confer experimental accuracy upon this branch of science. He took bars of various metals, and arranged

each of them as A B in fig. 14. At one end is a lamp L, the flame of which is preserved constant; this communicates heat to the end of the bars: at equal distances asunder, and fitting into cavities made in the bar, are the thermometers a, b, c, d, e. The heat imparted by the lamp travels through the bar, and the mercury in the thermometer a is soon observed to rise; b is next affected; the heat reaches the others in succession, and causes them to rise. Fix your attention upon any slice taken across the bar, say at s. This slice receives heat from the end A of the bar, and sends it on to the end B. As long as the quantity received from A exceeds that given up to B, the temperature of the slice will increase; but after a sufficient time these quantities will become exactly equal to each other, and thus, though there is a flow of heat through the slice, its temperature remains constant. M. Despretz waited until this condition had established itself throughout the whole bar his thermometers then ceased to rise ; the mercury then stood highest in a, and lowest in e, gradually diminishing from one to the other. From this he was enabled to calculate, and express in numbers, the conductibility of each bar for heat.

3. The following are the results of the most recent experiments made upon this subject :

Thus silver stands at the head of the list of metallic conductors and bismuth at the foot. The numbers in the second column express the relative conductive powers of the metals used. I have also added a third column, which shows the conductive powers of the same substances for electricity; and here the suggestive fact will be observed, that the best conductors of the one agent are also the best conductors of the other.

4. Next to the metals, crystals, stones, glass, and similar bodies are the best conductors of heat, but they differ as much among themselves as the metals do. A crystal of quartz, for example, has a conductive power greatly superior to that of a crystal of gypsum: rock salt conducts heat much better than sugar. Flint conducts better than marble. It is perhaps worthy of remark that substances which belong to the animal and vegetable kingdoms are extremely imperfect conductors; and both animals and vegetables are in some measure protected by this property from the injurious effects of sudden changes of temperature.

5. I have already referred to the fact that in a cold room, metals, when touched by the hand, seem coldest, and the woollen fabrics least cold. We are now in a

condition to understand this. The metal is a good conductor, and diffuses the heat communicated to it by the hand quickly through its mass, it thus abstracts the heat speedily from the hand and produces the sensation of cold. Wool, on the contrary, cannot thus dispose of the heat; the superficial layer in contact with the hand is all that the hand has to warm, and the quantity of heat necessary for this is almost inappreciable.

6. You will sometimes see metallic pipes used for the conveyance of steam, and which it is desirable to keep warm, wrapt round with straw bands, coarse flannel, or some other similar material. Were the pipe in contact with the atmosphere it would yield up its heat quickly, but the interposition of the non-conducting material prevents this. Precisely in the same manner the interposition of non-conducting woollen cloth saves our bodies from the abstraction of heat by the chill of the atmosphere. The philosophy of wooden handles to coffee-pots and kettles, the reason why a piece of ivory is interposed, where it is deemed desirable to have a metallic handle to a teapot, and other things of the same kind, will be so evident to you as not to need a word of explanation.

7. Not only do different bodies possess different conductive powers for heat, but in some cases the same body possesses different powers in different directions. Many crystals exhibit this peculiarity. Wood also exhibits it. If the surface of a plate of wood be thinly coated with wax, or with stearine, and a wire be introduced through a small hole in the plate, and heated while in this position, the heat passing from the wire into the wood will melt the wax; but it will not melt it equally well in all directions. Let a, b, c, d (fig. 15), be such a plate of wood, and h the aperture in which the wire is inserted; the heat passes most

Fig. 15.

freely in the direction of the fibre of the wood, which in the figure is parallel to the sides a b, and cd; the consequence is that the melted wax forms an oval, with its length along the fibre. If the wood conducted equally well in all directions, the figure melted away would form a circle.

8. I cannot quit this subject without pointing out a very common mistake made in treatises on Natural Philosophy, and repeated by lecturers upon heat. To illustrate the different conductive powers of different bodies, short cylinders of these bodies are taken and placed upright upon a heated surface-for example, upon the flat lid of a metallic vessel containing boilingwater. A bit of wax or of phosphorus is placed upon the upper end of each cylinder, and it is stated that the cylinder on which the wax is first melted, or the phosphorus first ignited, is the best conductor.

Now, if two cylinders an inch in length, one of iron and the other of bismuth, be each furnished with its bit of phosphorus, and placed at the same instant upon the heated surface, it will be found that the phosphorus upon the bismuth ignites sooner than that upon the iron. Hence, according to the lecturers and books referred to, bismuth ought to be a better conductor than iron. By reference to our table at page 267, it will, however, be seen that bismuth is greatly inferior to iron as a conductor of heat; it stands lowest among the metals, and is actually less endowed in this way than some nonmetallic bodies. How then are we to reconcile this apparent contradiction?

9. In fact, when such a mode of experiment was adopted, it was wholly forgotten that something else than mere conduction came into play and modified the result. It is undoubtedly true that the greater the conductive power of our cylinder, the greater will be the quantity of heat taken up by it in a given time from the hot surface upon which it rests, and thus iron will take up more heat than bismuth. But if you look at the table at page 250, you will see that while iron possesses a

specific heat of 0-11379, the specific heat of bismuth is only 0.03084. What is the consequence? Why, that though bismuth takes in far less heat than iron, yet on account of the low specific heat of the former metal, the small quantity which it takes in heats it considerably, and thus produces a speedy effect upon the phosphorus. I am the more induced to call your attention to this, from the fact that during a recent public examination nineteen out of twenty-two candidates, misled by what they had heard in lectures and read in books, fell into the error of supposing that the experiment to which I have referred was a true test of conduction. The proper way would be to wait until the body has taken in as much heat as it can: that body then which is heated to the greatest distance is the best conductor.

10. ON THE, RADIATION OF HEAT. -If you stand before a fire without touching it, you feel heat, and if a goose be suspended before it, you may, without at all bringing it into contact with the fire, roast the goose. Here there is no conductor between the goose and the fire, yet the heat reaches it and cooks it. You may reply that there is air between both, and that it may be the carrier of the heat. But you may allow a current of air to pass across between both; you may blow with a pair of bellows between them; you blow the air, away but you cannot blow away the heat; it will cross the current of air and reach the goose as before. Nay, you might by suitable means take away the air altogether, and still find that the heat would cross the vacuum. Heat which thus travels without the interposition of a conductor is called radiant heat: it passes from a warm body exactly as light is radiated from a luminous one.

11. There is a striking analogy between the action of radiant heat and of light. Heat can be reflected like light; it can be refracted, it can be polarized. You can cause two rays of light so to act upon each other that they shall mutually destroy each other's actions, or, in other words, by adding light to light you can produce darkness; and so in like manner by adding heat to heat