that time they could be kept from sexual intercourse the reproduction would cease.

Dr. Darwin, and all other advocates for spontaneous generation, speaks of some animals as simple and others as complete, some as imperfect and others as perfect; whereas, as far as we can discover, all animals, even the most minute that have been examined, appear to be as perfect, and to have a structure as wonderfully complicated, as the largest, though on account of their minuteness, we cannot dissect them to so much advantage. Their organs are equally adapted to their situations and occasions; and what is more, they have as great a degree of intelligence (which they discover by the methods of seeking their food, avoiding, or contending with their enemies) as the largest animals: besides, it is never pretended that any large species of animals, though called imperfect, as crabs and oysters, &c. are ever produced by spontaneous generation.

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The larger kinds of the more perfect animals Dr. Darwin does not pretend to have ever been "produced immediately "in this mode of spontaneous generation;" but he supposes, what is even more improbable, viz. that "vegetables and ani"mals improve by re-production; so that spontaneous vitality (p. 1.) is only to be looked for in the simplest organic beings, as in the smallest miscroscopic animalcules, which perpetually perhaps however enlarge themselves by re-produc"tion; and that the larger and more complicated animals "have acquired their present perfection by succesive genera"tions, during an uncounted series of ages."

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By this he must have meant to insinuate, for it is not clearly expressed (perhaps to avoid the ridicule of it) that lions, horses, and others, which he considers as more complicated animals, though they are not more so than flies and other insects, may have arisen from animals of different kinds, in the lowest state of organization, in fact, that they were once nothing more than microscopic animalcules.

But this is far from being analogous to any thing that we observe in the course of nature. We see no plants or animals, though ever so simple, growing to more than a certain size, and producing their like, and never any others organized in a

different manner. Is it at all probable that lions, horses or elephants, were ever any other than they now are? Were they originally microscopic? And if they come to be what they now are by successive generations, why does not the change and improvement go on? Do we ever see any small animal become a larger of a different kind? Do any mice become rats, rats become dogs, or wolves, wasps become hornets, &c. and yet this is precisely the analogy that the hypothesis requires.

In order to obviate the prejudice against this doctrine of spontaneous production, as favouring atheism, Dr. Darwin says of the objectors, p. 1. " They do not recollect that "God created all things which exist, and that these have "been from the beginning in a perpetual state of improve"ment, which appears from the globe itself, as well as from "the animals and vegetables which possess it. And lastly, "that there is more dignity in our idea of the Supreme "Author of all things, when we conceive Him to be the "cause of causes, than the cause simply of the events “which we see, if there can be any difference in infinity of power."

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The Supreme Being is, no doubt, the cause of all causes; but these causes have a regular connexion, which we are able to trace; and if any thing be produced in any different manner, we say it is not according to the course of nature, but a miracle. The world is, no doubt, in a state of improvement; but notwithstanding this, we see no change in the vegetable or animal systems, nor does the history of the most remote times favour the hypothesis. The plants and animals described in the book of Job are the same that they are now, and so are the dogs, asses, and lions &c. of Homer.

Vegetables and animals do not by any improvement, natural or artificial, change into one another, or into vegetables and animals of other species. It is, therefore, contrary to analogy, or the established course of nature, that they should do so. If miracles; which imply an omnipotent and designing power (and which to the generality of mankind are the most striking proofs of the existence of such a power, and a power distinct from the visible parts of nature, the laws of which

it counteracts) be denied, all changes that take place contrary to the observed analogy of nature must be events without a cause; and if one such event can take place, any others might, and consequently the whole system might have had no superior designing cause; and if there be any such thing as atheism, this is certainly it.

Dr. Darwin speaks of his organic particles as possessed of certain appetencies, or powers of attraction. But whence came these powers, or any others, such as those of electricity, magnetism, &c.? These powers discover as much wisdom, by their adaptation to each other, and their use in the general system, as the organic bodies which he supposes them to form; so that the supposition of these powers, which must have been impart ́ed ab extra, only removes the difficulty he wishes to get quit of one step farther, and there it is left in as much force as ever. There are still marks of design, and therefore the necessity of a designing cause.

No. XXV.

Observations on the Discovery of Nitre, in common Salt, which had been frequently mixed with Snow, in a Letter to Dr. Wistar, from J. Priestley, L. L. D. F. R. S.

DEAR SIR,

Read, December 2, 1803.

It was

WHEN I had the pleasure of seeing you at Northumberland, I mentioned a fact which I had just observed, but which appeared to me so extraordinary, that I wished you not to speak of it till I had more completely ascertained it. the conversion of a quantity of common salt into nitre. But having seen, in the last Medical Repository, an observation of Dr. Mitchell's, which throws some light upon it, I think it best upon the whole to acquaint experimentalists in general with all that I know of the matter; that, as the experiments must be made in the winter, they may take advantage of that which is now approaching.

T

In the winter of 1799 I made those experiments on the production of air from the freezing of water, an account of which is published in the 5th Vol. of the Transactions of the Philosophical Society of Philadelphia, p. 36; And having made use of the same salt, mixed with snow, in every experiment, always evaporating the mixture till the salt was recovered dry, I collected the salt when I had done with it, and put it into a glass bottle, with a label expressing what it was, and what use had been made of it.

This quantity of common salt having been frequently dissolved, and evaporated in an iron vessel, remained till the 26th of last October; when, having occasion to make a large quantity of marine acid, and this salt appearing to be of little value, I put to it an equal weight of acid of vitriol and about twice the quantity of water, and began the distillation in the usual way. But I was soon surprized to observe that red vapours rose from it, first filling the retort, and then the adopter, &c. and when the process was finished, returning to the retort, exactly as in the process for making spirit of nitre.

Not doubting, from this appearance, but that the produce was the nitrous acid (though having used much water, the acid was of course weak, and nearly colourless) I immediately dissolved copper in it, and found that it yielded as pure nitrous air as any that I had ever procured in the same way.

Examining the salt separately, I observed that when it was thrown upon hot coals, whether those portions of it that were white, or those that were brown from a mixture of the calx of iron, it burned exactly like nitre; so that from this appearance, I should have concluded that it had been wholly so. But that it contained some marine salt, and that the acid procured from it had a mixture of the marine acid, could not well be doubted; and this appeared to be the case both by the ́acid becoming turbid by a mixture of the solution of silver in nitrous acid, and by its dissolving gold with the application of heat, so that it was a weak aqua regia.

This conversion of common salt into nitre appeared so extraordinary, that I first thought there must have been some mistake in the label, though few persons I believe are more

careful in that respect than myself. But I never had any nitre of that appearance, and least of all any that had in it a mixture of common salt; so that I could not doubt but that this was the same salt that I had used before for the purpose above mentioned. That this change must have come from the snow with which it had been dissolved, could not be doubted; and therefore I resolved to repeat the experiment with the next that should fall, but seeing that Dr. Mitchell had procured an acid from hail stones, I was instantly determined to excite other persons to repeat the experiment as well as myself, having now more confidence in my own.

What was the acid that Dr. Mitchell procured he did not ascertain, mine was unquestionably the nitrous, and it must have displaced that of the common salt by a superior affinity to its base. This acid must be exceedingly volatile; for I could not produce the same effect by repeated solutions and evaporations of the same kind of salt in snow water of long standing, a quantity of which I have always had, to use occasionally instead of distilled water.

The manner in which nitrous acid may be formed in the atmosphere is easily explained on my hypothesis of the composition of that acid; since I have always procured it by the de-composition of dephlogisticated and inflammable air, together with a small mixture of marine acid (which must therefore be formed from some of the same elements) as Mr. Cavendish procured it by the de-composition of dephlogisticated air, both of us using electric sparks.

Now it is probable that, although most kinds of air, even those that have no chemical affinity, will remain diffused through each other, without any sensible separation, after being mixed together, yet in the upper regions of the atmosphere, above that of the winds, there may be a redundancy of inflammable air, which is so much lighter than any other kind of air, as Mr. Kirwan and others suppose, and that there is a proportion of dephlogisticated air, in the same region cannot be doubted. In this region there are many electrical appearances, as the aurora borealis, falling stars, &c. and in the lower parts of it thunder and lightening; and by these means,