112 Mr. G. F. Rodwell on some Effects 



the thread gathers itself into a sphere. The lower mass com- 

 mences its descent as an ellipsoid having its major axis at right 

 angles to the liquid surface, next becoming a sphere, then an 

 ellipsoid with its major axis parallel with the liquid surface, once 

 more a sphere, and so on until it reaches the bottom of the 

 vessel, while the upper mass of oil ascends to the surface, and 

 comes to rest as a hemisphere with its plane surface in contact 

 with the surface of the liquid. 



Let us consider the rationale of this experiment. 



It is universally stated that when a mass of liquid is heated 

 from beneath, the heat is conveyed to different parts by cur- 

 rents (a central ascending current of warm liquid, and descend- 

 ing side currents of cold liquid), by which means the whole mass 

 of liquid soon becomes of a uniform temperature. Now, if the 

 liquid in the above experiment rapidly acquired a uniform 

 temperature, it is obvious that the lengthening of the oil-sphere 

 into an ellipsoid and the transformation of the ellipsoid into 

 isolated masses could not take place. For if the density of the 

 liquid were uniformly lessened, the oil- sphere would descend 

 en masse, and rupture could not ensue between its particles, be- 

 cause before the occurrence of rupture gravity must act unequally 

 upon the sphere; that is to say, the upper part must be com- 

 petent, while the lower part is incompetent, to resist the action 

 of the force. I was hence led to examine whether, under the 

 conditions of the above experiment, the liquid does rapidly 

 acquire a uniform temperature. 



In the case of a homogeneous liquid, or of a uniform mix- 

 ture, I found that the same temperature was rapidly acquired 

 by all parts during heating from beneath. An oil-sphere 

 placed in a perfectly uniform mixture, did not lengthen itself 

 into an ellipsoid, but descended without change of form to the 

 bottom of the beaker. The manner of trying the experiment 

 described above was, to mix alcohol and water until the mixture 

 possessed nearly the specific gravity of oil, then to introduce 

 the oil, and to bring the oil-sphere to its desired position by 

 pouring in alcohol. Now alcohol and water do not very readily 

 mix, hence the upper layers of the liquid contained more alcohol 

 than the lower layers ; and this accounts at once for the different 

 temperatures observed, because the capacities for heat of alco- 

 hol and water differ considerably. It will be seen by the fol- 

 lowing results that, under the conditions of the above experi- 

 ment, the alcohol- and- water mixture does not rapidly acquire a 

 uniform temperature. 



The mixture was placed in a beaker, and it occupied a depth 

 of 175 millims. therein ; the beaker was placed upon a fiat piece of 

 copper heated by hot water. In order to measure the temperature 



