By a south-west exposure, then, we obtain for fruits what above all things they want, the most moisture, and the greatest heat. By no other exposure can these two articles be had combined to such a degree.

Observe two borders in a garden, one upon an east, the other upon a west wall. If they are of the same soil, the west border, all the summer over, is the driest, because it receives most heat. A west wall at Moffat, can produce a magnum plum, large and well ripened; but an east wall can with difficulty ripen a heart-cherry.

It is throwing away a wall to bestow it upon vines, figs, and nectarines. These three fruits should with us be confined to the hothouse: For a good plum is certainly better than a bad nectarine.

Whatever way a wall is fronted, it is a great matter to have a dry and a deep soil, and a full shelter at a proper distance, either from trees or from rising ground.

But I am afraid of tiring your Lordship. I wish the thing has not happened two or three pages ago. Whatever is in any letter I write, is entirely at your Lordship's service, for whatsoever purpose you please; nor will I grudge to write, while I have reason to think they can afford any entertainment to your Lordship.—I ever am, with the highest regard, your Lordship's most obliged and devoted servant,

JOHN WALKER.

[The

[The three following Letters from the late Dr JOSEPH BLACK, Professor of Chemistry, to Lord KAMES, ought to have been inserted immediately before the 5th Article of this Number of the Appendix.-The Remarks subjoined to them are from the same hand to which I am indebted for the Criticism on Lord KAMES's Gentleman Farmer.]

XI. From Dr BLACK to Lord KAMES.

On the Attraction between Clay and Water.

MY LORD,

Edinburgh, May 23. 1775.

I shall now give you what occurs upon the subjects of your Lordship's three letters, without further apology for the lateness of my

answer.

It is certain that one kind of earth is soluble in water,-the calcareous earth. I have made the experiment with the purest water that can be had,-water distilled with a gentle heat in glass vessels; and such water manifestly dissolved a small quantity of the calcareous earth in its ordinary natural state. Whether clay also can be perfectly dissolved by water, I cannot determine at present, not being acquainted with any experiment that is so nice a trial of this question as that by which I satisfied myself of the dissolution of the (k 2) calcareous

calcareous earth. That there is an attraction between clay and water is beyond doubt; but we have not yet any evidence that this attraction is of that kind that can produce a transparent dissolution of the clay in the water. It appears to be the same with the attraction of small vacuities for water and other fluids. It is manifestly stronger than the attraction of the particles of the clay for one another; by it the water penetrates the dry and hard clay, and separates to a certain distance the particles of it from one another, interposing itself every where between them, and occasioning the mass to swell considerably; but the same thing happens in other examples, which are considered as cases of the attraction of small vacuities. Dry wood, ivory, horn, &c. are also swelled by water. The particles of them are not removed to such a distance from one another, nor their mutual attraction so much diminished; but they are softened and rendered more pliable to a certain degree.

The different states to which clay is reduced, by being simply soaked with water, or kneaded with it, may, I think, be explained in this manner. When water is applied to a dry lump of clay, it must first penetrate the external parts of it. When these receive a certain quantity of water they swell. The external crust of the clay being thus enlarged in all its dimensions, the parts of it cannot remain united to one another, and to the internal parts of the mass which are not yet swelled. Innumerable cracks and divisions are therefore formed in it, which split it into small parts, and this in a very irregular manner; a mass of clay being seldom homogeneous, and equally penetrable to water. In the external crust itself there are numerous nuclei, which the water gets round and affects in the same manner as the whole mass. The external crust being thus shattered, the water during its progress inwards produces the same effect upon the parts of the clay that are next within it, and thus

splits and disunites the parts of the whole mass successively; the rents and divisions formed at first in the more external parts being enlarged afterwards by the irregular swelling of the internal. Wherever the smallest crack or flaw happens in a mass of this kind, the parts separated by such a crack or flaw are placed beyond the reach of one another's attraction. The particles of clay adhere to one another by the attraction of cohesion, which reaches only to a very small distance; so small, that we cannot perceive it by means of our senses. When we approach masses of matter to one another, in order to make the attraction of cohesion take place, we cannot perceive that it operates until such masses appear to our senses to be in the closest contact; though nice experiments shew that it does begin to act before they come into actual contact. The numerous rents, therefore, formed in the clay, while the water is penetrating it, place the parts which they separate beyond the reach of their mutual attraction, and the whole is a shattered pulpy and penetrable mass. But if this mass be compressed and wrought, as is done to prepare clay for the use of the potter, all the separate parts of it are brought again within the reach of one another's attraction; they are brought as near as the water, which is every where interposed between them, will permit; and as their attraction is stronger at smaller distances than at greater, the particles of the clay cohere the more firmly the less water is mixed with it, and most firmly when the whole water is evaporated: but if the mass be beaten to powder after it is dry, its cohesion is again destroyed, because the parts into which it is separated by the mechanical force, are by such separation placed beyond the reach of their attraction for one another: nor can they be properly applied to one another again, unless water be used, which, as a medium or gluten, makes them cohere, and brings

them

them together again, in consequence of the attraction which they have for it.

It is not easy to give a clear and satisfactory answer to your Lordship's query respecting the inequality between evaporation and rain, as it appears by experiments made at land. The only way I can account for it is by an hypothesis, viz. that the greatest part of the water which arises into the air by evaporation, arises from the sea; and that when it descends again in rain, it falls mostly upon the land. The first part of the hypothesis will be easily admitted. The second was suggested by an observation of Monsieur Bougainville's in his voyage through the Pacific Ocean. He says, that when they were far from land, the sky was always serene, and they had a moderate and regular trade-wind; but when they approached land, they had generally clouds, squalls and showers, and that thus they learned by experience to perceive by these signs their approach to land before they were in sight of it; that when they saw a cloud upon the horizon, and sailed towards it, they were sure to meet with land. How this happens, is a difficult question, which is not easily discussed.

Thus I have given your Lordship, without any ceremony, my views of these subjects, and shall be happy if they any way contribute to your examining them better, and seeing them more clearly than I do, or give you entertainment in any shape; but this I cannot hope to do, by continuing this letter beyond its present unreasonable length: so I shall conclude, and beg leave to subscribe myself, your Lordship's faithful humble servant,

JOSEPH BLACK.