ON PHYSICS OR NATURAL PHILOSOPHY. No. XIV. CAPILLARY ATTRACTION. Capillary Phenomena.-In the contact of solids and liquids, a series of phenomena are produced, to which the name of sapillary phenomena is given, because that they are particularly observed in tubes, whose diameter is so small that it is comparable to the thickness of a hair. The effects of capillary attraction are very various; but in all cases, they are the result of the mutual attraction of the liquid particles to each other, and to that attraction which subsists between these particles and solid bodies. Take, for example, the following phenomena: when we immerse a solid rod in a liquid which will wet it, the liquid, in opposition to the laws of hydrostatics, rises around the solid rod, as in the case of glass and water; and its surface, instead of being horizontal, takes a concave form, as shown in fig. 47; but, if a solid rod be immersed in a liquid which will not wet it, as in the case of glass and mercury, the liquid, instead of rising, sinks round the solid rod, and its surface takes a convex form, as shown in fig. 48. instead of a small diaAccording Laws of Ascent and Depression in Capillary Tubes.-M. GayLussac has proved by experiment that the ascent and depression of liquids in capillary tubes, are regulated according to 1st. There is an ascent when the the three following laws: liquid wets the tubes, and a depression when it does not wet them: 2nd, this ascent and depression are in the inverse ratio of the diameters of the tubes, so long as they do not exceed the tenth part of an inch : 3rd, the ascent and depression vary with the nature of the liquid and with the temperature; but they are independent of the substance of the tubes and of the thickness of their sides, if the latter be previously wetted. These laws hold good in a vacuum as well as in air. The method employed by M. Gay-Lussac in the discovery of these laws was the following: 1st, he measured the interior as these tubes are wetted or not wetted by the liquid, so do they produce an ascent or a depression. This ascent or depression increases as the diameter of the tube diminishes, see figs. 49 and 50. When the tubes are wetted by the liquid, Fig. 49. Fig. 50. the surface of the liquid takes the form of a concave meniscus diameter of the tubes, not directly, which would have been very difficult, but by selecting those which presented the same sectional area throughout their whole length, and weighing the quantity of mercury which completely filled them; the density of the metal being known, it was then easy to deduce, from its weight and the height of the column, the required diameter, as shown in a former lesson: 2nd, he then placed the liquid under consideration in a vessel ABCD, figure e, and vertically immersed in it, the capillary tubes which were successively submitted to experiment; close by each tube, he placed a rod BF, tapering to a point, which, by the motion of a screw, was made to reach the exact level of the liquid; then, by means of a cathetometer, he measured the vertical distance between the upper extremity of the column of liquid in the tube, and the lower extremity or point of the rod which came in contact with the liquid. The heights which different liquids reach are by no means the same, as may be seen in the following table; for, in a tube whose interior diameter was about one twenty-fifth part of an inch, the liquids rose to the different heights here mentioned, above the level of the liquid in the vessel : other so as to form an angle, and be immersed in a liquid which wets them, so that their line of contact be placed vertically, the liquid will rise towards the vertex of the angle between the two plates, and its surfâce, from the highest to the lowest point, will assume the form of the curve called an equilateral hyperbola. The asymptotés of this curve which is double, being traced on each plate, are the vertical straight line in which the edges of the plates meet, which is common to both, and the horizontal straight lines which determine the level of the liquid in which they are immersed, as shown by the dotted lines in the following figure H. : ⚫083 ⚫071 *061 ⚫053 ⚫047 ⚫041 ⚫036 *032 ⚫028 ⚫025 ⚫022 ⚫020 ⚫018 Water. Fig. 53. Mercury. Laws of Ascent and Depression between two Plates Parallel or Inclined.-Phenomena analogous to those presented by capillary tubes, are produced between two bodies of any form immersed in a liquid, when they are sufficiently near to one another. For example, if we immerse in water two parallel plates of glass so near each other that the two curvatures formed at their contact with the liquid, are united, it is observed: 1st, is attracted towards the vertex of the angle of the two that the water rises regularly between the two plates, in the plates; but if the liquid does not wet the plates as is the inverse ratio of the interval which separates them; and, 2nd, case with mercury, the drop of the liquid is rounded that the height of ascent for a given interval, is the half of that at both its extremities, into a convex meniscus, as in fig. 63, which would have taken place in a tube whose diameter is and is repelled from the vertex of the angle. The directions of equal to this interval. If parallel plates are immersed in mer-attraction and repulsion in these figures are indicated by the cury, depression takes place, but according to the same laws. arrow heads. Fig. 51. The force of attraction of a liquid to the sides of a vessel lies between two extreme cases; it is equal to that of the liquid to itself, or it is zero; in the former case, the ascent of the liquid in tubes is the consequence; in the latter, depression is the result. Between these two extremes, there must be the case in which there is neither ascent nor depression; this occurs when the force of the attraction of the liquid to the solid is exactly equal to half of the force of the attraction of the liquid to itself. Water brought in contact with well polished steel appears to realise this particular case; for the liquid seems, on the approach of the metal, to experience neither elevation nor depression. As already observed, every column of liquid elevated by capillary action is terminated by a concave surface; and every It two plates of glass, A B and ▲ C, fig. 51, be inclined to each column depressed, by a convex surface. In cylindric tubes of sufficiently small diameter, these surfaces are hemispherical. and if left to itself, it will be attracted in the direction of the Between two parallel plates they are semicylindric. Since the arrow heads. iquid columns in tubes rise in proportion to the smallness of their diameter, it follows that the meniscus which appears at the surface is proportionally increased in curvature, which furnishes us by its direction and force, or rather by the shortness of its radius, an expression for the force which acts at the extremity of the column; the concave meniscus indicating a force which acts from the interior to the exterior or a traction; and the convex meniscus, a force which acts from the exterior to the interior, or a compression. This view is verified by the following experiments. Take an inverted siphon, having two unequal branches both in length and in diameter, as shown in the following figures 1o, 2o, 3o, and such that the capillary action is very marked, in the narrow branch, and almost nothing in the other branch, on account of its great diameter. Pour water into it at three different times, so as to make it assume the levels indicated by these figures. Attractions and Repulsions of Capillary Action.--The attrac tions and repulsions which we observe among bodies floating at the surface of liquide, and which arise from capillary action, are regulated by the following laws: Ist, When two floating bodies are wetted by a liquid, as, for example, two balls of cork in water, a powerful attraction takes place as soon as they are put so near each other that a plane surface of water no longer exists between them. 2nd, When two floating bodies are not wetted by a liquid, as, for example, two balls of wax in water, a strong attraction takes place as soon as they are put in the bodies are such that one is wetted by the liquid and the other same circumstances as the former. 3rd, When two floating not, as a ball of cork and a ball of wax in water, repulsion is observed to take place as soon as they are so near each other, that the two contrary curvatures of the liquid are found in contact. The phenomena just described, depending on the concave or convex curvature assumed by the surface of the liquid in which the bodies are placed, we shall inquire into the cause which determines the form of this curvature in our next lesson. at B. In fig. 1o, the level being very low in the branch ▲, it is elevated in the branch в to a height corresponding to the capillary action at that point, and the meniscus is concave In fig. 2o, on pouring an additional quantity of water into the branch A, up to the exact level of the extremity B, the two surfaces are then of the same height, and both become plane; in fig. 3°, on pouring an additional quantity of water still into the branch ▲, up to the level which measures the capillary action in the branch B at that point, the water rises in the form of a convex meniscus, and exerts a force of compression sufficient to prevent any flow; but if the level at a be increased in height above this point, the water will then begin to issue by the narrow branch B. Again, if in a conical tube, of which the following figures marked w and м, are sections through their axes, we introduce Fig. W. Dabei (da+bei, there-by); Dahin i.e. to refer to. Dabeistehen, to stand close by. Daherschleichen, to sneak along. (bahin, there-thither); Dahineilen, to hasten away. Dagegen (dagegen, there-against); Dagegensein, to be against. Daran Danieber (da+nieter, there-below); (tartan, there-to); Daniederschlagen, to beat down. Daransegen, to put or lay thereto, i.e. to risk, to atake. Daraufgeben, to give there-on, i.e. to give an earnest. Dareinreden, to talk there-in, i.e. to interrupt. Davonlaufen, to run off, or Entzwei Herab Heranführen, to bring on or along. Herausfahren, to drive or urge on. Herausfahren, to drive out. Herbeirufen, to call by or towards. Hereinfahren, to drive in or into. UNIVERSITY OF LONDON.—No. IV. At the request of numerous students of the POPULAR EDUCATOR, we insert the following papers; they will give valuable information, not only to those who aspire to the honour of becoming members of the University, but they will form a body of useful exercises also to those who have been our STUDENTS since the commencement of our Lessons in the various branches of learning in this work. MATRICULATION EXAMINATION.-1853. [N.B.-Candidates are prohibited, under pain of instant dismissal, from introducing any book or manuscript into the Examination-Room, and from communicating with each other during the Examination. Candidates are required to attend in person on one of the last three days of the week immediately preceding the Examination, to pay their Fees and write their names in the Register. If the Candidate fail to pass, the Fee will not be returned to him, but he will be admissible to any future Matriculation Examination without the payment of any additional Fee.] PASS EXAMINATION. JUIY. 4. MONDAY. Afternoon, 2 to 4, French; 4 to 6, German. 5. TUESDAY. Morning, 10 to 1. Mathematics. Afternoon, 3 to 6, English History. 6. WEDNESDAY. Morning, 10 to 1, Greek Classic and History. Afternoon, 3 to 6, Chemistry. 7. THURSDAY. Morning, 10 to 1, Mathematics. Afternoon, 3 to 6, Natural Philosophy. 8. FRIDAY. Morning, 10 to 1, Roman Classic and History. Afternoon, 2 to 5, The English Language. J Monday, July 4.-Afternoon, 2 to 4. FRENCH.-(Examiner, M. DELILLE.) Translate into English: L'homme appelé à commander aux autres sur les champs de bataille a d'abord, comme dans toutes les professions libérales, une instruction scientifique à acquérir. Il faut qu'il possède les sciences exactes, les arts graphiques, la théorie des fortifications. Ingénieur, artilleur, bon officier de troupes, il faut qu'il devienne en outre géographe, et non géographe vulgaire, qui siit sous quel rocher naissent le Rhin ou le Danube et dans quel bassin ils tombent, mais géographe profond, qui est plein de la carte, de son dessin, de ses lignes, de leur rapport, de leur valeur. Il faut qu'il ait ensuite des connaissances exactes sur la force, les intérêts et le caractère des peuples; qu'il sache leur histoire politique, et particulièrement leur histoire milicaire: il faut surtout qu'il connaisse les hommes, car les hommes a la guerre ne sont pas des machines; au contraire, ils y deviennent plus sensibles, plus irritables qu'ailleurs, et l'art de les manier d'une main délicate et ferme fut toujours une partie importante de l'art des grands capitaines. A toutes ces connaissances supérieures, il faut enfin que l'homme de guerre ajoute les connaissances plus vulgaires, mais non moins nécessaires de l'administrateur. Il lui faut l'esprit d'ordre et de détail d'un commis; car ce n'est pas tout que de faire battre les hommes, il faut les nourrir, les vêtir, les armer, les guérir. Tout ce savoir si vaste, il faut le déployer à la fois et au milieu des circonstances les plus extraordinaires. A chaque mouvement il faut songer à la veille, au lendemain, à ses flancs, à ses derrières; mouvoir tout avec soi, munitions, vivres, hôpitaux; calculer à la fois sur l'atmosphère et sur le moral des hommes; et tous ces éléments si divers, si mobiles, qui changent, se compliquent sans cesse, les combiner au milieu du froid, du chaud, de la faim et des boulets. Tandis que vous pensez à tant de choses, le canon gronde, votre tête est menacée; mais ce qui est.pire, des milliers d'hommes vous regardent, cherchent dans vos traits l'espérance de leur salut; plus loin, derrière eux, est la patrie avec des lauriers ou des cyprès; et toutes ces images, il faut les chasser, il faut penser, penser vite; car une minute de plus, et la combinaison la plus belle a perdu son à-propos, et au lieu de la gloire, c'est la honte qui vous attend.-THIERS. La Chenille. Un jour, causant entre eux, différents animaux Un renard s'écria: "Messieurs, cela s'explique, Tuesday, July 5.-Morning, 10 to 1. ARITHMETIC AND ALGEBRA.-(Examiner, Rev. Prof. HEAVISIDE.) 1. In dividing one whole number by another, what does the quotient determine? Divide 243584 by 346, and explain the steps of the operation. 2. Show that any number will be divisible by 12, if its two last digits be divisible by 4, and the sum of its digits be divisible by 3 also. What are the prime factors of a number? resolve 54180 into its prime factors. 3. Find the simple interest on £4572 15s. for 9 years at 43 per cent. If the three per cent. stock be at 98, and the three and a quarter per cent. stock be at 101, which stock is it most advantageous to buy? What income will £5000 invested in the three per cent. stock produce? 4. Explain the principle on which vulgar fractions are added together. Find (x-2a)3 |