I am much Obliged to you for Dr. Priestley’s Experiments. They have thrown a good deal of Light upon the subject of fixed Air, Altho’ I can by no means assent to some of his inferences from them. The Experiment made with a Sprig of mint extends our Ideas of the Oconomy of Vegetables. But is all the fixed Air which is discharged from its various sources Absorbed and applied to the Nourishment of Plants? Does not common Air possess a power of destroying it either by Diffusion or Mixture? Is not much of it destroyed likewise by being restored to its Elasticity so as to become common Air? What water is to vapor, fixed Air appears to be to common Air. How diffirent are the Effects of Water and Vapor upon Animals, Vegetables and Fires, and yet we all know they are of the same Natures. The former possesses no narcotic stimulating or poisonous Quality, and yet Animals expire, and fires are extinguished in it. May not all the peculiar Qualities in like manner of fixed Air be ascribed to its containing common Air in a fixed state and not to any adventitious Matter of a poisonous and stimulating Nature?

The Doctor deserves great Credit for his Application of fixed Air to Medicine. To his Remarks upon this Subject I would beg leave to add One More. A large Quantity of fixed Air is discharged in Perspiration. This is proved by a Candle’s immediately ceasing to burn if it be put under the Bed Cloaths of a Person who is well covered in Bed. This fixed Air being of a recrementitious Nature is no doubt of a septic quality. If this be the Case we learn the great Necessity of obliging Patients to sit up as much as possible in putrid Diseases, and to change their Linnen and Bed Cloaths every Day.

You will receive with this Letter a small Pamphlet which the public have ascribed to me. It was written at the Request of Anth: Benezet to promote and accompany a Petition to our Assembly to put a more complete Stop to the Importation of Negro Slaves into the Province. I have sent a Copy of it together with all those News Papers which contain the late Dispute between Govr: Hutcheson and the Counsil and Assembly at Boston to our good Friend Monsr. Dubourg of Paris. They are directed to your Care. Please to forward them by the first Opportunity.

When shall we have the Pleasure of seeing your Treatise upon Colds? In Dr. Cullen’s late ingenious Arrangement of Diseases he speaks of a great many Catarhs or Colds from Contagion, but includes with these One Species a frigore. He does not say however that Moisture alone has any Share in producing them. I have found Shallopmen and Sailors in this City [who are] more subject to Intermitting fever, but much less to Colds than any Other People. With the sincerest Gratitude and the truest Respect I have the Honor to be Your Much Obliged Most Affectionate Humble Servant