lower end of the tube in which м hangs is covered by the board. The temperature in the enclosed space will rise gradually, and it will be some time before it becomes steady, After some considerable interval it will be found that the thermometer reading does not alter, the mercury remaining stationary somewhere near 100°.

Note the reading; this is the value of r in the above equation (1).

While waiting for the body to become heated the operation of finding the water equivalent of the calorimeter may be proceeded with.

The calorimeter consists of a copper vessel, E, which is hung by silk threads inside a larger copper vessel, F. The outside of the small vessel and the inside of the large one should be polished, to reduce the loss of heat by radiation.

This larger vessel is placed inside a wooden box, G, to the bottom of which slides are fixed. These slides run in grooves in the wooden baseboard of the apparatus, and the box can be pushed easily under the board to which the heater is attached, being just small enough to slide under it. When the box is thus pushed into position the calorimeter is under the hole in the board which has already been mentioned; and if the cylinder be turned so that its inner tube may come over this hole, the heated body can be dropped directly into the calorimeter. L is a sliding screen, which serves to protect the calorimeter from the direct radiation of the heater, and which must be raised when it is required to push the calorimeter under the heater.

A brass rod, н, is attached to the back of the box G, and carries a clip in which a delicate thermometer, K, is fixed. The thermometer bulb is in the calorimeter, a horizontal section of which is a circle with a small square attached to it; the thermometer is placed in the square part, and is thus protected from injury by the mass м when it is immersed, or by the stirrer. The stirrer is a perforated disc of copper, with a vertical stem. A wooden cover with a slot in it,

through which the stirrer and thermometer pass, fits over the box G. There is a long vertical indentation in the heater A, and the upper part of the thermometer can fit into this when the box G is pushed into position under the heater. Care must be taken to adjust the clip and thermometer so that they will come into this indentation.

In determining the water equivalent it is important that the experiment should be conducted under conditions as nearly as possible the same as those which hold when the specific heat itself is being found.

Let us suppose that it has been found, either from a rough experiment or by calculation from an approximate knowledge of the specific heat of the substance, that if the calorimeter be rather more than half full of water the hot body will raise its temperature by about 4°. Then, in determining the water equivalent, we must endeavour to produce a rise in temperature of about 4°, starting from the same temperature as we intend to start from in the determination of the specific heat.

Weigh the calorimeter. Fill it rather more than half full of water, and weigh it again. Let m' be the increase in mass observed; this will be the mass of water in the calori. meter; let be the temperature of the water. The experi. ment is performed by adding hot water at a known tempera. ture to this and observing the rise in temperature. If the hot water be poured in from a beaker or open vessel its temperature will fall considerably before it comes in contact with the water in the calorimeter. To avoid this there is provided a copper vessel with an outer jacket. The inner vessel can be filled with hot water, and the jacket prevents it from cooling rapidly. A copper tube with a stopcock passes out from the bottom of the vessel, and is bent vertically downwards at its open end. This tube can pass through the slot in the covering of the wooden box G close down to the surface of the water in the calorimeter. A thermometer inserted in a cork in the top of the vessel

For the

serves to read the temperature of the hot water. present purpose this may be about 30°. It is not advisable that it should be much higher.

Turn the tap of the hot-water vessel, and let some water run into a beaker or other vessel; this brings the tube and tap to the same temperature as the water that will be used. Turn the tap off, and place the calorimeter, which should be in the wooden box, with the thermometer and stirrer in position, underneath the tube, and then turn the tap again, and allow the hot water to run into the calorimeter rather slowly. The temperature of the water in the calorimeter rises. When it has gone up about 3° stop the hot water from flowing. Stir the water in the calorimeter well; the temperature will continue to rise, probably about 1° more; note the highest point which the mercury in the thermometer attains. Let the temperature be '. Note the temperature of the hot water just before and just after it has been allowed to flow into the calorimeter; the two will differ very little; let the mean be T'. This may be taken as the temperature of the hot water. Weigh the calorimeter again; let the increase in mass be м' grammes. This is the mass

of hot water which has been allowed to flow in, and which has been cooled from T' to '. The heat given out is

M'(T'0').

It has raised the temperature of the calorimeter, stirrer, &c., and a mass m' of water from to 0'. The heat required to do this is

and this must be equal to the heat given out by the hot water in cooling, m, being, as before, the required water equivalent.

In doing this part of the experiment it is important that the apparatus should be under the same conditions as when determining the specific heat. The measurements should be made, as we have said, with the calorimeter in the box, and the initial and final temperatures should be as nearly as may be the same in the two experiments. The error arising from loss by radiation will be diminished if the experiment be adjusted so that the final temperature is as much above that of the room as the initial temperature was below it.

Having found the water equivalent of the calorimeter we proceed to determine the specific heat of the substance. The mass of the empty calorimeter is known; fill the calorimeter with water from one-half to two-thirds full; weigh it, and thus determine m, the mass of the water. Replace the calorimeter in the wooden box on the slides of the apparatus, and take the temperature of the water two or three times to see if it has become steady; the final reading will be the value of t. Note also the temperature of the thermometer P; when it is steady raise the slide L, and push the box G under the heater, turning the latter round the axis D until the tube B is over the hole in the stand. Then by loosening the string which supports it drop the mass M into the calorimeter. Draw the box back into its original position, and note the temperature with the thermometer K, keeping the water well stirred all the time, but being careful not to raise the substance out of the water. When the mercury column has risen to its greatest height and is just beginning to recede read the temperature. This gives the value of 0, the common temperature of the substance and the water.

Thus all the quantities in the equation for the specific heat have been determined, and we have only to make the substitution in order to find the value.

The same apparatus may be used to determine the specific heat of a liquid, either by putting the liquid into a very thin vessel, suspending it in the heater, and proceeding in the same way, allowing, of course, for the heat emitted by the

vessel, or by using the liquid instead of water in the calorimeter, and taking for the mass м a substance of known specific heat. Thus c would be known, and if m be the mass of the liquid, its specific heat, we should have

MC(T―0)=mc(0− t) + m, (0−t).

t, 0, and T having the same meaning as above.

Experiment.-Determine by the method of mixture the specific heat of the given substance, allowing for the heat absorbed by the calorimeter &c.

Name and weight of solid. Copper 32'3 gms.

Temp. of solid in the heater

99'5 C.

65'4 gms.

12.0 C.

15'7 C.

2.0

Latent Heat of Water.

DEFINITION.--The number of units of heat required to convert one gramme of ice at o° C. into water, without altering its temperature, is called the latent heat of water.

A weighed quantity of water at a known temperature is contained in the calorimeter. Some pieces of ice are then dropped in and the fall of temperature noted. When the ice is all melted the water is weighed again, and the increase gives the mass of ice put in. From these data, knowing the water equivalent of the calorimeter, we can calculate the latent heat of the water.

The ice must be in rather small pieces, so as to allow it to melt quickly. It must also be as dry as possible. We may attain this by breaking the ice into fragments and Dutting it piece by piece into the calorimeter, brushing off