450 Professor Tijndall [May 3, 10, and 17, 



more influential than the action of the warm wind in producing the 

 observed effect. 



Various interesting experiments were made by Rumford on what 

 is now known as '' surface tension." From his experiments he in- 

 ferred that the surface of a liquid — of water for example — is covered 

 by a pellicle which can be caused to tremble throughout, by touching it 

 with tbe point of a needle. He proposed to the geometricians of 

 Paris to determine the shape of a drop resting on a horizontal 

 surface, and restrained solely by the resistance of a pellicle exerting a 

 given force on its surface. This pellicle he considers to be due to 

 the adhesion of the particles of liquids to each other, and he makes 

 various ingenious calculations to determine the size of a particle of 

 heavy matter, of gold for instance, which would rest suspended in 

 water because of its inability to force asunder the particles of the 

 liquid. The diameter of a sphere of gold which would behave in this 



way he found to be YTT^Virs" ^^ ^^ inch. 



Even among scientiiic men, probably few are aware that Rumford 

 experimented on the diffusion of liquids ; a field of investigation in 

 which Graham afterwards rendered himself so- eminent. Into a glass 

 cylinder. If inch in diameter and 8 inches hijzh, he poured a layer of 

 saturated aqueous solution of muriate of soda 3 inches thick, over 

 this he carefulh^ poured a layer of distilled water of the same thick- 

 ness, he then let a drop of the oil of cloves fall into the vessel. This 

 oil, being heavier than the pure water and lighter than the solution, 

 rested as a sphere at the common boundary of the two liquids. A 

 layer of olive oil four lines in thickness was then poured over the 

 water, in order to shut off the air. The object of the experiment was 

 to ascertain whether one liquid remained permanently superposed 

 upon the other without any mixing. If this proved to be the case 

 the position of the drop of oil would remain constant ; but if the 

 heavy mineral solution rose into the water overhead, the di*op of 

 oil, which Rumford called his "little sentinel," would warn him of 

 the event by rising in the liquid. After twenty -four hours he entered 

 the cellar in which the experiment was made, and found that the little 

 ball of oil had risen three lines. For six days it continued to rise at 

 the rate of about three lines a day. He afterwards experimented with 

 other solutions, the result being '" that the mixture went on continually, 

 but very slowly, between the various aqueous solutions employed and 

 the distilled water resting upon them." Rumford's experiments were 

 probably prompted by his views on molecular physics. He would 

 hardly have thought of the foregoing arrangement were not the intestine 

 motions of the ultimate particles of bodies present to his mind. He 

 is, moreover, quite aware of the importance of the result which he has 

 here established. He says that the subject has often occupied his 

 thoughts, and that he had at different times made " a considerable 

 number of experiments with a view of throwing light into the profound 

 darkness with which the subject is shrouded on every side." 



His manifold industry was devoted in part to steam considered as 



