78 Mr Darwin and Miss Pertz, On the effect of [Feb. 10, 



Having shown that alcohol added to the water in the funnel 

 increases the rate of bubbling by diminishing the surface-tension, 

 we expected to find that alcohol, added to the beaker in which the 

 leaves were, would produce either no effect or less than the normal 

 effect. It turned out however that the addition of alcohol to 

 the beaker produced nearly the usual effect. This proved that the 

 accelerating effect produced by alcohol when the cut stalk and 

 the leaves are in the same vessel of water is not simply due to 

 diminished surface-tension, although this cause undoubtedly has 

 some effect. 



At this point Mr Blackman made the useful suggestion that 

 water should be added instead of alcohol. It was found that 

 this treatment greatly increased the rate of bubbling, and it 

 at once became clear that this result and the chief part of the 

 alcohol effect were due to the currents set up in the water by 

 the act of pouring in the fluid, water or alcohol as the case may 

 be. This was confirmed by gently stirring the surface-water with 

 a rod, until, by the movement of small particles floating in the 

 water, it could be seen that the lower strata were in rotation. 



Under this treatment the rate of bubbling at once became 

 nearly doubled. It should be noted that the plant was not visibly 

 shaken or moved, so that the effect was not due to the bubbles 

 being helped to escape by mechanical shock. This point is worth 

 mention, because a blow on the table was found to cause a slight 

 escape of gas from the glass capillary of the above described 

 apparatus at a pressure which had ceased to produce a flow of 

 bubbles. 



Method. 



The experiments were made in a dark room illuminated by 

 one of the Incandescent Gas Company's burners, in which a 

 white-hot mantle, surrounding an argand flame, gives a brilliant 

 steady light. Between the gas and the vessel in which was the 

 bubbling plant we placed a glass trough through which a current 

 of water flowed. In this way the plants could be kept at a tempe- 

 rature sufficiently constant. 



The plant to be experimented on was tied to a glass rod sup- 

 ported vertically in a beaker of water. In all the earlier ex- 

 periments the yield of gas was estimated by taking with a 

 stop-watch the time which elapses during the evolution of a given 

 number of bubbles. The reciprocals of these times will be pro- 

 portional to the rate of bubbling. The " rates " given in our 

 experiments are simply derived from the reciprocals by moving 

 the decimal place. The " rates " are comparable throughout any 

 given experiment, but the rate in one experiment is not neces- 

 sarily comparable with that in any other. 



