II – 18,II+7; from whence it appears, that the air of this 14th of February was better than it had been for fix months before. There can be no doubt of the accuracy of the experiments, because I compared the air taken at different times with that which I had first used in the month of September, and which I had preserved in dry glafs bottles accurately stopped. Now if the formulæ expreffed above are compared together, it will be found, that the difference between the firft terms is of twelve parts, and that between the latter of feven; that is, of one tenth and one twenty-fourth of the whole quantity of air: which are much greater differences than those mentioned above. Notwithstanding this, I could not perceive any particular change of health, or facility of breathing, arifing from those changes of the falubrity of the atmospherical air; and I am informed, that no particular diseases appeared which could indicate any remarkable change of air.

Nature is not fo partial as we commonly believe. She has not only given us an air almoft equally good every where and at every time, but has allowed us a certain latitude or a power of living and being in health in qualities of air which differ to a certain degree. By this I do not mean to deny the existence of certain kinds of noxious

air in fome particular places; but only fay, that in general the air is good every where, and that the fmall differences are not to be feared so much as fome people would make us believe. Nor do I mean to speak here of those vapours and other bodies which are accidentally joined to the common air in particular places, but do not change its nature and intrinsical property. This state of the air cannot be known by the test of nitrous air, and those vapours are to be confidered in the fame manner as we should confider fo many particles of arfenic fwimming in the atmosphere. In this case it is the arsenic, and not the degenerated air, that would kill the animals, who ventured to breathe it.

In this place, therefore, I do not mean to speak of those changes which do not immediately alter the nature of the air itself. The other states of that fluid are of another kind, and they are not to be examined by means of nitrous or inflammable air (the uses of which laft, I fhall fhew on another occafion), The fame thing may be said of those vapours or particles which may be good for respiration, and do not change the nature of the air. Some vegetables, for instance, can diffuse through the air fuch exhalations as may be of real ufe to the animal economy when they

are

are breathed for a long time, or imbibed by the pores of the skin. I remember to have often put various flowers, as roses, pinks, &c. in veffels full of common air confined by quickfilver, where I left them for feveral hours; after which time I found, that the air was not at all altered, but that various animals feemed to breathe it very well, notwitstanding that the flowers filled the greatest part of the veffels. On the contrary, I have found, that the vapours arising from lime flacked in water, either do not alter the air at all, or very little; though when breathed with the air they occafion the death of animals.

I would not have any body suppose, that I think it of little importance to know the goodness of the atmospherical air, and the changes it undergoes. On the contrary, I believe it to be a very useful inquiry for mankind, because we do not yet know how far one kind of air more than another may contribute to a perfect state of health; nor at what time small differences may become very confiderable, when one continues to breathe the fame kind of air for whole years, especially in fome kind of diseases. An exact method of examining the goodness of common air may even be useful to posterity, in order to ascertain whether our atmosphere degenerates in a length of time. This curious inquiry, together

with the method, &c. are the production of this eighteenth century; and our defcendants must have fome gratitude for the philofophers who found out, as well as for those who improved it. If our ancestors had known and tranfmitted it to us, we fhould, perhaps, at present be able to judge of one of the greatest changes of our globe, of a change which very nearly interests human life.

XXIX. Account of fome Experiments in Electricity. In a Leter from Mr. William Swift to the Rev. Dr. King, F. R. S.

Read April 29, 1779

SIR,

Greenwich, April 28, 1779

EING encouraged by the attention the Royal So

BEING

ciety did me last year to give to the account of fome electrical experiments I had made, which you were fo good as to present to them, I take the liberty to beg their indulgence again, and hope you will be pleafed to lay before them fome new improvements I have made in my electrical apparatus. I fhall efteem myself very happy if what I have done is worthy the farther attention of that most learned and refpectable body.

One particular addition I have made to the apparatus confifts in what I call an anti-conductor: it is exactly like the prime conductor, but it is fixed to the cushion of the machine, and confequently, when the cylinder is put in motion, the anti-conductor is charged negatively, that is, the electric matter is diminished therein in the fame pro

portion