121 



FLUIDITY. 



FLUIDS, ELASTIC. 



122 



which it moves the law of resistance is nearly expressed by the square 

 of the velocity. This hypothesis was originally formed by considering 

 that the number of particles on which the moving body impinges in 

 a given time is nearly proportional to its velocity : we say nearly, 

 because the particles which have been struck form returning currents 

 which interfere with this simple law ; and, secondly, that the force 

 with which it impinges is also as its velocity, which must be modified 

 from the same consideration. The nature of these currents has not 

 been yet investigated, and therefore the law of the square of the 

 velocity is adopted generally as a first approximation, but the dis- 

 covery of the true law would appear to be within the limits of 

 calculation without aid from experiment, and is a subject worthy the 

 attention of physical mathematicians. 



The resistance of bodies only partly immersed in fluids, and having 

 a depth bearing a sensible ratio to that of the fluids, as in barges 

 towed along canals, is subject to laws far different from those which 

 we have considered, for the quantity immersed is itself a function of 

 the velocity, diminishing considerably with great velocities : thus, not- 

 withstanding the increase of resistance due to velocity, this diminution 

 due to less immersion permits the possibility of a minimum resistance. 

 ThU important subject will be further considered in the article 

 HYDRAULICS. 



The term fluid has been extended to the supposed media through 

 whirh the forces of electricity, galvanism, and magnetism act, but 

 little that can be relied upon has been deduced from their supposed 

 analogy with material fluids. [ELECTRICITY.] A surer source of cal- 

 culation is found in detecting the laws of their elementary actions by 

 experiment ; and indeed this process seems to point out the most 

 feasible methods for discovering the molecular laws even of material 

 fluids, manifested both in their tenacity and their capillary phenomena. 

 ility cannot be easily defined in the explicit terms of its exact 

 causes until more is known of the true laws of the forces which govern 

 the internal arrangement of bodies ; but taking the effect, we may with 

 Laplace say, that " mobility is the characteristic property of fluids." 

 Hence fluidity may be rendered imperfect by the admixture of solids 

 with fluids, as in mud, Ac. The effects of fluidity become still more 

 concealed in masses consisting of heterogeneous solids holding fluids in 

 their pores, aa in moist clays, dough, Ac. ; nor are they fully deve- 

 loped in solids which, through the action of heat, are tending to a 

 fluid state, as in melting tallow, wax, glass, Ac. In none of these cases 

 can the laws of perfect fluids be applied ; but as they belong only to 

 states of transition, their peculiar laws do not deserve, or at least have 

 not obtained, much consideration. 



FLUIDITY. All ponderable matter exists either in the gaseous, fluid, 

 or solid state ; and most solids, when heat is applied to them, may be 

 rendered fluid, or converted into liquids, under which circumstances 

 mutual repulsion of particles takes the place of cohesion. The degree 

 of heat required to produce this effect is different in different solids, 

 but, cccteru paribui, it is always the same in the same solid : in many 

 cases the transition from the solid to the fluid form is sudden, while in 

 other instances solids pass through various degrees of liquidity before 

 they become perfectly fluid. Of the first mode of becoming fluid ice 

 and the metals are examples, and wax or tallow of the second. 



As most solid bodies may be rendered fluid by heat, so many gaseous 

 and fluid bodies are converted into solids by diminishing their tempe- 

 rature. Solid bodies in becoming fluid render latent a large quantity 

 of heat ; and on the other hand, fluid bodies in becoming solid evolve 

 much sensible heat. The heat which is requisite to the fluid existence 

 of a body is termed the heat of fluidity. These facts are proved by two 

 simple experiments. Mix a pound of water at 32 Fahr. with a pound 

 of water at 172", and the resulting temperature will be the mean, or 

 102". If a pound of ice at 32 be dissolved in a pound of water at 

 172, the solution will not have the mean temperature of 102, but 

 only 32. As, then, the pound of ice, by being rendered merely fluid, 

 absorbs 140 of heat, so the quantity of heat which becomes sensible 

 when a pound of water at 32 is converted into ice at 32 amounts also 

 to 140. The actual quantity of heat rendered latent by different 

 fluids as they liquify depends upon the nature of the substance ; thus, 

 according to Person, the under-mentioned bodies contain the annexed 

 quantities of heat in the latent state when rendered fluid : 



Water 142-65 Fahr. 



Nitrate of Soda 113'34 



Nitrate of Potash .... 65-29 



Zinc 60-63 



Silver 87'92 



Tin 25-65 



Cadmium 24*44 



Bismuth 22.75 



t Sulphur 18-85 



Lead 9'65 



Phosphorus 9-05 



Mercury 5.11 



The nature of fluidity will be further considered under HEAT. 



FLUIDS, ELASTIC. This name may be applied to all fluids in 

 nature, since all are in certain degrees elastic ; but it belongs particu- 

 larly to such as are aeriform, liquid substances possessing the property 

 of elasticity only in a low degree. [ELASTICITY ; PEIZOMETER.] Among 



the aeriform fluids, however, those which are usually considered as per- 

 manently elastic are called gases and the term elastic fluid is fre- 

 quently confined to atmospheric air, and the vapours which are 

 produced from solids or liquids by the action of heat ; these last are 

 therefore such as may be rendered solid or liquid by reducing their 

 temperature, or by increasing the pressure under which they exist. 

 But the difference between these and the fluids which are called 

 permanently elastic is perhaps nominal, since many of the latter, by the 

 discoveries of Dr. Faraday, are found capable of being exhibited in a 

 liquid form. [GAS.] This philosopher, for example, obtained carbonic 

 acid in a liquid state from carbonate of ammonia, by subjecting it to 

 great compression in a sealed tube, one end of which was placed in a 

 freezing mixture. The liquor was colourless. This gas, with some 

 others, have also been reduced to the solid form. Many of the gases, 

 moreover, on being combined with one another and with other sub- 

 stances, form solids or liquids ; thus, oxygen gas unites with metals 

 and becomes solid ; ammoniacal g;>s and hydrochloric acid gas unite 

 and form the solid hydrochlorate of ammonia; while oxygen and 

 hydrogen gases unite to form water. 



Almost all gases are invisible ; but several which are so when they 

 exist alone, become visible on being mixed with one another. Thus, 

 binoxide of nitrogen being mixed with atmospheric air, the combi- 

 nation becomes visible and of a red colour. Several gases also become 

 visible when mixed with aqueous vapour. An augmentation of the 

 temperature of vapour may, by producing an increased rarefaction, 

 render it invisible ; and, on the other hand, a diminution of temperature 

 will cause such a condensation as may render visible a vapour which 

 before was imperceptible. These effects of heat and cold upon vapour 

 have been proposed as explanations of the apparent diminution of the 

 mass of a comet when near the sun, and of its apparent enlargement in 

 receding from that luminary. All elastic fluids are transparent, but 

 different quantities of light are absorbed in passing through those of 

 different kinds, and when the thickness of a stratum of fluid is con- 

 siderable, the absorption is so great as to render an object beyond it 

 invisible. 



The elastic forces of a dry gas at a given temperature are inversely 

 proportional to the volumes they occupy ; and this law holds good also 

 both for mixtures of elastic vapours with each other, and of vapours 

 with gases, provided no chemical action takes place between them. 

 Thus, diflerent fluids of equal temperatures and equal elastic forces 

 being introduced together in a close vessel whose capacity is equal to 

 the sum of the volumes of the fluids separately, the fluids for a time 

 remain separately in equilibrio ; but experience shows, that gradually 

 the fluids intermingle with one another, producing a homogeneous fluid 

 preserving the same temperature and elastic force. It has been found 

 also that if different fluids having equal temperatures with different 

 elastic forces, and occupying separately equal volumes V, be mixed 

 together in a close vessel whose capacity is v, the elastic force of the 

 mixed fluid will be equal to the sum of the elastic forces of the separate 

 fluids, and the temperature will remain constant. When a vapour at a 

 given temperature is compressed by being confined within a smaller 

 space than that which it previously occupied, part of the vapour 

 becomes condensed, and the remainder continues to possess the elastjc 

 force due to the temperature. And again, if the volume of a quantity 

 of vapour be increased, the vapour will expand, and, if not in contact 

 with the liquid from which it was produced, its elastic force will be 

 diminished; if in contact with the liquid new vapour will rise to 

 supply the void created by the dilatation, and the elastic force will 

 remain constant. 



The temperatures at which liquids become elastic fluids by the action 

 of caloric are very various ; hydrochloric and nitric ethers boil, under 

 the usual pressure of the atmosphere, the one at 51-9, and the other 

 at 185; acetic ether boils at 165 ; water boils at 212 ; while mercury 

 can be made to boil only at a temperature of 662. 



The quantity of vapour produced by heat from a liquid increases 

 with an increase of temperature, and while in contact with the liquid 

 its elasticity varies with its specific gravity. The elastic force of vapour 

 is increased when the vapour is mixed with air ; for if the interior of a, 

 barometer tube be moistened at the upper end with water, and air be 

 introduced in it above the column of mercury, the tube being inserted; 

 aa usual in a cistern of the latter fluid, the depression of the mercurial 

 column in the tube by the expansion of the vapour and air, in con- 

 sequence of an application of heat on the exterior, is greater than that 

 which results from the expansion of air when dry. 



The atmosphere which surrounds the earth is endowed with an 

 elastic power ; and partaking, moreover, of the earth's diurnal rotation, 

 its particles should, by their elasticity and centrifugal force combined, 

 recede from the earth till the whole is dissipated in space. Such is not 

 the fact ; and hence it is inferred, either that at a certain elevation 

 above the surface of the earth the elasticity of the atmosphere is totally 

 destroyed by the absence of caloric ; or that beyond the stratum in 

 which the centrifugal force of the particles is equal to their gravitation, 

 there may exist, in a state of rest, an ethereal fluid occupying the whole 

 extent of space, and preventing the atmosphere from being further 

 expanded by its own elasticity. 



Now, by mechanics, it may be found, that the distance from the 

 surface of the earth to the stratum of the atmosphere ia which the 

 centrifugal force of the particles is equal to their gravity, is about five, 



