Page:The American Cyclopædia (1879) Volume I.djvu/136

 116 ADHESION brick and mortar adhere chiefly by the cohe- sion of the mortar, which penetrates the pores of the brick ; stones without sensible pores do not adhere so well to mortar. 2. Liquids to solids. Taylor was the first who investigated this subject in a scientific manner. He sus- pended a polished plate on the balance as above described, and brought it carefully down on the surface of a liquid, when it adhered, and the adhesion was measured by the weight re- quired to separate the plate. After this method Guyton de Morveau in Paris found that plates of a French inch in diameter had the following adhesive power to mercury : gold, 446 grains ; silver, 429; tin, 418; lead, 397; bismuth, 372; zinc, 204 ; copper, 142 ; antimony, 126 ; iron, 115; cobalt, 8; cold platinum, 108; red-hot platinum, 10 grains. Taylor also believed that the pressure of the air was the main cause, but Guyton found nearly the same results in the vacuum of the air pump. Link took a polished plate of agate of nearly one inch diameter, and tested its adhesion to different liquids ; he found for water, 25 grains ; sulphuric acid, 29 ; hy- drochloric acid, 25 ; solution of saltpetre, 23 ; of lime, 21; almond oil, 16; petroleum, 16; turpentine and alcohol, 15 ; ether, 10. Where in many of these experiments drops of the liquid adhere to the plate used, it proves that the ad- hesion of the liquid to the solid is stronger than the cohesion of the liquid itself, and that the numbers obtained express rather the cohesion of particles of the liquid which were separated by the weight, than the adhesion of the plate to the liquid. The ascent of liquids in capillary tubes is also a result of adhesion, as well as the spreading out of liquids between two surfaces kept in close proximity. The chain pump, in which the water is carried up by a simple chain in a tube, is a practical application of adhesion. Prfivost made interesting experiments on elec- tive adhesion, showing how one fluid will drive another 'away from a surface for which it has more adhesion. He found that they displace one another in the following order : ether, alco- hol, oil of bergamot, poppy oil, olive oil, nut oil, and other oils, water. Pure water dis- places in its turn solutions of salts and alkaline earths. Camphor drives a film of water away from a surface, and pieces of camphor placed on water will show a peculiar motion ; the same is seen with camphor or phosphorus placed on pure mercury. These phenomena are due to the evaporation of the solid and the cohesion of its vapor. We see practical applications of the adhesion of liquids to solids in writing, paint- ing, printing, dyeing, washing, and elutriation, or separation of coarse from fine powders by suspension and Settling in a large quantity of water. 3. Liqufcls to liquids. If a drop of water is placed on mercury, or a drop of oil placed on water, it does not keep its round form, but spreads out at once, because its adhe- sion to the liquid surface is greater than the cohesion of its particles. A drop of water on an oily surface, however, will not spread out, as the cohesion of its particles is greater than its adhesion to the oil. The manner of dis- placement of one liquid by another having greater adhesive force to the liquid they float on, gives rise to a series of phenomena, for the study and exhibition of which Prof. Morton of the Stevens institute at Hoboken has recently contrived an apparatus in the style of a magic lantern. 4. Oases to solids. Many solids have the property of condensing gases on their sur- face (see ABSORPTION OF GASES), and polished metallic surfaces, even when long exposed to the air, will be covered with such a gaseous film, which is the first manifestation of chemi- cal affinity. In the process of daguerreotyping, the polished silver plate will be inert unless this film of air has been removed by a polishing process jnst before the operation. Such re- moval of air may be made visible on the surface of a glass mirror which has not been rubbed for some time, by drawing a few figures or let- ters on it with a clean finger; the invisible change of surface will become visible by breath- ing on the glass, when the appearance of the deposit of watery vapor will show where the air film' has been removed by friction. The adhesion of gases to solids is further illustrated by the small air bubbles which are often visi- ble in mineral waters and effervescent drinks, stickinfg to the sides of the glass vessel in which they are contained, and not rising to the sur- face notwithstanding they are some 600 times lighter than the liquid. This adhesion is also illustrated by heavy powders and even sewing needles floating on water ; the air adhering around the needle prevents the adhesion of the water, and the latter by its own cohesion forms a hollow depression in which the needle floats. The same adhesion of air around a piece of solid iron causes it to float on melted iron, notwith- standing it has not a less specific gravity than the fluid material. The mutual adhesion of solids and gases is also illustrated by the float- ing of particles of dust in the air ; subdivision of matter increasing the surface, a continued subdivision will at last cause a point to bo reached where the surface adhesion overcomes gravitation. In the vacuum of the air pump the dust falls down like a heavy body. It is the same with smoke, as this consists of solid particles carried upward by a current of heat- ed air ; the white smoke evolved by the burn- ing of magnesium, zinc, or phosphorus illustrates this point very plainly. 5. Gases to liquids. The adhesion of gases to the surface of liquids is stronger in proportion to their solubility or ab- sorption by the liquid. So carbonic acid ad- heres to water with greater force than air ; but air possesses very strong adhesion to water, as shown by the currents of air carried down by any considerable cataract. Use is made of this adhesion in the so-called water bellows, in which a stream of water falling through a wide tube carries air downward and produces a blast so strong that this principle was used for driv- ing the drills during the boring of the Mont