;i4 



SCIENTIFIC NEWS. 



[Sept. 2i, if 



parable to that of a thin elastic membrane which we 

 stretch by exerting a certain effort, whilst its cohesion is 

 at each moment opposed to any further extension ? This 

 is why we may say that the free superficial stratum of 

 a liquid is submitted to a contractile force or tension in 

 virtue of which it possesses a tendency to become as 

 small as possible. 



But what is the thickness of the stratum which is the 

 seat of this tendency ? Plateau and Quinete have found 

 by different methods that the thickness of this super- 

 ficial stratum does not exceed ^Uts of a millimetre in 

 thickness. And what is the intensity of the contractile 

 force? It varies from one liquid to another, and in one 

 and the same liquid it generally decreases as the tem- 

 perature rises. At 59? Fahr. the tension of distilled 

 water is about 7-5 milligrams per millimetre of length ; 

 olive oil has, under the same conditions, a tension of 3'6, 

 petroleum of 2'6, absolute alcohol of 2^5, and ether of 

 i-88 milligrams. 



But we must now demonstrate the existence of the 

 contractile force by some very simple experiments. 



First Experiment. — Let us take two pencils, one of 

 which must be of very light wood and not more than 

 three or four millimetres in thickness. We apply them 

 to each other, so that they may touch in a horizontal 

 straight line. In the space close to this line of contact 

 we put a few drops of pure common water, so that the 

 parts bordering on the line of contact may be well 

 wetted. There will then be formed, adhering to the two 

 pencils, a small liquid mass, of a concave curvature, 

 which is shown in section in fig. 1, a b. Then the 

 pencil of light wood can remain suspended to the other, 

 by reason of the tension of the concave surfaces a b 

 which prevail on either hand of the line of contact. For 

 instance, if the length of the pencils is twelve centi- 

 metres, the weight which can be sustained will be 

 2x120x7-5 = 1800 milligrammes. The lighter pencil 

 must therefore weigh less than i - 8 gramme. 



Second Experiment. — We cleanse perfectly a ring of 

 copper wire of about one millimetre in thickness and 

 eight centimetres in diameter, and place it with care 

 upon the surface of pure water in a capsule which has 

 previously been well washed. The copper ring will 

 float (fig. 2, section a), and this though its density is 

 8 - 8 times greater than that of water. This is because 

 all the tensions of the liquid which prevail on the one 

 hand and the other of the ring give a resultant acting 

 from below upwards. A very simple calculation shows 

 that the weight of the ring is about tt x (|) x ttX 80 x 8'8 

 milligrammes = 173 grammes. On the B other hand, the 

 maximum effect of the tensions is 2 XttX 80 x 8'5 milli- 

 grammes=3 - 77 grammes. We see, then, that even if 

 we disregard the thrust of the liquid, the effect of the 

 surface tension is more than double the weight of the ring. 



In the same manner we may float upon water needles, 

 globules of mercury, a slender platinum ring, etc. 



Third Experiment. — We procure a leaf of light un- 

 glazed paper (e.g., silk-paper), say seventeen centimetres 

 in length and three in width. All the edges are folded 

 (fig. 3) so as to form a rectangle of fifteen centimetres 

 in length and one centimetre in breadth ; all the edges 

 are turned up to the height of one centimetre, and four 

 small folds are made following a diagonal of each of the 

 squares formed by the first folds, and we thus obtain a 

 small box, the long sides of which are made very flat. 

 This being done, the apparatus is set upon a table ; all 

 the inner surfaces are well moistened by means of a 



brush, and water is poured in to the height of 4 — 5 milli- 

 metres. Immediately, the tension of the liquid surface 

 draws together the long sides which are opposite to each 

 other, and the vessel becomes closed automatically. 



Fourth Experiment. — Take a cylindrical cork (fig. 4) of, 

 e.g., two centimetres in thickness and four in 

 length ; in the centre of one of the ends we insert a 

 very fine iron wire, six or eight centimetres in length, 

 and having at its lower end a hook or a small basket 

 suitable for receiving a load. To the opposite end of the 

 cork is fixed a system composed of a ring of fine wire 

 ten centimetres in diameter, and supported by two ends 

 of the same wire which are inserted into the cork in 

 such a manner that the plane of the ring may be per- 

 pendicular to the axis of the latter and placed concen- 

 trically with respect to it. 



This being done, the small apparatus is plunged into 

 water contained in a vessel of sufficient depth ; if the 

 load be suitable, the cork will remain upright, and will 

 only project eight or ten millimetres above the level 

 (fig. 4). If the entire system be made to descend ver- 

 tically into the water and be left to itself, the ring will 

 not again leave the water; it will merely rise a little 

 above the level, producing a bi-concave meniscus. Here 

 the result of the surface tension gives rise to a resultant 

 acting from above downwards and sufficient to counter- 

 balance the increase of the thrust. If the load be suit- 

 able, this resultant, augmented by the weight of the 

 system, very little exceeds the upward thrust of the 

 liquid. It is then sufficient to bring near the water a 

 little wadding soaked in ether (which decreases the 

 surface tension) to see the ring, in appearance, spon- 

 taneously rise out of the liquid and the system resume its 

 original position of equilibrium. 



(To be continued.) 



PAPER BOTTLES. 



ONE of the most interesting of the many uses to 

 which paper has been put is the manufacture of 

 paper bottles. 



We have long had paper boxes, barrels, and car 

 wheels, and more recently paper pails, wash basins, and 

 other vessels ; but now comes a further evolution of 

 paper in the shape of paper bottles, which are already 

 quite extensively used for containing such substances as 

 ink, bluing, shoe dressing, glue, etc., and they would 

 seem to be equally well adapted for containing a large 

 variety of articles. 



They are made by rolling glued sheets of paper into 

 long cylinders, which are then cut into suitable lengths, 

 tops and bottoms are fitted in the inside coated with a 

 waterproof compound, and all this is done by machinery 

 almost as quickly as one can count. 



They are cheaper and lighter than glass, unbreakable, 

 and consequently very popular with consumers, while 

 the fact that they require no packing material, and are 

 clean, handy, and economical, commends them to manu- 

 facturers. Unlike glass, they can be manufactured and 

 shipped at all seasons ; and being made by machinery, 

 the supply is independent of labour troubles, which are 

 additional advantages to manufacturers who use bottles. 



«-^t^«^5*f-. — - 



Function of the Halteres or Balancers in Dip- 

 terous Insects. — It is contended that in the common fly 

 and in its congeners these parts contain the organs of 

 hearing. 



