the fable of the man blowing hot and cold: for in the instance of the lead, fire overcomes the attraction of cohesion; and the same power, heat, when applied to puddings, bread, &c. causes their parts to cohere more powerfully. How are we to understand this?

Father. I will endeavour to remove your difficulty. Heat expands all bodies without exception, as you shall see before we have finished our lectures. Now the fire applied to metals in order to melt them, causes such an expansion, that the particles are thrown out of the sphere, or reach of each other's attraction: whereas the heat communicated in the operations of cookery, is sufficient to expand the particles of flour, but is not enough to overcome the attraction of cohesion. Besides, your mamma will tell you that the heat of boiling would frequently disunite the parts of which her puddings are composed, if she did not take the precaution of enclosing them in a cloth, leaving them just room enough to expand without the liberty of breaking to pieces; and the moment they are taken from the

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water, they lose their superabundant heat, and become solid.

Emma. When Ann the cook makes broth for little brother, it is the heat then which overcomes the attraction which the particles of meat have for each other, for I have seen her pour off the broth, and the meat is all in But will not the heat overcome the attraction which the parts of the bones have for each other?

rags.

Father. The heat of boiling water will never effect this, but a machine was invented several years ago, by Mr. Papin, for that purpose. It is called Papin's digester, and is used in taverns, and in many large famiiies, for the purpose of dissolving bones, as completely as a lesser degree of heat will liquefy jelly. On some future day I will show you an engraving of this machine, and explain its different parts, which are extremely simple*.

* See Vol. IV. Conver. XIX.

CONVERSATION IV.

Of the Attraction of Cohesion.

FATHER. I will now mention some other instances of this great law of nature. If two polished plates of marble, or brass, be put together, with a little oil between them to fill up the pores in their surfaces, they will cohere so powerfully as to require a very considerable force to separate them. -Two globules of quicksilver placed very near to each other, will run together and form one large drop.-Drops of water will do the same. Two circular pieces of cork placed upon water at about an inch distant will run together.-Balance a piece of smooth board on the end of a scale beam; then let it lie flat on water, and five or six times its own weight will be required to separate it from the water. If a small

globule of quicksilver be laid on clean paper, and a piece of glass be brought into contact with it, the mercury will adhere to it, and be drawn away from the paper. But bring a larger globule into contact with the smaller one, and it will forsake the glass, and unite with the other quicksilver.

Charles. Did not you tell me that it was by means of the attraction of cohesion, that the little tea which is generally left at the bottom of the cup instantly ascends in the sugar when thrown into it?

Father. The ascent of water or other liquids in sugar, sponge, and all porous bodies is a species of this attraction, and is called capillary attraction; it is thus denominated from the property which tubes of a very small bore, scarcely larger than to admit a hair, have of causing water to stand above its level.

Charles. Is this property visible in no other tubes than those, the bores of which are so exceedingly fine?

From capillus, the Latin word for hair.

Father. Yes, it is very apparent in tubes whose diameters are one tenth of an inch or more in length, but the smaller the bore, the higher the fluid rises; for it ascends, in all instances, till the weight of the column of water in the tube balances, or is equal to the attraction of the tube. By immersing tubes of different bores in a vessel of coloured water, you will see that the water rises as much higher in the smaller tube, than in the larger, as its bore is less than that of the larger. The water will rise a quarter of an inch, and there remain suspended in a tube, whose bore is about one eighth of an inch in diameter.

This kind of attraction is well illustrated, by taking (Plate 1. Fig. 5.) two pieces of glass joined together at the side BC, and kept a little open at the opposite side AD, by a small piece of cork E. In this position immerse them in a dish of coloured water, FG, and you will observe that the attraction of the glass at, and near BC, will cause the fluid to ascend to B, whereas about the parts D, it scarcely rises above the level of the water in the vessel.