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1870_that a difference of 1° in the boiling-point corre. sponds to about 530 feet of ascent, and this difference in boiling will denote a fall of about 0-589 inch of barometric pressure-that, under the receiver of an air-pump, water may be made to boil at a very much lower temperature than in the air. This and other things of a similar kind I find, from experience, may be made most instructive and useful to them, and more particularly if a school is provided with philosophical apparartus with which the experiments can be shown. A table of the temperatures at which different fluids boil and freeze, should be suspended on the wall.

Heat water to boiling in a Florence flask, cork it well when boiling, and turn the flask upside down; having removed it from the lamp it now ceases to boil ; sprinkle water on the surface of the bottle, the steam within is condensed, and it again begins to boil ; when it again ceases to boil, from the elasticity of the steam within, repeat the sprinkling and it commences boiling again. Thus the application of cold makes the water boil.

Archdeacon Wollaston invented an apparatus of such delicacy for ascertaining this, that the difference of the height of a common table from the ground would produce a difference in the boiling-point, which was clearly shown by the instrument.

The different ways in which water and metals are heated - hot current ascending, the cold water descending, and metals from particle to particle ; point out also the difference in the process, in attempting to heat water by placing the fire above and not under the vessel containing it. The conducting power of fluids is very small, and it has been found that water may be made to boil in the upper part of a tube, without imparting much heat to the water below it, and that it may be brought to the boiling-point within one fourth of an inch of ice without the latter immediately melting; and that ice is melted eighty times slower when it is fixed at the bottom of a cylindrical vessel with water above it, than when it floats upon the surface of warm water. · Salt is got from sea water by exposing it to the air in

large pans; the water goes off in vapour and leaves the salt behind; the greater the surface exposed to the air the more rapidly the water goes off. Shallow pans better than deep, and why: Do you not observe the water lessen very much in summer in your sheep-ponds, even when you do not take cattle to drink at them? It is taken up by the air ; in the same way a good brisk wind rapidly dries the hay, corn, and clothes after washing; and if you want anything that has been washed to dry fast, you unfold it as much as you can in order to expose all its surface to the air. For the same reason you spread out the grass and leave the corn in the field, in order that the fluid matter contained in them may be taken off.

Salt also is found as a mineral in Cheshire, Poland, etc.; and salt-springs are very often found in the coal-mines in some districts, particularly in Durham and Newcastle, where a great part of the salt used by the miners for their own domestic purposes is supplied by the salt springs in the mines.

The following is an easy instructive experiment: Take a small quantity of rock-salt and also of saltpetre, the crystals of which differ very much, dissolve them together in water, they form a clear limpid fluid. Pour this solu. tion of the two into a small dish and let it evaporate; crystals of pure salt and saltpetre will be the result, the beautiful long crystals of saltpetre being totally devoid of salt. This shews clearly that the atoms of salt have an attraction for and seek for their own atoms — the same of the saltpetre, and that if there is any attraction of the one for the other, it is less than that among themselves.

Dew. When it is once undersood that the air of the atmosphere holds up a considerable quantity of vapour, and that the greater its temperature the greater is the quantity which it holds, it will be easily understood that, when any portion of air comes in contact with a body colder than itself, that it will throw down some of its moisture.

During the daytime, the earth, plants, etc., absorb heat from the sun ; when he goes down, they radiate or give off part of the heat they have absorbed, and consequently cool ; -- this cools the air in contact with them, and when

cooled below the point which enables it to hold up all the vapour which it had taken up during the day, it lets it fall again -- this is called the dew-point. Now, some plants and some leaves, and earths give off heat faster than others – on such a more copious dew will be deposited. . On the contrary, gravelled walks, stone, etc. give off heat less rapidly, and on them little or no dew falls.

This all know from experience, or at least may easily ascertain it:— then to call their attention to the beautiful drops of dew formed on the leaves the service they are to the plants - the beautiful provision of the Almighty in causing the dew to fall more copiously on the vegetable world, which wants it, than on the mineral - attraction of cohesion keeping the globules together, etc. Why they disappear in the morning, again becoming vapour.

Little or no dew on cloudy nights : why ? An umbrella overhead in an evening prevents the falling of dew on the person — on the clothes-the philosophy of this the clouds are an umbrella, and the reason why no dew falls on a cloudy night applies to the umbrella-held over the head.

Any schoolmaster taking an interest in this subject, will see some very simple but curious and instructive experiments in Griffiths' “ Chemistry of the Four Seasons." They consist in taking equal portions of dry wool of a given weight, and placing them in the evening - one on gravel, another on glass, another on grass, but sheltered by a slight covering a little elevated above it, and then at sunrise taking them up and weighing them; of course the increased weight, which will in all these positions vary very much, is the weight of water deposited in the shape of dew. These and a variety of phenomena connected with this subject, easy of explanation—such as the mists—the fogs rising in damp, marshy places-following the course of a river, and many appearances of a like kind, which those living in the country are in the habit of witnessing, may be studied with great interest : but, as it is merely my object to throw out what I conceive to be useful hints, I will not pursue it further.

The force with which the absorption of moisture by porous bodies causes them to expand, is much greater than those who have never thought on the subject have an idea of. · As an instance of this, and of turning it to practical purpose, Sir John Herschel, in his “ Discourse on the Study of Natural Philosophy,” gives the following very interesting one, as a process which is had recourse to in some parts of France, where millstones are made : “When a mass of stone sufficiently large is found, it is cut into a cylinder several feet high, and the question then arises how to subdivide this into horizontal pieces, so as to make as many millstones. For this purpose horizontal indentations or grooves are chiselled out quite round the cylinder, at distances corresponding to the thickness intended to be given to the millstone, into which, wedges of dried wood are driven. These are then wetted or exposed to the night dew, and next morning the different pieces are found separated from each other by the expansion of the wood arising from its absorption of moisture."

This is a very curious instance of a simple natural power doing what would require great trouble and expense to effect; either by chiselling through, or by any machinery of sawing, sometimes used for dividing blocks of stone. The same author also mentions another instance where a knowledge of the laws of nature, although acting here in a different way, is called into action. In this case the heat first expanding, and then the application of the water causing a sudden contraction. In the granite quarries near Seringapatam the most enormous blocks are separated from the solid rock by the following neat and simple process. The workmen having found a portion of the rock sufficiently extensive, and situated near the edge of the part already quarried, lay bare the upper surface, and mark on it a line in the direction of the intended separation, along which, a groove is cut with a chisel, about a couple of inches in depth. Above this groove a narrow line of fire is then kindled and maintained till the rock below is thoroughly heated ; immediately on which a line of men and women, each provided with a potful of cold water, suddenly sweep off the ashes, and pour the water into the heated groove, when the rock at once splits with a clean

fracture. Square blocks of six feet in the side, and upwards of eighty feet in length, are sometimes detached by this method.

The following practical way of giving an insight into the principle on which bodies float in fluids lighter than themselves, and of estimating their weight by the quantity of fluid displaced, has been found very serviceable :

They* have two tin vessels, a larger and a smaller one, the large one having a small spout level with the top, so that, when filled with water and running over, it may discharge itself into the small vessel placed by the side of it; the small one of known dimensions, say nine inches square at the bottom and six inches high, with a graduated line on one of the sides, so that it may be immediately seen to what height the water rises when flowing into it, and of course knowing the area of the base, and multiplying this into the height at which the water stands, will give its volume.

Then they are provided with a number of cubes of wood the woods of the parish, oak, elm, ash, etc., four inches on a side — together with other pieces of any irregular shapes, for the purpose of experiment. · Having filled the larger vessel with water up to the spout, and placed the smaller one under it, the teacher takes a cube of oak, for instance, floats it on the water, which immediately begins to flow into the smaller vessel, and when it has ceased to do so, the height at which it stands is observed. They then calculate the number of cubic inches of water displaced.

This they know is equal to the number of cubic inches of oak under water - (the teacher should show them the proof of this — that it is equal in weight to the piece of oak. Proof – then knowing that the weight of à cubic foot of water, temperature about 62°, is 1,000 ozs., and why it is necessary to specify the temperature - they calculate, for instance, the weight of a cubic inch, by dividing 1,000 by 1,728, the number of inches in a foot.

Then multiplying the weight of one inch by the number of inches, this gives the weight of water displaced, and the weight of the wood.

* This is speaking of the boys in King's Somborne school.

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