouse living 14 minutes, without being at all hurt, in little more than two ounce measures of another quantity of noxious air in which a plant had grown.

I have completely ascertained the restoration of air in which, tallow or wax candles, spirit of wine, or brimstone matches have burned out by the same means.

The nitrous air, which I shewed you, I find to be an admirable test of air that is fit for breathing. It makes this air red and turbid, but no other that I have tried. I took air in which a mouse had putrified, which was in the highest degree noxious and fetid and also a quantity of fixed air. The nitrous air admitted to each of these kinds of air separately made no sensible alteration in them but when they were mixd (which I discovered to make a wholesome air) the nitrous air made the mixture turbid, and diminished the bulk of it, as in common air, tho not in the same degree. A mouse put to this mixture lived five minutes without uneasiness, when, if it had been put to either of them separately, a few minutes before, it would have died in a few seconds.

Air that has passed thro’ hot charcoal has many, perhaps, all the properties of air that has been diminished by other processes. It extinguishes flame, kills animals, and is not diminished or made turbid by a mixture of nitrous air.

But the observation that pleases me more than any I ever made, is the diminution of air by the crystallization (I believe) of quick-silver and the nitrous acid. This effect both precedes and follows the generation of nitrous air from the same mixture. This I suspect to be the case with other crystallizations.

I have observed many other things, which I have not room to mention at present. I am with great respect Dear Sir yours sincerely