of closing the vessel. Hence the mass of this volume of hydrogen must be subtracted from the denominator of the fraction. The mass of this volume of hydrogen is v × H2 × ('0000896) 3 (1 + ·00367t2) × 760 grams. Hence the complete expression becomes 760 W-w+ 0000896 760 (1+00367) v × H2 (1+00367t2) If great accuracy is not required this formula may be considerably simplified; thus if t1 =t, and both are small and also if H1 = H ̧= H ̧ the formula becomes The following results were obtained for common alcohol (CH,O= 46) by Dumas' method :— Mass of globe and dry air at 13.9° C. Volume of residual air at 13.9° C. 77.047-76.9174 + 000464 × 739 × W = = 76.9174 grams. W=77.0470 grams. 449.32 c. c. V v = = 3.277 c. c. H=739 m.m. 446.043 286.9 D= = 23.97 instead of the more nearly correct 23.4. L. C. A. 4 (23) HEAT. Heat is one of the forms in which energy shews itself, and probably consists in the relative motion of the particles of matter. When heat is communicated to a body, it may raise the temperature of the body, increase or rarely decrease its volume, change its physical state from solid to liquid or from liquid to gas, or cause it to undergo chemical change. The unit of heat or calorie is the amount of heat re-. quired to raise one kilogram of water from 0 C. to 1o C. The Specific Heat of a substance is the number of units of heat required to raise one kilogram of it from 0° C. to 1° C. With few exceptions the specific heats are less than unity. Dulong and Petit's Law. The specific heat of an element in the solid condition multiplied by its atomic weight is called the atomic heat of the element, and is found in the majority of cases to be nearly 6.4. Hence, if 64 be divided by the specific heat of an element, a number approximately equal to its atomic weight is obtained. Thus the specific heat of solid mercury is 03192. Elements (except oxygen, carbon, hydrogen, boron, and silicon), when they have entered into combination, appear to keep the same atomic heat, which they possessed when in the free condition. Thus Potassium Chloride KCl 6·4+6·4 12.8. Specific heat 17295 × 74·6 (molecular weight) = 13·2. But many more experiments are needed before any accurate law can be hoped for. 1 The specific heat of gases under constant pressure is the one usually given in tables. The specific heat at constant volume of gases, the molecules of which consist of two atoms, may be found by dividing the specific heat under constant pressure by 1.4. When a solid passes to the liquid or a liquid to the gaseous condition, a considerable amount of heat is absorbed and converted into a greater kinetic energy of the particles of the body. The heat so absorbed is spoken of as the latent heat of the liquid, or the gas, since it produces no change in the temperature of the body. Heat can be converted into work and conversely work into heat. When proper precautions to avoid loss are taken, the one form of energy always produces the same equivalent quantity of the other. The amount of heat which raises one kilogram of water from 0° C. to 1° C. is able to do work equivalent to lifting 425-4 kilograms through a metre in opposition to gravity. Hence the mechanical equivalent of the unit of heat is 425-4 kilogram-metres. This is signified by J. is Conversely the thermal equivalent of the unit of work = ⚫00235 units of heat. 13 J Thus to find how many kilograms could be raised through 950 metres by the heat given off by 50 kilos. of copper (sp. ht. 095) in cooling from 200° C. to 0° C. The heat evolved is 2 50 × 200 × 095 units, which can do 50 × 200 × 095 × 425·4 units of work. If x be the number of kilograms which can be raised, x=425.4 kilos. *(24) HEAT OF CHEMICAL ACTION. When chemical action takes place heat is either evolved (exothermic reactions) or absorbed (endothermic reactions) according as the substances resulting from the reaction are more or less stable than those which have entered into it. It is usual to express the results of the experiment by giving the amount of heat evolved or absorbed either in units of heat (kilogram-degrees) or in gram-degrees, when a number of grams of each substance is taken equal to the multiple of the atomic weight of the substance, which enters into the reaction. Thus (H, Cl) = 22 means that 1 gram of hydrogen in uniting with 35.5 grams of chlorine evolves 22 units of heat. == (H2, 0) = 69 means that 2 grams of hydrogen in uniting with 16 grams of oxygen give off 69 units of heat. If the reaction takes place in presence of excess of water, or if a substance is dissolved in excess of water, the indeterminate quantity of water is expressed by Aq. Thus (NH,, Aq) = 8 means that 17 grams of ammonia, while dissolving in water, give off 8 units of heat. When during a reaction heat is not evolved but absorbed, its amount is preceded by a minus sign. Thus (C2, H)= == -10-8 means that during the formation of ethene from 24 grams of carbon and 4 grams of hydrogen 10.8 units of heat are absorbed. In the important class of cases in which substances burn or combine with oxygen the following special terms are used. The calorific power of a substance is measured by the number of units of heat evolved when 1 kilogram of it is burnt in oxygen. The calorific intensity of a substance is measured by the temperature to which it can raise the products of its own combustion. If a kilogram of a substance in burning evolves Q units of heat, and forms products the masses of which are m1, m, m ̧, &c., and their specific heats 8,, 82, 83, &c., Thus to find the calorific intensity of carbon, the calorific power of which is 8080, the specific heat of carbon dioxide being .202. 12 kilos. of carbon form 44 kilos. of carbon dioxide, From the calorific intensity the pressure exerted by a mixture of gases exploded in a closed space may be calculated by finding first the temperature to which the mixture is raised by its own combustion, next the volume which the products of combustion would occupy at that temperature, and lastly the pressure required to compress the products of combustion within the volume of the enclosing vessel. Thus to find the explosive force of a mixture of carbon monoxide and oxygen:— A kilogram of carbon monoxide in burning evolves 2400 units of heat and forms 4 kilos. of carbon dioxide having the specific heat at constant volume 0.17. |