Page:Encyclopædia Britannica, Ninth Edition, v. 5.djvu/496

Rh 484 CHEMISTRY contraction represents the hydrogen, and one-third the oxygen, which has disappeared. This enables us to use hydrogen to determine the amount of free oxygen in air, or iu any gaseous mixture, one- third of the contraction which occurs when a measured quantity of the gas is ex ploded with a measured excess of hydrogen representing the amount of oxygen present. The affinity of oxygen for hydrogen, as measured by the heat developed by their combination, is very great, 68,376 units of heat, according to Thomsen, being evolved in the combination of 16 grammes of oxygen with 2 &quot;005 grammes of hydrogen, the product being liquid water at 18 D C. By burning the two gases together from a jet the most intense artificial heat that is known, except that of the discharge of a powerful galvanic battery, is produced. It is well here to call attention to the circumstance that the equations ordinarily employed to express the formation of water from its elements, and vice versa the resolution of water into its elements, viz. and do not take into account that in the one case a large amount of heat is developed, and that in the other case a corresponding amount of energy is expended. These equations merely represent, in fact, the distribution of weight in the changes, and therefore are but imperfect ex pressions of what really occurs, since the development of heat in the formation of water, and the absorption of heat, or the expenditure of a corresponding amount of some other form of energy in its decomposition, are integral parts of the changes, the amount of heat developed or ab sorbed, under given conditions, being as definite and con stant as the weights of the substances which enter into reaction and are produced. The same may be said of all equations employed to represent chemical change. The composition of water may be determined by burn ing a known weight of hydrogen in an excess of oxygen, anl weighing the water produced; then the difference be tween the weight of the hydrogen burnt and of the water produced is the amount of oxygen combined with the hy drogen. Or hydrogen is passed over a weighed quantity of copper oxide in a tube heated to redness ; the hydrogen then reduces the oxide or removes the oxygen from it, forming water, which is carefully collected and weighed, and the loss of weight which the tube of copper oxide suffers is carefully determined. The loss of weight of tbo copper oxide gives the amount of oxygen, and the differ ence between this and the amount of water produced is the amount of hydrogen combined with this amount of oxygen. The amount of oxygen combined with 2 parts by weight, or 2 atoms, of hydrogen is usually stated to be 16 parts, and 1 6 is generally regarded as the atomic weight of oxygen. But from the examination of the determinations which have been made by various elements of the com position of water, Staas, to whom we are indebted for the most exact determinations of atomic weights yet made, arrives at the conclusion that this number is too high, and that the atomic weight of oxygen is certainly not higher than 15 96. Thomsen has recently determined the amount of water produced by burning 2 litres of hydrogen in an excess of oxygen, and taking Regnault s numbers for the specific gravities of oxygen and hydrogen, he obtains a number for the atomic weight of oxygen which is in com plete accordance with that given by Staas. At the ordinary temperature of the air water is a clear, transparent, tasteless, and odourless liquid ; it appears colourless when seen in small quantity, but that it really has a pale blue colour is apparent when a shining white object is viewed through a column several feet in thickness. Water is solid at temperatures below C., C. being the temperature at which frozen water or ice melts ; the melting point is diminished by increase of pressure to the slight extent of - 00757 C. for each additional atmosphere. Water expands in freezing, its density compared with water at C. being -92. The conversion of liquid water at C. into solid water or ice is accompanied by the libera tion of heat, and heat is rendered latent or absorbed to the same extent in the melting of ice, the quantity of heat absorbed or liberated in the melting of ice or freezing of water being sufficient to raise the temperature of 79 times its weight of water from to 1 C. Water evaporates at all temperatures when in contact with atmospheric air or other gases, and the vapour given off has a density and tension determined by the tempera ture ; the tension of the vapour rapidly rises with the temperature, until at 100 C. it is equal to the ordinary at mospheric pressure (760 mm.), and the water boils. The boiling point, however, rapidly rises with increase of pres sure, and sinks when the pressure is diminished ; thus under the pressure of two atmospheres water boils at ]21 C., and under the pressure of twelve atmospheres at 190 C. When water boils under the ordinary atmo spheric temperature it is converted into 1600 times its volume of vapour. The conversion into vapour is attended with the absorption of a large amount of heat, the quan tity of heat absorbed or rendered latent in the conversion of water at 100 C. into steam of the same temperature being sufficient to raise the temperature of 536 times the weight of water converted into steam from to 1 C. Water, chemically speaking, is a remarkably neutral substance, and hence its great value to the chemist as a solvent. There are very few substances which are not to some extent dissolved by it, but the solubility of different substances is very unequal. Heat generally increases its solvent power, whilst cold diminishes it ; there are many exceptions to this rule, however. The dissolution of sub stances which may again be separated from the solution undecomposed is accompanied, in the majority of cases, with an absorption of heat, as will be evident on inspection of the table on p. 485. In the first column the name of the substance is given, and in the second its formula ; the third exhibits the number of molecules of water (in grammes) in which one molecule (in grammes) of the sub stance is dissolved at about 18C. ; the fourth column shows the number of units of heat developed or absorbed, the sign indicating that heat is absorbed, and the + sign that it is developed. It is extremely difficult to interpret the meaning of num bers such as are contained in the table, especially as we are almost entirely ignorant of the condition of substances in solution in water. But there is no doubt that the heat developed or absorbed on dissolving a solid substance is the mean result of several distinct operations, which partly, perhaps, involve an absorption, and partly a de velopment of heat. Thus, in the first place, there is a change of state from the solid to the liquid, which in most if not all cases involves an expenditure of energy ; then, many substances on dissolving in water combine with it, the combination probably being always attended with de velopment of heat. Contraction also generally takes place in the dissolution of salts in water, and is accompanied by a considerable development of heat, arising from the great resistance which water offers to compression. Chemists, moreover, are inclined to the belief that very many if not all substances, even those which are ordinarily regarded as stable in presence of water, enter to a greater or less ex tent into reaction with water when dissolved in it ; a solu-