ON WATKK AND ITS r< >M I'< H'NDS 77 



excess, water already begins to come off at a temperature slightly 

 above 100, but if it be in but a small proportion there is such a 

 relation between it and the sulphuric acid that at 120, 150, 200, and 

 even at 300, water is still held by the sulphuric acid. The bond 

 between the remaining quantity of water and the sulphuric acid is 

 evidently stronger than the bond between the sulphuric acid and the 

 excess of water. The force acting in solutions is consequently of 

 different intensity, starting from so feeble an attraction that the proper- 

 ties of water as, for instance, its power of evaporation are but very 

 little changed, and ending with cases of strong attraction between the 

 water and the substance dissolved in or chemically combined with it. In 

 consideration of the very important signification of the phenomena, and 

 of the cases of the breaking up of solutions with separation of water 

 or of the substance dissolved from them, we shall further discuss them 

 separately, after having acquainted ourselves with certain peculiarities 

 of the solution of gases and of solid bodies. 



The solubility of gases, which is usually measured by the volume 

 cf gas 29 (at and 760 mm. pressure) per 100 volumes of water, varies 

 not only with the nature of the gas (and also of the solvent), and 

 with the temperature, but also with the pressure, because gases them- 

 selves change their volumes considerably with the pressure. As might 

 be expected, (1) gases which are easily liquefied (by pressure and cold) are 

 more soluble than those which are liquefied with difficulty. Thus, in 

 100 volumes of water there dissolve at and 760 mm. only two volumes 

 of hydrogen, three volumes of carbonic oxide, four volumes of oxygen, 

 &c., for these are gases which are liquefied with difficulty ; whilst 



- p If a volume of gas v be measured under a pressure of // mm. of mercury (at 0) 

 and at a temperature t Centigrade, then, according to the laws of Boyle, Mariotte, and 

 of Gay-Lussac combined, its volume at and 760 mm. will equal the product of v into 

 760 divided by the product of h into l + at, where a is the co-efficient of expansion of 

 gases, which is equal to 0'00367. The weight of the gas will be equal to its volume at 

 and 760 mm. multiplied by its density referred to air and by the weight of one volume 

 of air at and 760 mm. The weight of one litre of air under these conditions being = 

 1-293 grams. If the density of the gas be given in relation to hydrogen this must be 

 divided by 14'4 to bring it in relation to air. If the gas be measured when saturated 

 with aqueous vapour, then it must be reduced to the volume and weight of the gas when 

 dry, according to the rules given in Note 1. If the pressure be determined by a 

 column of mercury having a temperature /, then by dividing the height of the column by 

 1 + 0'00018 the corresponding height at is obtained. If the gas be enclosed in a 

 tube in which a liquid stands above the level of the mercury, the height of the column 

 of the liquid being = H and its density = D, then the gas will be under a pressure which 



TTITv 



is equal to the barometric pressure less , where 13'59 is the density of mercury. By 



13' 59 



these methods the quantity of a gas is determined, and its observed volume reduced to 

 normal conditions or to parts by weight. The physical data concerning vapours and 

 gases must be continually kept in sight in dealing with and measuring gases. The student 

 must become perfectly familiar with the calculations relating to gases. 



