122 Prof. Buff on the Electricity of Plants. 



When the fixed temperature is that of meltlug ice, M. Re- 

 gnault's experiments give 606*5 Centigrade degrees, applied to 

 liquid water as the value of this constant ; so that 

 L + K,T=Kw(606°-5 + -305TO) ^ 



for the Centigrade scale, I 



= Kw(1091°-7 + -305(T0-32°)) | ' ' ' ^ ) 

 for Fahrenheit's scale, J 



is the complete expression for the heat required to raise unity of 

 weight of water from the temperature of melting ice to T*^ above 

 the ordinary zero, and to evaporate it at the latter temperature. 

 It must be remarked, that the unit of heat in M. Regnault's 

 tables is not precisely the specific heat of water at O'' Centigrade, 

 but its mean specific heat between the initial and final tempera- 

 tures of the water in the calorimeter. The utmost -error, how- 

 ever, which can arise from this circumstance, is less than yg^^o 

 of the total heat of evaporation, so that it may safely be neglected. 

 [This section formerly concluded with a computation of the 

 specific heats of steam on the supposition of its being sensibly a 

 perfect gas; but the error of this supposition, though not mate- 

 rial in calculating the power of steam-engines, is in all proba- 

 bility sufficiently great to vitiate the computation in question, 

 which has therefore been cancelled.] 



[To be continued.] 



XVII. On the Electricity of Plants. By Professor H. Buff*. 



ABOUT twenty-seven yeara ago Pouillet {Ann. de Chim. et de 

 Phys. XXXV. 401) described a number of experiments, from 

 which he concluded that plants, during their germination and 

 growth, impart positive electricity to the air and negative electri- 

 city to the soil. The majority of physicists accepted this con- 

 clusion without further proof, inasmuch as it appeared to render 

 a satisfactory account of the origin of atmospheric electricity. 

 A repetition of Pouillet's experiments, with more delicate appa- 

 ratus, was undertaken by P. lliess, who however was unable to 

 obtain the results of the fonner investigator. Two yeara ago, 

 Wartmaim (Phil. Mag. [4] i.578.) and Becquerel {Ann.de Chim. 

 et de Phys. [3] xxi. 10) communicated observations, according 

 to which growing plants impart negative electricity to the atmo- 

 sphere and positive electricity to the soil, the two electricities 

 thus moving in directions opjiosite to those inferred from the 

 observations of Pouillet. 



The mode of experiment pursued by the last two physicists 

 was as follows ; — Homogeneous platinum wires were brought 

 * Communicated by the Author. 



