Water equivalent of calorimeter, stirrer, con denser and thermometer ... ... = = Final temperature corrected according to method of Section XXIV. ... condenser Weight of steam condensed... 17.36 x 140·2 Latent heat of steam = 67.6 = 532 4:11 Repeat the experiment, using a different quantity of water in the calorimeter, and if the two results are nearly alike take the mean as the final value. Experiments on latent heat of vaporisation are liable to a number of errors owing to the difficulty of taking account of the gain and loss of heat at the point where the steam is led into the apparatus. Hence the results obtained when the experiments are made on a small scale are very uncertain. The above represents an average determination with the apparatus used. SECTION XXIX. HEAT OF SOLUTION OF A SALT. Apparatus required: Small calorimeter with suspending hook, larger calorimeters, thermometers, and salts. If p grams of a salt, the molecular weight of which is m, be dissolved in P grams of a solvent of molecular weight M, the solution formed has p/m gram molecules of the salt to P/M gram molecules of the solvent, or 1 gram molecule of the salt to every n Pm/pM gram molecule of the solvent. = If the specific heat of the solution formed be c, and if during the process the temperature of the solution decreases from to to t, the quantity of heat absorbed by the solution of the salt is {(P + p) c + w} {to — t}, where w is the water equivalent of the calorimeter and thermometer. The quantity is the heat of solution of 1 gram of the salt, and the quantity is the heat of solution of 1 gram molecule of the salt, and is called the "molecular heat of solution." The molecular heat of solution of a salt is nearly constant for weak solutions, but diminishes as a rule as the strength of the solution increases. Determine the molecular heats of solution of Sodium Chloride and of Ammonium Chloride in water and their variations with concentration by mixing = 16 NaCl (m = 585) in 98.5 grams water (M = 18), n = NH,Cl (m = 53.5) in 101 The specific heats of the solutions may be taken as '84, 92, 97, •87, 93 and 96* respectively. Proceed as follows: Place the requisite quantity of water at about 18°.5 C. in a calorimeter surrounded by an air space and water-jacket at the temperature of the room. Weigh the salt, put it into one of the small calorimeters, and suspend it by means of its hook in the water of the large calorimeter. Place a thermometer graduated to tenths of a degree in the water. After about 10 minutes take observations of temperature for 6 minutes. If the change of temperature is regular, unhook the small calorimeter and upset it in the water so that the salt and water come into contact with each other. Stir the mixture well and observe the temperature every half minute till the change has been regular for at least 6 minutes. From your observations determine the molecular heat of solution in each case, recording in the usual way and making the proper corrections for cooling. Tabulate your results as follows: * Von Buchka, Tabellen, pp. 275, 276. + Berthelot, Thermochimie, II. p. 202, gives for n=100 Q=1260. Ibid. p. 222, gives for n=120 Q=4000. SECTION XXX. THE MECHANICAL EQUIVALENT OF HEAT OR SPECIFIC HEAT OF WATER IN WORK UNITS. Apparatus required: Puluj's friction cones with rotating pulley, jar of water, float, thermometer. WHEN a gram degree of heat, i.e. the heat necessary to raise 1 gram of water at 15° C. to 16° C., is generated by the performance of mechanical work, the work done is called the mechanical equivalent of heat, or the specific heat of water in work units (ergs per degree). To determine this quantity, the work may be done in a variety of ways; the one adopted in what follows depends on one solid being made to slide along another against friction. In order that the sliding motion may be continuous the solids. are circular, one, a small hollow cone of steel, fits into another similar cone slightly larger. The lower outer cone is held in, but thermally insulated from, a frame which can be set in rapid rotation about a vertical axis coincident with that of the cone. The smaller cone is filled with mercury and is placed in the rotating cone, but is prevented from rotating by a light wooden To one end of this arm a thread is attached which passes over a pulley and carries a float placed in a jar of water. arm. The moment of the couple which the tension in the thread exerts on the inner cone is equal and opposite to that which the rotating outer cone exerts on the inner cone. The work done by the frictional couple in any interval of time is equal to this moment multiplied by the angle through which the outer cone has in the interval been rotated with respect to the inner cone. To determine this angle the apparatus is provided with two dial wheels which register the number of revolutions of the outer cone. The angle of rotation is 27 times the number of revolutions. The tension in the thread is equal to the effective weight of the float which is numerically equal to g x volume of float pulled out of the water. Take the two cones out of the supporting frame, see that their surfaces are clean and weigh them together. Weigh the screws by which the wooden pointer is attached to the inner cone. Fill the inner cone to within 3 mms. of the top with clean mercury and weigh again to get the weight of mercury. Taking the specific heat of the steel of the cones to be 119, that of mercury to be 033 and that of brass '09, calculate the water equivalent of the cones and contents Replace the outer cone in the supporting frame, taking care that it does not touch any of the metal of the frame. By means of the adjusting screws at the sides of the frame, centre the cone accurately so that it revolves about its own axis. Attach the wooden pointer to the inner cone and place the cone in the outer as in the figure. Adjust the position of the float cylinder and the length of the thread so that when the cone spindle is rotated at a convenient speed the thread and the wooden rod to which it is attached are perpendicular to |