HYDRODYNAMICS. 



473 



Jan, and Messrs 



Hauy and Tremery found the following de- 

 of mercury : 



Diameter of the tube. 

 1 millimetres. 

 1.333 



Depression of the mercury. 



3.666 millimetres. 

 5.5 



,: -., .. 



The ultimate product or constant quantity inferred 

 by Dr T. Young from Lord C. Cavendish'* experi- 

 ments is 0.015, whereas Hauy's experiments make it 



0.01137. 



The results of the experiments of Gellert have al- 

 ready been given in p. 468. 



Water suffers also a depression like mercury in tubes 

 of glass that have been coated with grease. This was 

 first observed by Carre, and was afterwards verified 

 by the experiments of Cigna and Dutour. 



If. Dutour took two tubes, each of which was about 

 two lines in diameter, and having lined one of them 

 with a thin film of wa, ami the other with grease, he 

 plunged them about four lines deep in water. The 

 water was depressed in both the tubes, but less in the 

 first than in the second. 



4. On Ike Adhetia* of Duct to tke Surface of Fluid*. 



In our article ADHHIOW, w have already given an 

 account of the experiments of Taylor, Morveau, Ach- 

 anl, and Dutoar. 



The following mutts were obtained by M.Gay Lus- 

 sac for a circular plate of white glass, with water, al- 

 cohol, and oil of turpentine. 



I.us- 



Water 

 Alcohol 



Alcohol 

 Alcohol 



Diameter of UM 

 circular pUte. 

 MilliSMtna, 

 . 118.366 

 . 118.366 

 . 118.366 

 118.566 



w, ,-. .. 



toniwit. 



>;,-, .'- 



Oil of turpentine 118.366 



' ..to 

 31.08 

 34.87 

 37.152 



04*141 



The following result was obtained for a disc of cop- 

 per : 



r of disc of Weight nfrfsssry Temperature 

 capper. to rais* lU ccntijrrad*. 



Water . 110.604 67.945 18'.5 



M. Gay Lussac made many experiments on the ad- 

 hesion of a disc of (has to mercury, but the result* which 

 be obtained differed widely from one another. In making 

 his experiments on the adhesion of disc* of glass, the 

 disc was suspended at one scale of a balance, and raixd 



VOL. II. FABT II. 



sion of 

 Fluids. 



vertically by placing, successively and slowly, small On 

 weights in the other scale. The sum of these weights Ur 7 

 at the moment when the disc detached itself, indicated 

 the force of adhesion. In making these experiments on 

 mercury, however, he observed, that the sum of the 

 weights was more or less great according to the slow- 

 ness with which they were successively added ; and in 

 adding them at very great intervals, the sum varied 

 from 158 grammes to 2y6 grammes for a disc 118.366 

 millimetres in diameter. 



Resile found, that the adhesion of 25 square lines ot B es ;] e ' F cx . 

 mercury was 32 French grains, while that of the same perimenu. 

 surface of water was 8 1 grains. In some cases, he 

 found that the apparent adhesion was diminished under 

 the exhausted receiver of an air pump. 



5. On the Magnitude and Form of Dropt of Fluids. 



The effect of the cohesion of fluids is very finely ex- p henome . 

 emplified in the formation of drops. It is obvious, that n O f 

 drops of fluids that have the least force of cohesion, drop*. 

 will have the least magnitude, provided their specific 

 gravities are the same ; for the effect of the force of co- 

 hestori must be diminished by the weight of the drop 

 which will be sooner detached, and therefore of a less 

 magnitude than if the fluid had less weight. Dr Young 

 infers, from the law of the superficial cohesion of fluids, 

 " that the linear-dimensions of similar drops depending 

 from a horizontal surface, must vary precisely in the 

 same ratio as the heights of ascent of the respective 

 fluids against a vertical surface, or as the square roots 

 of the heights of ascent in a given tube. Hence the mag- 

 nitudes of similar drop* of different fluids must vary a* 

 the cubes of the square roots of the heights of ascent in 

 a tube." In water, for example, Dr \ ounp found the 

 weight of a drop to be 1.8 grains, and the weight of a 

 drop of diluted alcohol 0.85 of a grain ; whereas the 

 height of the same alcohol was to the height of water 

 in the tame tube as 100 to 64. The weight of the 

 drop should have been .82, as inferred from the consi- 

 deration of the heights of ascent combined with that 

 of the specific gravities This result is widely different 

 from that which was obtained by Dr Brewster (See p. 

 448. ) with his capillary hydrometer. The magnitude 

 of a drop of water was to the magnitude of a drop of 

 spirit nearly proof as 8.93 to 1 ; and. therefore, taking 

 the specific gravity of spirit at 0.920, the weights of 

 the drops wen to one another as 3.255 to 1, or as 100 

 to 3 1 nearly. 



The magnitude of drops of fluid* depends also upon 

 UM form of the surface from which they fall. It the 

 fluid is collected at th extremity of a very minute fibre 

 of glass, the drop will fall when its weight balance* 

 the attractive force exerted by the glass, and therefore, 

 in the present case, the drop will be very small ; but 

 if the fluid is collected on a hemispherical surface, the 

 surface of glass which is in contact with the fluid is 

 greater ; and, therefore, the drop must contain a much 

 greater quantity of water before its weight balances the 

 attractive force of the hemispherical surface. 



The form of a drop of fluid, abstracting the consi- 

 deration of its weight, would always be that of a per- 

 fect sphere; and we accordingly find that the drops of 

 rain by which the rainbow is formed, and very small 

 drops of fluids lying upon a surface which does not at- 

 tract them, have a shape almost perfectly spherical. 

 In other cues the form of a drop is modified by its 

 Jo 



