56 I'KINCIPLKS OF CHEMISTRY 



water we will begin with the most feeble, which are determined by 

 purely mechanical superficial properties of the reacting substances. 12 



Water is mechanically attracted by many substances ; it adheres to 

 their surfaces just as dust adheres to objects, and one polished glass 

 adheres to another. Such attraction is termed ' moistening,' ' soaking,' or 

 * absorption of water.' Thus water moistens clean glass and adheres to 

 its surface, is absorbed by the soil, sand, and clay, and does not flow 

 away from them but lodges itself between their particles. Similarly, 

 water soaks into a sponge, cloth, hair, or paper, etc., but fat and greasy 

 substances in general are not moistened. Attraction of this kind does 

 not alter the physical or chemical properties of water. For instance, 

 under these circumstances water, as is known from everyday experi- 

 ence, may be expelled from objects by drying. Water which is in any 

 way held mechanically may be dislodged by mechanical means, by fric- 

 tion, pressure, centrifugal force, <fcc. Thus water is squeezed from wet 

 cloth by pressure or centrifugal machines. But objects which in prac- 

 tice are called dry (because they do not wet people's hands) often still 

 contain moisture, as may be proved by heating the object in a glass 

 tube closed at one end. By placing a piece of paper, dry earth, or any 

 similar object (especially porous substances) in such a glass tube, and 

 heating that part of the tube where the object is situated, it will be 

 remarked that water condenses on the cooler portions of the tube. The 

 presence of such absorbed, or, as it is termed, ' hygroscopic,'' water is 

 generally best recognised in non- volatile substances by drying at 100, 



12 When it is necessary to heat a considerable mass of liquid in different vessels, it 

 would be very uneconomical to make use of metallic vessels and to construct a separate 

 fire grate under each one ; such cases are continually met with in practice. A considerable 

 mass of water, for instance, may have to be heated for making solutions, or it may be 

 required to expel volatile liquids from different vessels at intermittent periods ; as, for 

 instance, alcohol from partially fermented liquors, &c. In such cases one boiler or 

 vessel containing water is made use of. Steam from this boiler is introduced into the 

 liquid, or, in general, into the vessel which it is required to heat. The steam, in con- 

 densing and passing into a liquid state, parts with its latent heat, and as this is very 

 considerable a small quantity of steam will produce a considerable heating effect. If it 

 be required, for instance, to heat 1,000 kilos, of water from 20 to 50, which requires 

 approximately 30,000 units of heat, steam heated to 100 is passed into the water from 

 a boiler. Each kilogram of water at 50 contains about 50 units of heat, and each kilo- 

 gram of steam at 100 contains 637 units of heat ; therefore, each kilogram of steam in 

 cooling to 50 gives up 587 units of heat, and consequently 52 kilos of steam are capable 

 of accomplishing the required heating of 1,000 kilos, of water from 20 to 50. Water is 

 very often applied for heating in chemical practice. For this purpose metallic vessels 

 or pans, called ' water-baths,' are made use of. They are closed by a cover formed of 

 concentric rings lying on each other. The objects such as beakers, evaporating basins, 

 retorts, &c. containing liquids are placed on these rings, and the water in the bath is 

 heated. The steam given off heats the bottom of the vessels to be heated, and thus 

 accomplishes the evaporation or distillation or other required process. A water-bath 

 may also be used for heating a vessel directly immersed in the water. 



